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ScienceWeek
ScienceWeek - May 3, 2002 Vol. 6 Number 18
A Online Research Digest Published Weekly Since 1997
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A star is simpler than an insect.
-- Martin Rees
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Top Graphic: The Siesta -- Paul Gaugin (1848-1903)
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Section 1
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Contents of this Issue (Full reports in Section 2):
[(*) = includes background reports from ScienceWeek] [Subheads
under each report (e.g., 2a) provide supplementary material from
other archives related to main report.]
Basic Sciences:
1. On Quantum Entanglement of Protons in Organic Molecules
1a. On Quantum Entanglement
2. Long Range Order and Surfaces
3. On the Facets of Crystals
4. Statistical Mechanics of Complex Networks
4a. On Complex Networks
5. On Marie Curie (1867-1934) (*)
6. Black Holes, Magnetic Fields, and Relativistic Jets (*)
7. Function of a Nonclassical Photopigment in the Human Eye
7a. On the Human Retina
8. Vulnerability of the Human Spermatozoon 8a. Human Sperm-Egg
Cell Interaction
9. On Hydrophobic Clusters and Protein Folding (*)
9a. Hydrophobicity Analysis of Amino Acids
10. Proteins: On Beta-Sheet Structures
11. Fine Structure of DNA Toroids
12. Species Diversity and Ecosystem Functioning
Praxis:
13. On Quantum Physics and Information Theory (*)
14. Aerosols, Climate, and the Hydrological Cycle (*)
14a. Aerosol Pollution and Earth's Water Supply
15. Chiral Architectures in Synthetic Macromolecules
16. On Luminescent Molecular-Level Devices
17. On Construction of Circular Oligonucleotides
18. On Surfactant-Mediated Synthesis of Ordered Mesoporous
Solids
19. On Fish Fatty Acids and the Risk of Sudden Death
20. Air Pollution: Long-Term Exposure and Disease (*)
21. On Plant-Insect Biochemical Warfare (*)
22. NMR Spectroscopy and Structural Proteomics
23. Hospital Volume and Surgical Mortality
24. On Cell Barriers to Drug and Gene Delivery
Miscellany:
25. In Focus: Supramolecular Polymers
26. ScienceWeek Notices and Subscription Information
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Section 2
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1. ON QUANTUM ENTANGLEMENT OF PROTONS IN ORGANIC MOLECULES
C.A. Chatzidimitriou-Dreismann et al (Technical University of
Berlin, DE) discuss quantum entanglement in organic molecules,
the authors making the following points:
1) According to the conventional viewpoint, an elementary
chemical reaction can be theoretically represented by the
"motion" of nuclei (treated either as classical mass points or
as quantum wave packets) on Born-Oppenheimer (B-O) potential
energy surfaces. These potential energy surfaces are determined
by solving the electronic time-independent Schroedinger equation
under the approximation of the nuclei as classical mass points
while keeping their positions fixed at various spatial
configurations. But nuclei are also quantum objects, and thus
they can occupy nonclassical states, which are caused by
interactions between the particles themselves, as well as
interactions of the particles with electronic charges in their
vicinity. These states are also described as quantum entangled,
quantum correlated, "Schroedinger's cat",
Einstein-Podolsky-Rosen correlated, and so forth.
2) At present, multiparticle quantum entanglement and its decay
dynamics, called "decoherence", are the focus of several
experimental and theoretical fields of physics and engineering,
e.g., quantum optics, quantum computation and information,
quantum cryptography, and teleportation. These investigations
usually deal with pairs of quantum particles (photons, atoms,
ions, etc.) that are carefully isolated from their environment
in order to keep quantum entanglement intact for a time
sufficiently long to become measurable. In condensed systems at
ambient experimental conditions, however, quantum entanglement
is widely believed to be unimportant and/or not accessible to
experiments, because of its extremely fast decoherence caused by
environmental disturbances.
3) The authors present experimental evidence for quantum
entanglement between well-localized protons of C--H bonds of
2-isobutoxyethanol dissolved in D(sub2)O. The applied
experimental method is neutron Compton scattering, which has a
characteristic time window in the subfemtosecond time range. The
results of these experiments reveal that in the subfemtosecond
time scale the measured cross-section density (i.e., the
effectively present concentration) of the H atoms is
"anomalously" reduced by approximately 20 percent. The authors
suggest that this novel effect, involving the microdynamics of
protons of covalent C--H bonds, may have a broad range of
chemical and biological applications.
J. Am. Chem. Soc. 2001 123:11945
References (abridged):
1. Editorial. J. Am. Chem. Soc. 1997, 7/9, 6213.
2. (a) Zewail, A. H. Femtochemistry: Ultrafast Dynamics of the
Chemical Bond; World Scientific: Singapore, 1994. (b) Femtost
Chemistry, Manz, J., Woste, L. Eds.; VCH Verlagsgesellschaft:
Wein 1995.
3. Bohm, A. Quantum Mechanics: Foundations and AppMcation, ed.;
Springer-Verlag: New York, 1993.
4. (a) Zurek, W. H. Phys. Today 1991, 44 (10), 36-44. (b)
Haroche, S. Phys. Today 1998, 57 (7), 36-42. (c) Anglin, J. R.;
Paz, J. P.; Zurek, W. H. Phys. Rev. A 1997, 55, 4041-4053. (d)
Tegmark, M. Phys. Rev. E 2000, 67, 4194-4206.
5. (a) Zeilinger, A. Sci. Am. 2000, 252 (4), 32-41. (b) The
Physics of Quantum Information', Bouwmeester, D., Ekert, A.,
Zeilinger, A., Eds.; Springer-Verlag: Berlin, 2000. (c) Nielsen,
M. A.; Chuang, I. L. Quantum Computation and Quantum
Information', Cambridge University Press: Cambridge, 2000.
Also:
Ex Link: On Quantum Entanglement
ScienceWeek http://www.scienceweek.com
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2. ON LONG RANGE ORDER AND SURFACES
R. Drautz et al (Max Planck Institute for Metals Research
Stuttgart, DE) discuss long range order and surfaces, the
authors making the following points:
1) The spontaneous organization of long-ranged ordered
structures is an important part of condensed matter physics. It
is now understood that the appearance of order depends on the
one hand, in general, on the symmetry and dimensionality of the
system, and on the hand crucially on the individual details of
the underlying microscopic interactions. Surfaces and interfaces
are ideal objects to access this interplay between general
(universal) effects and interaction-dependent effects on
long-range order. The presence of a free surface gives rise to a
symmetry break leading to ordering phenomena between 2 and 3
dimensions, and simultaneously allows surface-modified
interactions and surface fields to enter the scenario to
influence the surface ordering and even lead to new classes of
universal effects.
2) Binary alloys that exhibit bulk ordered structures and bulk
order-disorder transitions have served as model systems to study
surface-ordering phenomena. At the surface of the alloy, the
relative strength of the competing bulk interactions may be
significantly disturbed, and new phases may be allowed at the
surface that are forbidden in the bulk. For a microscopic
understanding of the stability of such surface phases, rigorous
calculation of both bulk and surface-altered interactions is
necessary but still not achieved.
3) The authors report a study of NiAl alloys that show
remarkable bulk and surface properties. The bulk phase diagram
exhibits a variety of ordered and disordered structures. A
delicate competition between the tendency for Al segregation and
ordering in the Ni-Al system, induced by the symmetry break at
the surface, stabilizes a long-range ordered surface in the
entire concentration range [Ni] > 0.75.
Phys. Rev. Lett. 2001 87:236102
References (abridged):
1. H. Nakanishi and M. E. Fisher, J. Chem. Phys. 78, 3279 (1983).
2. H. W. Diehl, in Phase Transitions and Critical Phenomena,
edited by C. Domb and J. L. Lebowitz (Academic, London, 1988),
Vol. 10.
3. H. Dosch, Critical Phenomena at Surfaces and Interfaces,
Springer Tracts in Modern Physics Vol. 126 (Springer, Berlin,
1992); H. Dosch, Appl. Phys. A 61, 475 (1995); M. Polak and L.
Rubinovich, Surf. Sci. Rep. 38, 127 (2000).
4. G. Kostorz, in Dynamics of Ordering Processes in Condensed
Matter, edited by S. Komura and H. Furukawa (Plenum Press, New
York, 1988), p. 199.
5. M. Polak, J. Deng, and L. Rubinovich, Phys. Rev. Lett. 78,
1058 (1997); Phys. Rev. Lett. 74, 4059 (1995).
ScienceWeek http://www.scienceweek.com
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3. ON THE FACETS OF CRYSTALS
H. Alles et al (Helsinki University of Technology, FI) discuss
crystal facets, the authors making the following points:
1) The crystals that we find in nature usually have a polyhedral
shape. Their surfaces are covered with smooth faces (facets)
that correspond to high-symmetry crystallographic orientations.
These facets have developed on the crystal surfaces during
growth when the crystals were formed. Studies of crystal growth
dynamics and the equilibrium shape of ordinary crystals are
difficult, the difficulties caused by large differences in
entropy and density between the solid and the adjacent melt or
vapor phases, and finite thermal conductivities of these bulk
phases. As a result, the relaxation processes are highly
dissipative and the corresponding time scales extremely long.
The equilibrium shapes have therefore been obtained with
microscopic metallic crystals, which after annealing for several
days show facets surrounded by rough (rounded) areas on their
surfaces.
2) In the middle of the 20th century, Lev Landau (1908-1968)
predicted that in the limit of low temperature, any ideal
crystal should be completely faceted, with an infinite number of
facets on its surface. This is the so-called "devil's staircase"
phenomenon. The most complicated crystals, however, have shown
at most six types of facets. Only very recently, Pieranski et al
(2000) were able to grow lyotropic monocrystals that revealed
more than 60 types of facets on their surfaces, experimentally
confirming the devil's staircase phenomenon. Pieranski et al
interpreted their results as the coincidence of a quite large
surface tension and interplanar distance with an exceptionally
low elastic modulus. But with liquid lyotropic crystals,
faceting can be studied in only a very limited temperature
range, and it is difficult to measure the equilibrium shape of
the crystals.
3) The experimental situation is more promising in helium
crystals, which exist at low temperatures and high pressures.
The dynamics of helium crystals, which are surrounded by a
superfluid with high thermal conductivity, can be extremely
fast. For example, He-4 crystals have been observed to relax to
their equilibrium shape within a fraction of a second at
temperatures below 1.5 kelvins. Also observed in He-4, and not
observed in ordinary systems, is the phenomenon of
"crystallization waves", periodic melting-freezing waves on the
superfluid-solid interface that are characterized by a very
small dissipation.
4) The authors report experiments in which a total of 11
different types of facets were identified on growing He-3
crystals at the temperature of 0.55 millikelvins by using a
low-temperature Fabry-Perot interferometer.
Proc. Nat. Acad. Sci. 2002 99:1796
References (abridged):
1. Chernov, A. A. (1984) Modern Crystallography III: Crystal
Growth (Springer, Berlin).
2. Landau, L. D. (1971) Collected Papers (Pergamon, Oxford).
3. Schulz, H. J. (1985) J. Physique 46, 257-269.
4. Burkov, S. E. (1985) J. Physique 46, 317-327.
5. Pieranski, P. , Sotta, P. , Rohe, D. & Imperor-Clerc, M.
(2000) Phys. Rev. Lett. 84, 2409-2412.
ScienceWeek http://www.scienceweek.com
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4. STATISTICAL MECHANICS OF COMPLEX NETWORKS
R. Albert and A-L. Barabasi (University of Notre Dame, US)
discuss complex networks, the authors making the following
points:
1) Complex weblike structures comprise a wide variety of systems
of high technological and intellectual importance. For example,
the biological cell is best described as a complex network of
chemicals connected by chemical reactions; the Internet is a
complex network of routers and computers linked by various
physical or wireless links; fads and ideas spread on the social
network, whose nodes are human beings and whose edges represent
various social relationships; the World Wide Web is an enormous
virtual network of Web pages connected by hyperlinks. These
systems represent just a few of the many examples that have
recently prompted the scientific community to investigate the
mechanism that determine the topology of complex networks, and
the desire to understand such interwoven systems has encountered
significant challenges.
2) Physics, a major beneficiary of reductionism, has developed
an arsenal of successful tools for predicting the behavior of a
system as a whole from the properties of its constituents. For
example, we now understand how magnetism emerges from the
collective behavior of millions of spins, or how quantum
particles lead to such spectacular phenomena as Bose-Einstein
condensation or superfluidity. The success of these modeling
efforts is based on the simplicity of the interactions between
the elements: there is no ambiguity as to what interacts with
what, and the interaction strength is uniquely determined by the
physical distance.
3) We are at a loss, however, to describe systems for which
physical distance is irrelevant or for which there is ambiguity
as to whether two components interact. While for many complex
systems with nontrivial network topology such ambiguity is
naturally present, in the past few years it has been
increasingly recognized that the tools of statistical mechanics
offer an ideal framework for describing these interwoven systems
as well. These developments have introduced new and challenging
problems for statistical physics and unexpected links to major
topics in condensed matter physics, ranging from percolation to
Bose-Einstein condensation. Concerning complex networks in
general, while traditionally such systems have been modeled in
random graphs, it has become increasingly recognized that the
topology and evolution of real networks that deviate from random
graphs are governed by robust organizing principles that need
quantitative formulation.
Rev. Mod. Phys. 2002 74:47
References (abridged):
1. Aiello, W., F. Chung, and L. Lu, 2000, Proceedings of the
32nd ACM Symposium on the Theory of Computing (ACM, New York),
p.171.
2. ben Avraham, D., and S. Havlin, 2000, Diffusion and Reactions
in Fractals and Disordered Systems (Cambridge University Press,
Cambridge/New York).
3. Bollobas, B., 1985, Random Graphs (Academic, London). Bunde,
A., and S. Havlin, 1996, Eds., Fractals and Disordered Systems
(Springer, Berlin).
4. Crisanti, A., G. Paladin, and A. Vulpiani, 1993, Products of
Random Matrices in Statistical Physics (Springer, Berlin).
Also:
Ex Link: On Complex Networks
ScienceWeek http://www.scienceweek.com
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5. ON MARIE CURIE (1867-1934)
Roger M. Macklis (Cleveland Clinic Foundation, US) discusses
Marie Curie, the author making the following points:
At left: Marie Curie (1867-1934)
1) With the possible exception of Albert Einstein, Marie Curie
was the most famous scientist of her era and is almost certainly
the most celebrated female scientist in history. Although known
primarily for her discovery of radium, her true gift to science
was her realization that radioactivity is an intrinsic atomic
property of matter rather than the result of superficial
chemical processes. She was one of the exceedingly rare Nobel
laureates to win the prize twice (physics and chemistry). Her
life will forever reflect dogged determination, unswerving
devotion to work, political tenacity, and an optimistic belief
is scientific positivism. On a more personal note, she has also
come to symbolize a cautionary tale concerning the unfortunate
difficulties encountered when a woman enters and succeeds
dramatically and publicly in a sphere traditionally dominated by
men. Initially viewed as a mere research assistant "riding" on
the coattails of her more talented husband Pierre, his death in
1906 confronted her with the need and the opportunity to both
establish her own scientific identity and to insist, despite her
critics, on her place in the annals of the dawning technological
age. She succeeded brilliantly, although she paid a personal
price for her temerity.
At left: Pierre Curie (1859-1906)
2) After the death of her husband, Curie, now a distraught widow
at 38, resolved to carry on the work that had been so important
to her and her husband. She was promoted, amid some grumbling,
to her husband's old chair. She continued her work on the
chemical purification and characterization of radium and its
byproducts, but her colleagues noted she had lost the fire that
had imbued her previous research. A clandestine affair with the
physicist Paul Langevin (1872-1946), a married student of her
dead husband's and 5 years her junior, apparently renewed her
zeal for life, but legal action by Langevin's wife brought the
affair to the scandal sheets. Then, in the midst of all of this,
the Swedish Academy announced that Marie Curie had been awarded
an unprecedented second Nobel Prize, this time in chemistry, for
the discovery of radium. Curie traveled to Stockholm to accept
the 1911 prize, but the public scandal concerning her affair
with Langevin followed her. That year, Curie collapsed both
physically and psychologically, and her subsequent scientific
career was devoted primarily to organizing a national radium
institute. When Curie died in 1934, she became the first woman
to be granted the great honor of perpetual interment at the
Pantheon in Paris. A year after her death, her daughter Irene
won the family's third Nobel Prize.
Science 2002 295:1647
Related background:
THE CURIES AND URANIUM
In the rush to build industries and make profits from the
application of scientific knowledge, one must remember that
Nature does not easily give up her secrets, and that such
knowledge is often gained only by arduous efforts and dedicated
lives. Here are two excerpts concerning the Curies and their
work on uranium:
"On April 19, 1906, the 47-year-old Nobel laureate Pierre Curie
was run over by an oversize horse-drawn wagon filled with bales
of army uniforms. He was negotiating that tricky Parisian
intersection where traffic from the Rue Dauphine, the Quai
Conti, the Quai des Grand Augustins, and the Pont Neuf have
created Gallic havoc for over a century. Curie had just quit a
meeting of reform-minded university professors where he argued
for legislation to improve the lot of junior faculty and to
prevent laboratory accidents. He had planned to stop at his
publisher's office on the Quai, but the office was shut because
of a strike by equally reform-minded trade unionists.
Absent-minded and somewhat radium-sick, he turned away in the
spring rain, and was on his way to the library of the Institut
when that 6-ton wagon rumbled down the bridge from the Ile de la
Cite to crush his skull. The death brought to an end two
remarkably creative careers in physical science, his own and
that of his wife, Maria Salomea Sklodowska -- known to the world
as Madame Curie. She later recollected that on the Rue Dauphine,
"I lost my beloved Pierre, and with him all hope and all support
for the rest of my life." She was right: for although Madame
Curie was to survive her husband until 1934, her contributions
to science after his death were less than innovative; she turned
her tough mind to the application of their discoveries, to
teaching young scientists, and to construction of the Radium
Institute which she turned into a world center of physical
science... The story of Pierre and Marie Curie is a tribute to a
dazzling set of discoveries jointly made by a man and a woman of
genius... Among the most memorable photographs... is a late one
of an intense Marie Curie on the balcony of her Institute behind
the Ecole Normale. Her lined face looks forward into the future,
her hands are scrumbled by the scars of radium; it's an image
that sums up the hope and the harm of her discovery. She would
have been pleased that she is shown overlooking the street that
we now call the Rue Pierre et Marie Curie."
Gerald Weissmann: _Darwin's Audubon: Science and the Liberal
Imagination_
Plenum, New York 1998, p.81,85
Related Background:
ON PERSEVERANCE IN RESEARCH
"The School of Physics could give us no suitable premises, but
for lack of anything better, the Director permitted us to use an
abandoned shed which had been in service as a dissecting room of
the School of Medicine... Yet it was in this miserable old shed
that we passed the best and happiest years of our life, devoting
our entire days to our work. Often I had to prepare our lunch in
the shed, so as not to interrupt some particularly important
operation. Sometimes I had to spend a whole day mixing a boiling
mass with a heavy iron rod nearly as large as myself. I would be
broken with fatigue at the day's end. Other days, on the
contrary, the work would be a most minute and delicate
fractional crystallization, in the effort to concentrate the
radium... Thus the months passed, and our efforts hardly
interrupted by short vacations, brought forth more and more
complete evidence. Our faith grew ever stronger, and our work
being more and more known, we found means to get new quantities
of raw material and to carry on some of our crude processes in a
factory, allowing me to give more time to the delicate finishing
treatment. At this stage I devoted myself especially to the
purification of the radium, my husband being absorbed by the
study of the physical properties of the rays emitted by the new
substances. It was only after treating one ton of pitchblende
residues that I could get definite results. Indeed we know today
that even in the best minerals there are not more than a few
decigrams of radium in a ton of raw material. At last the time
came when the isolated substances showed all the characters of a
pure chemical body. This body, the radium, gives a
characteristic spectrum, and I was able to determine for it an
atomic weight much higher than that of the barium. This was
achieved in 1902. I then possessed one decigram of very pure
radium chloride. It had taken me almost four years to produce
the kind of evidence which chemical science demands, that radium
is truly a new element. One year would probably have been enough
for the same purpose, if reasonable means had been at my
disposal."
Marie Curie: _Pierre Curie_ (1927) (trans. Charlotte and Vernon
Kellogg)
[Editor's note: Marie Curie (1867-1934) and her husband Pierre
Curie (1859-1906) shared the Nobel Prize for Physics in 1903.
Marie Curie also won the Nobel Prize for Chemistry in 1911.
Three years after receiving the Nobel Prize, Pierre Curie died
at the age of 47 in a traffic accident, run over by a
horse-drawn vehicle. During her work in the shed, Marie Curie
also had to care for her 5-year-old daughter Irene, later known
as Irene Joliot-Curie. Irene and her husband Frederic Joliot
shared the Nobel Prize in Chemistry in 1935. Marie Curie died at
the age of 67 of a leukemia apparently caused by her exposure to
high-energy radiation (her notebooks are apparently still too
contaminated to handle). She died one year before her daughter
was awarded the Nobel Prize. Despite their poverty, and the
chance for obvious riches, the Curies refused to patent their
radium isolation process. Concerning the patenting of the
process, Marie Curie stated: "It would be impossible, it would
be against the scientific spirit... Physicists should always
publish their researches completely. If our discovery has a
commercial future, that is a circumstance from which we should
not profit. If radium is to be used in the treatment of disease,
it is impossible for us to take advantage of that."]
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6. BLACK HOLES, MAGNETIC FIELDS, AND RELATIVISTIC JETS
1) S. Koide et al (Toyama University, JP) discuss black holes,
the authors making the following points:
1) Relativistic jets have now been discovered in several
different classes of astrophysical objects, including active
galactic nuclei, microquasars, and gamma ray bursts. A rapidly
spinning black hole may exist at the center of each of these
objects, and energetic reactions that occur near the hole may be
responsible for the jets. One of the most promising processes
for producing relativistic jets is the extraction of rotational
energy from a spinning (Kerr) black hole. One method of
extraction is the "Penrose process", which uses fission of a
particle near the black hole to extract the black hole
rotational energy. However, this process may not be applicable
to most astrophysical objects, because the particle fission must
occur near the black hole, and the relative velocity of the
particles produced by the fission should be near the speed of
light. On the other hand, Blandford and Znajek (1977)
demonstrated that a large-scale magnetic field around a Kerr
black hole also could extract rotational energy. They assumed a
magnetic force-free condition, which corresponds to an extremely
strong magnetic field or an extremely low inertia plasma case.
2) The authors report that using numerical simulations, they
modeled the general relativistic magnetohydrodynamic behavior of
a plasma flowing into a rapidly rotating black hole in a
large-scale magnetic field. The results demonstrate that a
torsional Alfven wave is generated by the rotational dragging of
space near the black hole. The wave transports energy along the
magnetic field lines outward, causing the total energy of the
plasma near the hole to decrease to negative values. When this
negative energy plasma enters the black hole horizon, the
rotational energy of the black hole decreases. Through this
process, the energy of the spinning black hole is extracted
magnetically, and this process may be applicable to the
formation of relativistic jets.
Science 2002 295:1688
References (abridged):
1. T. J. Pearson, et al., Nature 290, 365 (1981)
2. J. A. Biretta, W. B. Sparks, F. Macchetto, Astrophys. J. 520,
621 (1999)
3. I. F. Mirabel and L. F. Rodriguez, Nature 371, 46 (1994)
4. S. J. Tingay, et al., Nature 374, 141 (1995)
5. S. R. Kulkarni, et al., Nature 398, 389 (1999)
Notes:
In this context, a "jet" is a long thin linear feature of bright
emission extending from a compact object. Jets are very common
at radio wavelengths, but have also been observed in optical and
x-ray emissions. A "relativistic jet" is a jet moving at close
to the speed of light.
A "Kerr black hole" has angular momentum but no charge (i.e., a
rotating black hole with no charge).
In general, an "Alfven wave" is a disturbance transmitted
through a plasma (a fully ionized gas) in the presence of a
magnetic field. The direction of propagation is parallel to the
mean magnetic field, with the plasma particles vibrating at
right angles to this direction. The speed of propagation, the
"Alfven speed", depends on the magnetic field strength and
plasma density. Such waves are a type of magnetohydrodynamic
wave, and they have been directly observed in solar wind
high-speed streams from the Sun and in planetary magnetospheres.
The boundary of a black hole is called the "event horizon"
(black hole horizon), because any event within the boundary is
invisible outside, the invisibility resulting from the fact that
no radiation can escape to be detected.
Some galaxies are known to have very "active" central regions
from which enormous amounts of energy are emitted each second,
and it is believed that these "active galactic nuclei" are
probably powered by accretion of matter into a supermassive
black hole of 10^(6) to 10^(9) solar-masses. Astronomers have
recently discovered that many active galactic nuclei eject
clouds of ionized gas with velocities of up to 10 percent of the
speed of light over a wide range of angles, in contrast to the
previously known collimated jets. These mass outflows are
considered to be intriguing because they provide information
about the dynamical forces (such as radiation and wind pressure)
near an active supermassive black hole.
Quasars (quasi-stellar objects) are extremely luminous sources
radiating energy over the entire spectrum from x-rays to radio
waves, and which are apparently the oldest and most distant
objects in the universe. They are believed to involve massive
black holes. Microquasars are quasars of apparent stellar mass.
Gamma ray bursts are intense flashes of gamma rays detected at
energies up to 10^(6) electronvolts. They were discovered by US
Air Force satellites in 1967 but not declassified until 1973.
The detection of these bursts averages about 1 per day, and
measurements indicate the distribution of bursts is isotropic,
i.e., they are uniformly distributed across the sky. The current
consensus is that gamma ray bursts are produced by the merger of
two neutron stars, and up to this point, the bursts that have
been noted apparently originate outside our own galaxy.
Related background:
PLASMA JETS IN ACTIVE GALACTIC NUCLEI
A.P. Lobanov and J.A. Zensus (Max Planck Institute for Radio
Astronomy Bonn, DE) discuss plasma jets in active galactic
nuclei. One of the most intriguing features observed in active
galactic nuclei is highly collimated and relativistic plasma
outflows (jets) that originate in the immediate vicinity of the
center of activity and propagate at distances of up to several
megaparsecs (1 parsec = 3.26 light years). Observations of jets
in active galactic nuclei probe the behavior of extremely
relativistic matter in the Universe and provide a unique and
remote "laboratory" for studying the most powerful cosmic
phenomena such as supermassive black holes and extragalactic
accretion disks. The quasar 3C273 is one of the closest and most
luminous and best studied active galactic nuclei, with a
prominent relativistic outflow observed in the x-ray, optical,
and radio wave bands. The relativistic jet observed in this
quasar is one-sided, with no signs of emission on the counterjet
side at dynamic ranges of up to 16,000:1. This is evidence for
strong relativistic boosting in an intrinsically double-sided
outflow powered by an accretion disk around a black hole. The
enhanced emission features (jet components) identified in the
jet on scales of up to approximately 20 milli-arc seconds are
moving at apparent speeds exceeding the speed of light by
factors of 5 to 8. These jet components may result from the
flares observed in this quasar in the optical and radio
wavelengths and also reflect the precession of the jet axis. The
structure and kinematics of such outflows are typically
explained in terms of shock waves and Kelvin-Helmholtz
instability.
Science 2001 294:128
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7. FUNCTION OF A NONCLASSICAL PHOTOPIGMENT IN THE HUMAN EYE
M.W. Hankins and R.J. Lucas (Imperial College London, UK)
discuss human vision, the authors making the following points:
1) The mammalian eye shows marked adaptation to time of day.
Some of these modifications are not acute responses to
short-term light exposures, but rely upon assessments of the
photic environment made over several hours. In the past, all
attempts at a mechanistic understanding have assumed that these
adaptations originate with light detection by one or the other
of the classical photoreceptor cells (rods or cones). However,
it has been demonstrated that the mammalian eye contains
non-rod, non-cone photoreceptors.
2) In the human retina, 2nd-order processing of signals
originating in cones takes significantly longer at night than
during the day. Long-term light exposure at night is capable of
reversing this effect. The authors report they used the cone
electroretinogram as a tool to examine the properties of the
irradiance measurement pathway driving this reversal. The
findings indicate that this pathway a) integrates irradiance
measures over time periods ranging from at least 15 to 120
minutes; b) responds to relatively bright light, having a
dynamic range almost outside the sensitivity of rods; c) acts on
the cone pathway primarily through a local retinal mechanism;
and d) detects light via an opsin:vitamin A photopigment with
maximum absorption at a wavelength of 483 nanometers.
3) The authors conclude that a photopigment with a spectral
sensitivity profile quite different from those of the classical
rod and cone opsins, but matching the standard profile of an
opsin:vitamin A-based pigment, drives adaptations of the human
primary cone visual pathway according to time of day.
Current Biology 2002 12:191
References (abridged):
1. Birch D.G., Berson E.L. and Sandberg M.A. (1984) Diurnal
rhythm in the human rod ERG. Invest. Ophthalmol. Vis. Sci.,
25:236-238.
2. Bassi C. and Powers M. (1986) Daily fluctuations in the
detectability of dim lights by humans. Physiol. Behav.,
38:871-877.
3. Roenneberg T., Lotze M. and von Steinbuchel N. (1992) Diurnal
variation in human visual sensitivity determined by incremental
thresholds. Clin. Vis. Sci., 7:83-91.
4. Hankins M.W., Jones R.J. and Ruddock K.H. (1998) Diurnal
variation in the b-wave implicit time of the human
electroretinogram. Vis. Neurosci., 15:55-67.
5. Stockton R.A. and Slaughter M.M. (1989) B-wave of the
electroretinogram. A reflection of ON bipolar cell activity. J.
Gen. Physiol., 93:101-122.
Also:
Ex Link: On the Human Retina
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8. ON THE VULNERABILITY OF THE HUMAN SPERMATOZOON
J. Aitken and J.A. Graves (University of Newcastle, UK) discuss
the human spermatozoon, the authors making the following points:
1) The impact of environmental toxicants and the innate
inadequacy of human spermatozoa are compounded by the advent of
effective contraception and the introduction of
assisted-conception technologies. This lifting of the selection
pressure on fertility means that those endowed with genes for
high fecundity have lost their advantage over those without. As
a result, future generations are bound to experience a further
decline in semen quality and, ultimately, human fertility.
2) The authors consider the mechanisms responsible for the poor
fertilizing potential and genetic damage shown by human
spermatozoa. Two main causes of germ-cell dysfunction have
recently been discovered: gene deletions on the long arm of the
male sex-determining Y chromosome, and oxidative stress. The
authors suggest these etiologies may be associated. The Y
chromosome is particularly vulnerable to gene deletions because
it is not a matching partner for the X chromosome, so it cannot
retrieve lost genetic information by homologous recombination.
Over the past 300 million years, the mammalian Y chromosome has
been reduced from a pairing partner to the X chromosome to a
shadow of its former self, rescued only by a large addition from
a non-sex-determining chromosome in "placental" mammals. Many of
the remaining genes have acquired functions essential for sex
determination and spermatogenesis.
3) The original Y chromosome contained approximately 1500 genes,
but during the ensuing 300 million years all but about 50 were
inactivated or lost. Overall, this gives an inactivation rate of
five genes per million years. The presence of many genes that
have lost their function (pseudogenes) on the Y chromosome
indicates that this process of attrition is continuing, so that
even these key genes will be lost. At the present rate of decay,
the Y chromosome will self-destruct in approximately 10 million
years. This has already occurred in the mole vole, in which the
Y chromosome (together with all of its genes) has been
completely lost from the genome.
Nature 2002 415:963
References (abridged):
1. Aitken, R. J. J. Reprod. Fertil. 115, 1–7 (1999).
2. Marshall Graves, J. A. Biol. Reprod. 63, 667–676 (2000).
3. Just, W. et al. Nature Genet. 11, 117–118 (1995).
4. Kuroda-Kawaguchi, T. et al. Nature Genet. 29, 279–286 (2001).
5. Kamp, C. et al. Mol. Hum. Reprod. 7, 987–994 (2001).
Also:
Ex Link: Human Sperm-Egg Cell Interaction
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9. ON HYDROPHOBIC CLUSTERS AND PROTEIN FOLDING
Robert L. Baldwin (Stanford University, US) discusses protein
folding, the author making the following points:
1) There is currently a debate about whether a hydrophobic
collapse precedes or occurs concomitantly with the formation of
secondary structures at the beginning of protein folding. The
role of secondary structures in guiding the folding pathway is
readily understood because such structures provide the framework
for the final native structure. It has also long been recognized
that burial of nonpolar (hydrophobic) side chains out of contact
with water provides the major source of the free energy change
that drives folding. But compelling models have been lacking for
how this burial might produce a "hydrophobic collapse" that
initiates folding.
2) The most plausible model has been the "hydrophobic zipper",
in which clusters of nonpolar side chains stabilize secondary
structures such as alpha helices or beta hairpins, and examples
are known in which such hydrophobic clusters guide the folding
process. The clusters persist until folding is complete and can
be visualized in the native structure of proteins. Hydrophobic
patches have also been observed in native protein structures at
sites where two alpha helices interact, and there is good
evidence that hydrophobic clusters of this kind can also guide
the folding process.
3) In these examples, hydrophobic clusters and secondary
structures are formed concomitantly and produce native-like
structures. But hydrophobic clusters have also been found in
denatured proteins, under conditions where secondary structures
are unstable. Some of these clusters must reflect non-native
interactions, since they cannot be found in the structures of
the native proteins. At present, it remains unclear how these
hydrophobic clusters affect the folding process.
Science 2002 295:1657
References (abridged):
1. S. Akiyama et al., Proc. Natl. Acad. Sci. U.S.A. 99, 1329
(2002)
2. W. Kauzmann, Adv. Protein Chem. 14, 1 (1959)
3. K. A. Dill, K. M. Fiebig, H. S. Chan, Proc. Natl. Acad. Sci.
U.S.A. 90, 1942 (1993)
4. K. Zdanowski, M. Dadlez, J. Mol. Biol. 287, 433 (1999)
5. M. E. Hodsdon, C. Frieden, Biochemistry 40, 732 (2001)
Also:
Ex Link: Hydrophobicity Analysis of Amino Acids
Related background:
ON EXPLANATIONS OF PROTEIN FOLDING
Since the 3-dimensional configuration of a protein is an
essential determinant of what the protein does in a biological
system, protein "folding", the process that leads to this
configuration, is a central focus in biophysical chemistry.
... ... William A. Eaton (National Institutes of Health, US)
presents a review of current research in this field, the author
making the following points:
1) There are two aspects to the problem of protein folding. The
first is predicting the 3-dimensional structure of a protein
from its amino acid sequence; the second is to understand _how_
proteins fold. The problem of protein folding has recently
assumed additional importance as more and more human diseases
(e.g., Alzheimer's and Parkinson's diseases) are believed to be
caused by aggregation of misfolded proteins.
2) The question of _how_ a protein folds can be phrased more
precisely as follows: What are the sequences of structural
changes that occur in a polypeptide as it finds its way from the
myriad of possible structures in the *denatured state to the
final unique *native structure? How many different folding
routes exist, and what are their relative probabilities?
3) Until approximately a decade ago, the problem of
understanding how proteins fold was addressed by identifying and
characterizing one or two metastable structures believed to be
obligatory intermediates in a sequential process along a well-
defined protein-folding pathway. The prevailing view was that
structural characterization of such intermediates would give the
clue to the basic underlying mechanism, as in the study of
organic chemical reactions. However, unlike small-molecule
chemical reactions, in which covalent bonds are broken and new
bonds formed in a structurally well-defined transition state,
the many degrees of freedom of a polypeptide chain demand a
different approach. A polypeptide of 100 amino acids has a huge
number of conformations, even if only a tiny fraction of the
more than 2^(100) (= 10^(30)) possible conformations are
thermally occupied. Understanding the complexities of protein
folding at the microscopic level, and developing models that
make quantitative predictions, therefore requires a statistical
approach, i.e., the theoretical and computational tools of
modern statistical mechanics.
4) Nonexponential kinetics have played an important role in
understanding conformational changes in native proteins. They
are particularly interesting for protein folding because they
could arise from a process that is "downhill" in free energy,
i.e, one in which the overall free energy barrier separating the
native from the denatured state is very small or nonexistent.
For large barriers, only the structures of the initial and final
states are observable, because structures along the folding
route are too sparsely populated. If, however, the barrier
becomes very small or disappears altogether, all of the
structures can in principle be detected and characterized by
spectroscopy.
5) At the present time, there exists the exciting prospect of
performing single molecule experiments for direct exploration of
the energy landscape and folding routes. Finding proteins that
fold with a "downhill scenario" is an essential first step in
this quest. That some proteins will exhibit downhill folding,
moreover, is one of the novel theoretical predictions of an
energy landscape analysis of protein folding.
Proc. Nat. Acad. Sci. US 1999 96:5897
Notes:
... ... *denatured state: In biochemistry, the term
"denaturation" refers to the complete unfolding and loss of
catalytic activity of a protein.
... ... *native structure: The "native" structure or
configuration of a biological macromolecule is the functional
state or configuration ordinarily assumed by the molecule in the
biological system in which the molecule occurs.
Related background:
ON THE CHEMICAL PHYSICS OF PROTEIN FOLDING
... ... C.L. Brooks et al present a short review of protein
folding from the perspective of chemical physics, and with a
focus on the work of their own group, the authors make the
following points: 1) The question of the mechanism of protein
folding was once thought to be entirely analogous to the
question of mechanism in intermediary metabolism or classical
organic chemistry: the essential classical idea was that a
protein folding pathway involves a series of discrete
intermediates. Such discrete intermediates do occur in the late
stages of protein folding, but to answer the practical questions
of structure prediction and design, a new viewpoint on folding
is required. 2) The authors suggest this new viewpoint is that
of chemical physics rather than that of classical chemistry, and
that the chemical physics view requires a new set of theoretical
ideas, computational techniques, and major advances in
experimental methodology. 3) The authors suggest the theoretical
framework for the new chemical physics approach to protein
folding should be that of "*energy landscape theory", which
asserts that "a full understanding of the folding process
requires a global overview of the energy landscape." 4) The
authors propose that the protein folding energy landscape
resembles a partially rough funnel riddled with energy traps
where the protein can transiently reside. There is no unique
pathway but a multiplicity of convergent folding routes toward
the native state... The authors state that the essence of the
funnel energy landscape idea is competition between the tendency
toward the folded state and trapping because of "ruggedness" of
the funnel. 5) Concerning theoretical modeling, the authors
point out that simulations with detailed atomic models are
extremely intensive numerically, so that the number and size of
systems that can be studied is limited. Simulation models of
intermediate complexity have therefore been used. 6) Concerning
experimental approaches to exploring the energy landscape of
protein folding, there are various new methods involving the
physical monitoring of folding from an unfolded state, for
example, monitoring in the microsecond range following
initiation of folding by a nanosecond-scale step-change in
ambient temperature. The authors conclude: "Experiments are
beginning to build up a *phase diagram of folding kinetics that
can be used to test and refine theoretical models."
Proc. Nat. Acad. Sci. 1998 95:11037
Notes:
... ... *energy landscape: The "energy landscape" here refers to
the contours of what is essentially a classical energy/entropy
diagram, with the native configuration state positioned at the
bottom of a deep potential well, in this case a funnel with
sides containing miniature energy wells or "traps".
... ... *phase diagram: A classical graphical representation of
the equilibrium relationships between phases of a chemical
system.
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10. PROTEINS: ON BETA-SHEET STRUCTURES
J. Kubelka and T.A. Keiderling (University of Illinois Chicago,
US) discuss protein beta-sheets, the authors making the
following points:
1) Globular proteins, despite the vast diversity of their
structures, have two main secondary structural types,
alpha-helix and beta-sheet. Unlike the alpha-helix, the
beta-sheet cannot be truly classified as a secondary structure,
since it does not occur as a single -strand, but rather as
beta-sheets which incorporate two or more associated strands.
The beta-sheet structure is stabilized by both intra- and
interstrand (tertiary) interactions involving hydrogen-bonding
and side-chain (hydrophobic) interactions, as well as by
intersheet association of the hydrophobic sheet surfaces. While
the alpha-helix forms a relatively uniform and well-defined
conformation, the beta-sheet encompasses a much broader
conformational class, the diversity of which is further
multiplied by a wide range of topologies resulting from
different assemblies of multistranded arrays. Due to these
complications, understanding of the beta-sheet conformation lags
behind that of the alpha-helix. An example of this difference is
the lack, until recently, of designed, solution-stable
beta-sheet peptides, as compared to the number of water-soluble
alpha-helical peptide models.
2) Infrared absorption (IR) spectroscopy is a classical
technique long used for peptide and protein conformational
analysis. Even though it lacks the detailed residue-specific
structural resolution of x-ray or NMR, the wide applicability,
high sensitivity, and fast time response of IR spectroscopy have
made it a valuable tool for estimation of average secondary
structural content and, importantly, for following structural
changes in protein denaturation studies. Early efforts at
computing beta-sheet IR spectra have predominantly focused on
ideal, infinite periodic structures, where the spectral pattern
is fully determined by the single repeating unit. Later, full
calculations on the finite ideal beta-sheet structures predicted
a strong dependence of the amide I band shape on the number of
associated strands as well as the strand length, the
"characteristic" split pattern being approached in the limit of
the infinite sheets. However, finite beta-sheets no longer
assume idealized periodic structures but develop a twist, where
the smaller beta-sheets typically have larger deviations. Thus,
relatively small beta-sheets that occur in native globular
proteins are invariably twisted, assuming a wide variety of
strand conformations as well as interstrand configurations. If
the band shapes are dependent on the detailed conformation of
the beta-strands, their lengths, and the number of associated
strands in the beta-sheet, a "typical" beta-sheet spectral
signature may not exist. No systematic study of predicted
spectral band shapes is available for realistic twisted
structures such as those found in proteins.
3) The authors compare quantum-mechanically based simulations of
infrared and vibrational circular dichroism spectra of model
antiparallel and parallel beta-sheets in different conformations
and sizes. The heart of these simulations is the ab initio
density functional theory calculation of vibrational absorption
and vibrational circular dichroism parameters for a small
oligopeptide and the transfer of these parameters onto a much
larger structurally related molecule.
J. Am. Chem. Soc. 2001 123:12048
References (abridged):
1. Nesloney, C. L.; Kelly, J. W. Bioorg. Med. Chem. 1996, 4, 739.
2. Salemme, F. R. Prog. Biophys. Mol. Biol. 1983, 42, 95.
3. Marqusee, S.; et al; Proc. Nat. Acad Sci. 1989, 86, 5286.
4. Scholtz, J. M.; Baldwin, R. L. Annu. Rev. Biophys. Biomol.
Struct. 1992,27,95
5. Yoder, G.; Pancoska, P.; Keiderling, T. A. Biochemistry 1997,
36, 15123.
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11. ON THE FINE STRUCTURE OF DNA TOROIDS
N.V. Hud and K.H. Downing (Georgia Institute of Technology, US)
discuss DNA toroids, the authors making the following points:
1) DNA in living cells is highly condensed and rarely assumes
the extended state that it does when free in solution. In both
bacteria and eukaryotic cells, the active regulation of DNA
condensation is known to be an integral part of the cell cycle.
In sperm cells and viruses, where DNA transcription and
replication are inactive, DNA can be packaged at densities that
approach the limits of molecular compaction. In most vertebrate
sperm cells DNA is condensed by arginine-rich proteins into
thousands of toroidal structures, each measuring 100 nanometers
in outside diameter. The DNA of some bacteriophages also is
packaged into a single toroid, or spool, with similar
dimensions. Thus, the toroid represents a fundamental morphology
selected by nature for the high-density packaging of DNA. In
addition to the relevance of DNA toroids to cell biology and
virology, DNA condensation is presently of great interest for
the development of gene therapies, because controlling DNA
condensation is viewed as a key step in the improvement of
artificial gene delivery systems.
2) Twenty-five years ago it was discovered that the polyamine
spermidine can cause the condensation of DNA into toroidal
structures in vitro with dimensions similar to those expelled
from bacteriophage capsids. Numerous subsequent studies
demonstrated that toroidal DNA condensates are produced by a
wide range of multivalent cations, and even by monovalent
cations in the presence of dehydrating or crowding agents.
Toroidal DNA condensates also have attracted the attention of
theoreticians who have shown toroid formation to be a general
phenomenon resulting from the self-association (or collapse) of
a semiflexible polymer in solution. DNA toroids are most often
reported to measure approximately 100 nanometers in outside
diameter, with a hole of approximately 30 nanometers in
diameter, although significantly larger toroids also have been
observed. Several theories have been presented that attempt to
explain why DNA toroids favor particular dimensions or a
particular amount of DNA per toroid. These theories have used
numerous physical factors, including the percentage of DNA
charge neutralized within a toroid, the number of nearest
neighbors for a DNA polymer in a toroid, the size of the DNA
loop that initiates toroid formation, toroid surface tension,
and the density of imperfections in the packing of DNA polymers.
However, no single theory at present is able to faithfully
predict both the size and size distribution observed for DNA
toroids.
3) The authors report that in a study using cryo-electron
microscopy, they obtained direct evidence that DNA within some
toroidal condensates is organized in a hexagonal close-packed
lattice, and that regions of regular but non-hexagonal DNA
packing also exist in toroids. Furthermore, both hexagonal and
non hexagonal DNA packing can be present within the same toroid.
The authors have developed 3-dimensional computer models of DNA
toroids, the models reproducing virtually all structural
features of toroids observed in cry-electron microscopy, and
therefore representing reasonable models for the fine structure
of DNA toroids.
Proc. Nat. Acad. Sci. 2001 98:14925
References (abridged):
1. van Holde, K. (1989) Chromatin (Springer, New York).
2. Holmes, V. & Cozzarelli, N. (2000) Proc. Nat. Acad. Sci. 97,
1322-1324
3. Allen, M. , Lee, J. , Lee, C. & Balhorn, R. (1996) Mol.
Reprod. Dev. 45, 87-92
4. Tikchonenko, T. (1975) in Comprehensive Virology, eds.
Fraenkel-Conrat, H. & Wagner, R. (Plenum, New York), pp. 1-117.
5. Hud, N. V. , Allen, M. J. , Downing, K. H. , Lee, J. &
Balhorn, R. (1993) Biochem. Biophys. Res. Commun. 193, 1347-1354
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12. SPECIES DIVERSITY AND ECOSYSTEM FUNCTIONING
B.J. Cardinale et al (University of Maryland, US) discuss
ecosystem functioning, the authors making the following points:
1) Rapid rates of species extinction and homogenization of the
world's biota provide compelling reasons for determining how
changes in biodiversity might affect the functioning of
ecosystems. Current ecological theory predicts that species
diversity can influence the consumption of resources that govern
ecosystem processes through two types of effects: a
"complementarity effect", which occurs through either resource
partitioning or facilitative interactions between species; and a
"selection effect", which occurs whenever species diversity is
correlated with the chance of resource use being dominated by a
single productive taxon.
Although facilitation between species is thought to be a key
mechanism by which biodiversity affects the rates of resource
use that govern the efficiency and productivity of ecosystems,
there is no direct empirical evidence to support this
hypothesis. The authors report that increasing the species
diversity of a functional group of aquatic organisms induces
facilitative interactions leading to non-additive changes in
resource consumption. In their study, the authors increased the
richness and evenness of suspension-feeding caddisfly larvae
(Insecta, Trichoptera) in stream mesocosms and found that the
increased topographical complexity of the benthic habitat alters
patterns of near-bed flow such that the feeding success of
individuals is enhanced. Species diversity reduces "current
shading" (i.e., the deceleration of flow from upstream to
downstream neighbours), allowing diverse assemblages to capture
a greater fraction of suspended resources than is caught by any
species monoculture. The fundamental nature of this form of
hydrodynamic facilitation suggests that it is broadly applicable
to freshwater and marine habitats. In addition, the authors
suggest there are several analogues in terrestrial ecosystems
where fluxes of energy and matter can be influenced by
biophysical complexity. Thus, changes in species diversity may
alter the probability of positive species interactions,
resulting in disproportionately large changes in the functioning
of ecosystems.
Nature 2002 415:426
References (abridged):
1. Hector, A. et al. Plant diversity and productivity
experiments in European grasslands. Science 286, 1123-1127
(1999).
2. Loreau, M. & Hector, A. Partitioning selection and
complementarity in biodiversity experiments. Nature 412, 72-76
(2001).
3. Mulder, C., Uliassi, D. & Doak, D. Physical stress and
diversity-productivity relationships: The role of positive
interactions. Proc. Natl Acad. Sci. USA 98, 6704-6708 (2001).
4. Tilman, D. et al. Diversity and productivity in a long-term
grassland experiment. Science 294, 843-845 (2001).
5. Jones, C. G., Lawton, J. H. & Shachack, M. Positive and
negative effects of organisms as physical ecosystem engineers.
Ecology 78, 1946-1957
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13. ON QUANTUM PHYSICS AND INFORMATION THEORY
V. Vedral (University of Oxford, UK) discusses quantum physics
and information theory, the author making the following points:
1) Quantum physics not only provides the most complete
description of physical phenomena known to man, it also provides
a new philosophical framework for our understanding of nature.
It enables us to accurately model systems ranging in size from
quarks and atoms to large cosmic objects such as black holes.
Information theory, on the other hand, teaches us about our
physical ability to store and process information. Without a
formalized information theory, many of the recent developments
in telecommunications, computer science, and engineering would
simply not have been possible. Although quantum physics and
information theory initially developed separately, their recent
integration can be seen as yet another important step towards
understanding the fundamental properties and limitations of
nature.
2) One of the central information-theoretic concepts, universal
in science, is that of distinguishability. Inevitably, for
example, the survival of an animal depends on its ability to
distinguish a mate from a predator or prey. In the same way,
physical experiments aim to be sensitive enough to be able to
distinguish one hypothesis from another. It is, however, no
surprise that the influence of the concept of distinguishability
is felt far beyond science. Life consists of a series of
decisions that have to be made. This we do, consciously or
unconsciously, by evaluating all the alternatives and
distinguishing the consequences of various alternative actions.
3) The author proposes that the apparently simple concept of
distinguishability is at the root of information processing.
Ultimately, how well we can distinguish different physical
states determines how much information we can encode into a
certain system and how quickly we can manipulate it..
Distinguishability, in turn, is completely dependent upon the
laws of physics, and quantum physics naturally allows for more
versatile information processing than does classical physics.
The reasoning behind this is that unlike classical states, two
different quantum states are not necessarily fully
distinguishable. Although this at first seems like a limitation,
it in fact presents us with significantly more possibilities for
information encoding and transmission. The author proposes that
"relative entropy" (first introduced in 1951 by Kullback and
Leibler) is the most appropriate quantity for measuring
distinguishability between different states, and the author
demonstrates that, as defined in his theoretical approach,
relative entropy, both classical and quantum, does not increase
with time.
Rev. Mod. Phys. 2002 74:197
References (abridged):
1. Ambainis, A., 2000, in Proceedings of the 32nd Annual ACM
Symposium on the Theory of Computing, May 2000 (Association for
Computing Machinery, New York), p. 636.
2. Bell, J., 1987, Speakable and Unspeakable in Quantum
Mechanics (Cambridge University, Cambridge).
3. Brillouin, L., 1956, Science and Information Theory
(Academic. New York).
4. Cover, T. M., and J. A. Thomas, 1991, Elements of Information
Theory (Wiley, New York).
5. Csiszar, I., and J. Korner, 1981, Coding Theorems for
Discrete Memoryless Systems (Academic, New York).
6. Davies, E. B., 1976, Quantum Theory of Open Systems
(Academic, London)
7. Deutsch, D., 1998, The Fabric of Reality (Viking-Penguin,
London).
8. DiVincenzo, D. P., C. A. Fuchs, H. Mabuchi, J. A. Smolin, I
Thapliyal, and A. Uhlmann, 1999, Quantum Computing and
Communications, Lecture Notes in Computer Science (Springer,
Berlin), Vol. 1505, p. 247.
Related background:
QUANTUM DECOHERENCE AND QUANTUM INFORMATION PROCESSING
D. Bacon et al (University of California Berkeley, US) discuss
quantum information processing, the authors making the following
points:
1) One of the most severe experimental difficulties in quantum
information processing is the fragile nature of quantum
information. Every real quantum system is an open system which
readily couples to its environment, and this coupling causes
quantum information in the system to become entangled with its
environment, which in turn results in the system information
losing its intrinsic quantum nature. This process is known as
"decoherence". Circumvention of this "decoherence problem" has
been shown to be theoretically possible with the development of
the theory of fault-tolerant quantum error correction. However,
the set of requirements to reach the threshold for such fault-
tolerant quantum computation is extremely daunting.
2) In the absence of coupling between a system and its
environment, the system and environment have separate temporal
evolutions determined by their individual energy spectra. When a
small interaction (relative to these energy scales) is switched
on between the two, the resulting evolution is dominated by
pathways that conserve the energy of the unperturbed
system-plus- environment. Under the assumption of such a
perturbative interaction, energetics play a key role in
determining the rate of decoherence processes. Such
energy-conserving decoherence has three possible forms: a)
energy is supplied from the system to the environment (cooling);
b) energy is supplied from the environment to the system
(heating); c) or no energy is exchanged at all
(non-dissipative). Thus, even when the environment is a heat
bath at zero temperature, cooling, and especially non-
dissipative interactions, can be a major source of decoherence.
3) The authors present a quantum informatics method for
suppressing the detrimental effects of decoherence, while at the
same time allowing for robust manipulation of the quantum
information, the objective to aid in breaching the threshold for
robust quantum computation.
Phys. Rev. Lett. 2001 87:247902
Related background:
ON NOISE AND QUANTUM INFORMATION PROCESSING
L. Viola et al (Los Alamos National Laboratory, US) discuss
noise and quantum information processing. Quantum information is
represented in terms of superposition states of elementary two-
level systems known as "qubits". The coherence properties of
such superpositions are essential to the extraordinary
capabilities that quantum mechanics promises for quantum
simulation, computation, and communication. At the same time,
these coherence properties are also extremely vulnerable to the
decoherence processes that real-world quantum devices undergo
due to unwanted couplings with their surrounding environment.
Thus, achieving noise control is indispensable for practical
quantum information processing. While a variety of strategies
have been devised to meet this challenge, no single method can
compensate for a completely arbitrary noise process. Rather,
constructing a reliable quantum-information-processing scheme
depends crucially on the errors that occur. If the interaction
with the environment is sufficiently weak, then to a good
approximation a restricted set of errors dominates the
information loss, and active quantum error correction can be
successfully implemented. Another instance where the relevant
errors are a subset of all possible errors occurs when the
system-environment interaction, no matter how strong, exhibits a
symmetry. This has provoked the development of passive
noise-control schemes based on encoding quantum information into
"noiseless" subspaces.
Science 2001 293:2059
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14. AEROSOLS, CLIMATE, AND THE HYDROLOGICAL CYCLE
The term "hydrological cycle" (water cycle) refers to the
continuous circulation of water between the oceans, atmosphere,
and land. Water evaporates from the oceans (and lakes and
rivers) as water vapor in the atmosphere, where it may condense
into clouds. Subsequently, clouds release precipitation (rain,
snow, or hail), which falls on the land and either evaporates
back into vapor, or is absorbed by living systems, or runs off
into streams or rivers.
V. Ramanathan et al (University of California San Diego, US)
discuss global aerosols, the authors making the following points:
1) One of the most visible impacts of human activities is the
brownish haze that pervades many industrial regions as well as
the rural areas of the tropics and the subtropics that are
subjected to heavy biomass burning. Long-range atmospheric
transport transforms this haze into a regional-scale aerosol
layer. Well-known examples are the Arctic haze, the Indo-Asian
haze, the east Asian dust and haze traveling across the Pacific,
and the biomass burning and dust plumes from North Africa
(Sahara and Sahel regions) that spread over most of the
subtropical Atlantic. Unlike the long-lived greenhouse gases,
which are distributed uniformly over the globe, aerosol
lifetimes are only a week or less, resulting in substantial
spatial and temporal variations with peak concentrations near
the source.
2) On account of the large spatial and temporal variability of
these aerosols, remote sensing from satellites delivers the most
reliable information about global aerosol distributions. The
measurable quantity from space is the "aerosol optical depth"
(AOD), which is derived from the solar radiation reflected to
space. The AOD is the vertical integral of the aerosol
concentration weighted with the effective cross-sectional area
of the particles intercepting (by scattering and absorption) the
solar radiation at the wavelength of interest. The globally and
annually averaged value of AOD (at 0.55 microns wavelength) is
approximately 0.12 (+- 0.04). Anthropogenic sources contribute
almost as much as natural sources to the global AOD.
Anthropogenic aerosols are typically in the submicrometer-to
micrometer-size range and are composed of numerous inorganic and
organic species falling under four broad categories: sulfates,
carbonaceous aerosols [black carbon and organic carbon, dust,
and sea salt. Global anthropogenic emissions of sulfates,
organics, and black carbon even exceed natural sources. Such a
large perturbation of the global aerosol loading is a major
environmental concern.
3) The authors suggest that in addition to human-made aerosols
enhancing scattering and absorption of solar radiation, they
also produce brighter clouds that are less efficient at
releasing precipitation. These in turn lead to large reductions
in the amount of solar irradiance reaching Earth's surface, a
corresponding increase in solar heating of the atmosphere,
changes in the atmospheric temperature structure, suppression of
rainfall, and less efficient removal of pollutants. These
aerosol effects can lead to a weaker hydrological cycle, which
connects directly to availability and quality of fresh water, a
major environmental issue of the 21st century.
Science 2001 294:2119
References (abridged):
1. J. E. Penner et al., in Climate Change 2001: The Scientific
Basis [Working Group I to the Third Assessment Report of the
Intergovernmental Panel on Climate Change (IPCC), Cambridge
Univ. Press, Cambridge, 2001], pp. 289-348.
2. J. Haywood and O. Boucher, Rev. Geophys. 38, 513 (2000)
3. Y. J. Kaufman, et al., J. Geophys. Res. 102, 17051 (1997)
4. W. D. Collins, et al., J. Geophys. Res. 106, 7313 (2001)
Also:
Ex Link: Aerosol Pollution and Earth's Water Supply
Related background:
ANTHROPOGENIC ATMOSPHERIC AEROSOLS AND GLOBAL CLIMATE CHANGE
S.E. Schwartz and P.R. Buseck (2 installations, US) present a
commentary on recent research on anthropogenic atmospheric
aerosols, the authors making the following points:
1) Most considerations of global climate change caused by human
activities have focused on the warming influence of greenhouse
gases. However, aerosols are another important atmospheric
constituent that influences climate and that has been affected
by human activities. In general, aerosol particles increase
scattering and absorption of shortwave (solar) radiation,
increase cloud reflectance, enhance cloud lifetimes, and
suppress precipitation. These phenomena are all thought to exert
a cooling influence on climate. Recent data indicate that
anthropogenic aerosols reduce cloud droplet size and suppress
precipitation downward of major urban areas and industrial
facilities, which is consistent with earlier hypotheses.
2) The influences of aerosols on climate are more complex than
those of greenhouse gases. Bulk aerosol composition is highly
variable spatially and temporally because of different sources
and production mechanisms and short atmospheric residence times
(from less than a day to more than a month). Particles sizes
range from nanometers to microns, and within the same size
class, particles can exhibit widely different compositions and
morphologies, with different constituents present within the
same particle (e.g., 10 nanometer carbon spherules can be found
embedded within much larger sulfate particles). The
inhomogeneities in properties and geographical distribution of
aerosols make it difficult to characterize their influences on
climate and to represent these influences in models.
3) Recent analysis of the consequence of absorption of shortwave
radiation by aerosols indicates that the heating of the
atmosphere can evaporate clouds. Clouds exert both cooling and
warming influences on climate: cooling in the shortwave (because
of their reflectance), and warming in the longwave (because of
absorption and re-emission of thermal infrared radiation). The
shortwave component dominates, so a reduction in cloud coverage
would result in a net warming influence.
4) The authors conclude: "Recent studies demonstrate both the
importance of aerosol effects on climate and the complexity of
aerosol-cloud interactions. Unfortunately for those would like a
quick and accurate assessment of anthropogenic climate forcing
over the industrial period, the studies also demonstrate that
there is much to be learned before such an assessment can
confidently be given."
Science 2000 288:989
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15. ON CHIRAL ARCHITECTURES IN SYNTHETIC MACROMOLECULES
J.J. Comelissen et al (University of Nijmegen, NL) discuss
synthetic macromolecules, the authors making the following
points:
1) In the course of evolution, nature has developed a multitude
of biomacromolecules tailored to deal with complicated tasks
such as information storage, support of tissue, transport, and
the performance of localized chemical transformations. Although
large numbers of researchers in the fields of chemistry and
physics have been, and still are, pursuing the same goals using
synthetic systems, nucleic acids and proteins still outclass
man-made materials. This has made an increasing number of
scientists over the past decades turn their eye to nature to
design and synthesize increasingly precise nanoscopic and even
mesoscopic structures using polymeric materials.
For the structuring of matter, nature uses the self-assembly of
both low- and high-molecular-weight compounds as a tool. Many
biological architectures, the dimensions of which may range over
several orders of magnitude, rely for their robustness on two
structural components: the alpha-helix and beta-sheet structures
of peptides. It is the secondary structure of these two
components which, in a delicate interplay among steric,
hydrophobic, electrostatic, and hydrogen-bonding interactions,
gives rise to the tertiary structure of nature's main building
blocks, the proteins. To achieve the high levels of
organization, information must be built into the smallest
building blocks, i.e., the amino acids. Indeed these building
units do contain this information in the form of chirality,
hydrogen-bonding capacity, steric demands, electrostatic
properties, hydrophilic or hydrophobic character, or metal ion
binding capability. Supramolecular chemistry since its early
days has been inspired by biological assembly methods and has
already delivered a large number of architectures of
macromolecular size based on these secondary interactions.
The authors propose that the same principles, when applied to
polymer chemistry, allow the construction of large and complex,
but precise, macromolecular architectures. The authors focus on
chiral polymers with a defined secondary structure in solution
and on chiral macromolecular architectures arising from the
aggregation of polymers. By building-in structural information,
various researchers have designed and synthesized
self-assembling low-molecular-weight surfactants preprogrammed
to form chiral superstructures such as "cigars", twisted
ribbons, helices, tubes, braids, boomerangs, and superhelices in
aqueous media. Comparable structures have been generated by
aggregation of other low-molecular-weight compounds in organic
solvents. The sensitivity to molecular geometry is highlighted
by dramatic differences in the aggregation behavior of two
regioisomeric phospholipid analogues, one of which forms
platelike aggregates, whereas the other, which has a more linear
shape, is able to pack in such a way that its molecular
chirality is expressed on the supramolecular level, leading to
the formation of helices, all with a diameter of 22 nanometers
and a regular pitch of 92 nanometers.
Chem. Rev. 2001 101:4039
References (abridged):
1. Feiters, M.C.; Nolte, R.J.M. In: Advances in Supramolecular
Chemistry. Vol. 6. Chiral Self-Assembled Structures of
Biomolecules and Synthetic Analogues; Gokel, G.W. Ed.; JAI Press
Inc., Stamford, CT; Vol. 6 pp 41-156.
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16. ON LUMINESCENT MOLECULAR-LEVEL DEVICES
T. Gunnlaugsson et al (University of Dublin, IE) discuss
molecular devices, the authors making the following points:
1) Over the past decade, advances in supramolecular chemistry
have led to the development of miniaturized devices based on
assemblies of single molecules that can execute functions on a
"molecular level", devices driven by photon, magnetic, or
electronic stimuli. Based principally upon molecular recognition
and concomitant modulation of physical properties such as the
redox potential, magnetism, photochemical, and photophysical
properties, these molecular-level devices can mimic some of the
basic functions of macroscopic electronic and mechanical
machinery. Examples of such supramolecular-level devices include
shuttles, wires, gears, ratchets, molecular pistons, machines,
and motors, giving rise to the fast growing field of
nanotechnology. Of particular interest are those devices
exhibiting switching between two states, off-on switching,
through host-guest interactions, observable as changes in
luminescent properties such as lifetime, quantum yield, and
wavelength. Such simple devices can be employed as chemosensors
for the detection of ions and molecules in physiological systems
and can conveniently be described as logic operations.
2) Mimicking logic functions, the basis of modern computing, is
of particular interest; the relationship between the input and
the output may be described by truth tables, where 1 represents
an active input/output and 0 an inactive one. Molecular logic
devices have recently been demonstrated, in each case using
fluorescence or phosphorescence as an output, triggered by ionic
(H+, Na+, etc.) or molecular inputs [e.g., O(sub2)]. Being able
to modulate the nature of these devices in a controllable way
could, in principle, lead to the construction of arrays of
molecular-level systems which may be important in the
development of molecular information processing. The potential
of such a supramolecular system has recently been illustrated by
de Silva and McClenaghan (2000), who demonstrated a
molecular-level arithmetic addition operation on 2-bit integers.
3) The authors report that the Eu(III) tetraazamacrocyclic
complexes [Eu·1] and [Eu·2], and the Tb(III) and Yb(III)
complexes [Tb·1] and [Yb·2], have been synthesized as
luminescent molecular-level devices. The Eu complexes exhibit
unique dual pH switching behavior in water under ambient
conditions. The delayed Eu emission is reversibly switched on in
acid, with an enhancement factor of several hundred for [Eu·1].
These observations are consistent with the protonation of the
quinoline aryl nitrogen moiety (pKa 5.9 for [Eu·1]). The
fluorescence emission spectra of these complexes are unaffected
by acid, but pronounced changes occur in alkaline solution due
to the deprotonation of the aryl amide nitrogen (pKa 9.4 for
[Eu·1]). [Tb·1] shows a more intriguing pH dependence; Tb
emission is switched "on" only in the presence of H+ and in the
absence of molecular oxygen, whereas the fluorescence emission
properties are similar to those observed with [Eu·1]. This
behavior can be conveniently described as a molecular-level
logic gate, corresponding to a two-input INHIBIT function. The
analogous [Yb·2] complex shows no such pH or O(sub2) dependence.
J. Am. Chem. Soc. 2001 123:12866
References (abridged):
1. Steed, J. W.; Atwood, J. L. Supramolecular Chemistry; Wiley:
Chichester, England, 2000.
2. Schneider, H.-J.; Yatsimirsky, A. Principles and Methods in
Supramolecular Chemistry; Wiley: Chichester, England, 1999.
3. Lehn, J.-M. Supramolecular Chemistry, Concepts and
Perspectives; VCH: Weinheim, 1995.
4. Jortner J., Ratner, M. Eds. Molecular Electronics, Chemistry
for the 21st Century; Blackwell Science: London, 1997.
5. Atwood, J. L., Davies, J. E. D., Macnicol D. D., Vogtie, F.,
Eds. Comprehensive Supramolecular Chemistry; Pergamon: England,
1996; Vols. 1, 2, and 10.
6. Sienicki, K., Ed. Molecular Electronics and Molecular
Electronic Devices; CRC Press: Boca Raton, FL, 1993; Vols. 1 and
2.
7. Balzani V.; Scandola, F. Supramolecular Photochemistry; Ellis
Horwood, Chichster, 1991.
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17. ON CONSTRUCTION OF CIRCULAR OLIGONUCLEOTIDES
T. Li et al (Hong Kong Polytechnic University, HK) discuss
circular oligonucleotides, the authors making the following
points:
1) Circular oligonucleotides possess many distinctive properties
when compared to their linear counterparts: higher DNA-binding
affinity, greater sequence selectivity, enhanced resistance to
degradation by exonuclease, and an ability to serve as efficient
templates for DNA and RNA polymerase. The earliest preparation
of this type of circular oligonucleotide relied on solid-phase
synthesis and had the limitation that only circular sequences
with less than 10 nucleotides could be made efficiently. For the
preparation of longer sequences of circular oligonucleotides,
template-directed methodologies were subsequently developed in
conjunction with enzymatic or chemical ligation to facilitate
the formation of phosphodiester linkages. Double-helical
complexes of linear oligonucleotides in a "dumbbell"
conformation were, for example, utilized as open precursors to
generate self-paired circular oligonucleotides of over 20
nucleotides in length. Most significantly, a strategy for
constructing unpaired circular oligonucleotides was effected by
using the triple helical conformation of oligonucleotides as
intermediates. These assemblies do not possess strong internal
structures and are therefore readily able to form complexes with
their target nucleic acids. These unpaired circular
oligonucleotides have since played significant roles in
molecular diagnostics, hybridization, and sequence-specific
inhibition of gene expression.
2) The authors now report for the first time that beyond the
scope of the previous duplex and triplex strategies, the
i-motif, a four stranded assembly, can direct the
sequence-specific formation of a phosphodiester linkage and thus
represents a new type of structural template for constructing
circular oligonucleotides. The i-motif is a structural entity
composed of two parallel-stranded duplexes zipped together in an
antiparallel orientation held together by hemiprotonated C-C+
base pairs. Similar to the double and triple helices, the
structural feature of the i-motif is capable of directing the
formation of circular oligonucleotides with high efficiency and
high sequence-selectivity. Unlike the double and triple helical
structures assembled via complementary base-pairings, the
formation of the i-motif is at the structural level of
individual bases, a self-recognition process. The authors
suggest that utilization of the unique self-recognition pattern
of the i-motif in the current study consequently not only
represents a distinctive strategy for constructing circular
oligonucleotides but also opens up a new method for the
synthesis of oligonucleotide sequences which are not accessible
via the double and triple helical methodologies.
J. Am. Chem. Soc. 2001 123:12901
References (abridged):
1. Kool, E. T. Compr. Nat. Prod. Chem. 1999, 7. 341-369.
2. Kool, E. T. Ace. Chem. Res. 1998, 31, 502-510.
3. Kool, E. T. Annu. Rev. Biophys. Biomol. Struct. 1996, 25,
1-28.
4. Giovannangeli, C.; Sun, J.-S.; Helene, C. Compr. Supramol.
Chem. 1996, 4, 177-192.
5. Kool E. T. J. Am. Chem. Soc. 1991, 113, 6265-6266.
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18. ON SURFACTANT-MEDIATED SYNTHESIS OF ORDERED MESOPOROUS SOLIDS
S. Inagaki et al (Toyota Central R&D Laboratories, JP) discuss
synthesis of ordered mesoporous solids, the authors making the
following points:
1) Surfactant-mediated synthesis strategies are widely used to
fabricate ordered mesoporous solids in the form of metal oxides,
metals, carbon and hybrid organosilicas. These materials have
amorphous pore walls, which could limit their practical utility.
In the case of mesoporous metal oxides, efforts to crystallize
the framework structure by thermal and hydrothermal treatments
have resulted in crystallization of only a fraction of the pore
walls.
2) The authors report the surfactant-mediated synthesis of an
ordered benzene–silica hybrid material. This material has an
hexagonal array of mesopores with a lattice constant of 52.5
angstroms, and crystal-like pore walls that exhibit structural
periodicity with a spacing of 7.6 angstroms along the channel
direction. The periodic pore surface structure results from
alternating hydrophilic and hydrophobic layers, composed of
silica and benzene, respectively. The authors suggest that this
material is formed as a result of structure-directing
interactions between the benzene–silica precursor molecules, and
between the precursor molecules and the surfactants. The authors
suggest that other organosilicas and organo-metal oxides can be
produced in a similar fashion to yield a range of hierarchically
ordered mesoporous solids with molecular-scale pore surface
periodicity.
3) To produce the material, a benzene-bridged organosilane
monomer was added to an aqueous solution of
alkyltrimethylammonium surfactant containing sodium hydroxide,
and kept at 95 °C for approximately 20 hours. The benzene–silica
hybrid material was obtained by collecting the white precipitate
(as-made materials), and removing surfactant by solvent
extraction. The material is in the form of plate-like particles
(0.5–30 microns in side length and 1 micron in thickness),
composed of needle-like single crystals aligned perpendicular to
the plate surface.
Nature 2002 416:304
References (abridged):
1. Yanagisawa, T., Shimizu, T., Kuroda, K. & Kato, C. The
preparation of alkyltrimethylammonium-kanemite complexes and
their conversion to microporous materials. Bull. Chem. Soc. Jpn
63, 988-992 (1990).
2. Kresge, C. T., Leonowicz, M. E., Roth, W. J., Vartuli, J. C.
& Beck, J. S. Ordered mesoporous molecular sieves synthesized by
a liquid-crystal template mechanism. Nature 359, 710-712 (1992).
3. Beck, J. S. et al. A new family of mesoporous molecular
sieves prepared with liquid crystal templates. J. Am. Chem. Soc.
114, 10834-10843 (1992).
4. Inagaki, S., Fukushima, Y. & Kuroda, K. Synthesis of highly
ordered mesoporous materials from a layered polysilicate. J.
Chem. Soc. Chem. Commun. 680-682 (1993).
5. Firouzi, A. et al. Cooperative organization of
inorganic-surfactant and biomimetic assemblies. Science 267,
1138-1143 (1995).
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19. ON FISH FATTY ACIDS AND THE RISK OF SUDDEN DEATH
C.M. Albert et al (Harvard University, US) discuss fish fatty
acids and cardiac health, the authors making the following
points:
1) It has been reported that fish consumption is associated with
a reduced risk of sudden death from cardiac causes, but not a
reduced risk of myocardial infarction, and it has been
hypothesized that the long-chain n–3 polyunsaturated fatty acids
found in fish, primarily eicosapentaenoic acid and
docosahexaenoic acid, may be responsible for this association.
Experimental data from studies in animals and at the cellular
level suggest that these n–3 fatty acids have antiarrhythmic
properties, and a recent randomized trial testing supplements of
these n–3 fatty acids in survivors of myocardial infarction
found a statistically significant 45 percent reduction in the
risk of sudden death, with no effect on nonfatal myocardial
infarction. However, prospective data on blood levels of
long-chain n–3 fatty acids and sudden death from cardiac causes
are sparse, and there have been no randomized trials of the
effects of long-chain n–3 fatty acids in the diet or as
supplements among persons without a history of cardiovascular
disease, who represent over half of all cases of sudden death
from cardiac causes.
2) The authors report a prospective, nested case–control
analysis among apparently healthy men who were followed for up
to 17 years. The fatty-acid composition of previously collected
blood was analyzed by gas–liquid chromatography for 94 men in
whom sudden death occurred as the first manifestation of
cardiovascular disease and for 184 controls matched with them
for age and smoking status. The authors report that base-line
blood levels of long-chain n–3 fatty acids were inversely
related to the risk of sudden death both before adjustment for
potential confounders and after such adjustment. As compared
with men whose blood levels of long-chain n–3 fatty acids were
in the lowest quartile, the relative risk of sudden death was
significantly lower among men with levels in the third quartile
and the fourth quartile. The authors conclude that the n–3 fatty
acids found in fish are strongly associated with a reduced risk
of sudden death among men without evidence of prior
cardiovascular disease.
New Engl. J. Med. 2002 346:1113
References (abridged):
1. Albert CM, Hennekens CH, O'Donnell CJ, et al. Fish
consumption and risk of sudden cardiac death. JAMA
1998;279:23-28.
2. Kang JX, Leaf A. Antiarrhythmic effects of polyunsaturated
fatty acids: recent studies. Circulation 1996;94:1774-1780.
3. Billman GE, Kang JX, Leaf A. Prevention of sudden cardiac
death by dietary pure -3 polyunsaturated fatty acids in dogs.
Circulation 1999;99:2452-2457.
4. GISSI-Prevenzione Investigators (Gruppo Italiano per lo
Studio della Sopravvivenza nell'Infarto Miocardico). Dietary
supplementation with n-3 polyunsaturated fatty acids and vitamin
E after myocardial infarction: results of the GISSI-Prevenzione
trial. Lancet 1999;354:447-455. [Erratum, Lancet 2000;357:642.]
5. Kannel WB, Schatzkin A. Sudden death: lessons from subsets in
population studies. J Am Coll Cardiol 1985;5:Suppl B:141B-149B.
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20. AIR POLLUTION: LONG-TERM EXPOSURE AND DISEASE
C. Arden Pope III et al (Brigham Young University, US) discuss
health effects of air pollution, the authors making the
following points:
1) Based on several severe air pollution events, a temporal
correlation between extremely high concentrations of particulate
and sulfur oxide air pollution and acute increases in mortality
was well established by the 1970s. Subsequently, epidemiological
studies published between 1989 and 1996 reported health effects
at unexpectedly low concentrations of particulate air pollution.
The convergence of data from these studies, while controversial,
prompted serious reconsideration of standards and health
guidelines and led to a long-term research program designed to
analyze health-related effects due to particulate pollution. In
1997, the Environmental Protection Agency adopted new ambient
air quality standards that would impose regulatory limits on
fine particles measuring less than 2.5 microns in diameter.
These new standards were challenged by industry groups, blocked
by a federal appeals court, but ultimately upheld by the US
Supreme Court.
2) Although most of the recent epidemiological research has
focused on effects of short-term exposures, several studies
suggest that long-term exposure may be more important in terms
of overall public health. The new standards for long-term
exposure were originally based primarily on 2 prospective cohort
studies, which evaluated the effects of long-term pollution
exposure on mortality. Both of these studies have been subjected
to much scrutiny, including an extensive independent audit and
reanalysis of the original data. The larger of these 2 studies
linked individual risk factor and vital status data with
national ambient air pollution data. The analysis of the authors
uses data from the larger study (involving 1.2 million adults)
and a) doubles the follow-up time to more than 16 years and
triples the number of deaths; b) substantially expands exposure
data, including gaseous co-pollutant data and new data, which
have been collected since the promulgation of the new air
quality standards; c) improves control of occupational
exposures; d) incorporates dietary variables that account for
total fat consumption, and consumption of vegetables, citrus,
and high-fiber grains; and e) uses recent advances in
statistical modeling, including the incorporation of random
effects and nonparametric spatial smoothing components in the
Cox proportional hazards model.
3) The authors report that fine particulate and sulfur
oxide-related pollution were associated with all-cause, lung
cancer, and cardiopulmonary mortality. Each
10-microgram-per-cubic-meter elevation in fine particulate air
pollution was associated with approximately a 4 percent, 6
percent, and 8 percent increased risk of all-cause,
cardiopulmonary, and lung cancer mortality, respectively.
Measures of coarse particle fraction and total suspended
particles were not consistently associated with mortality. The
authors conclude that long-term exposure to combustion-related
fine particulate air pollution is an important environmental
risk factor for cardiopulmonary and lung cancer mortality.
J. Am. Med. Assoc. 2002 287:1132
References (abridged):
1. Firket J. The cause of the symptoms found in the Meuse Valley
during the fog of December, 1930. Bull Acad R Med Belgium.
1931;11:683-741.
2. Ciocco A, Thompson DJ. A follow-up of Donora ten years after:
methodology and findings. Am J Public Health. 1961;51:155-164.
3. Logan WPD, Glasg MD. Mortality in London fog incident, 1952.
Lancet. 1953;1:336-338.
4. Pope CA III, Dockery DW. Epidemiology of particle effects.
In: Holgate ST, Koren H, Maynard R, Samet J, eds. Air Pollution
and Health. London, England: Academic Press; 1999:673-705.
5. Kaiser J. Showdown over clean air science. Science.
1997;277:466-469.
Related background:
HEALTH BENEFITS OF GREENHOUSE GAS REDUCTION
L. Cifuentes et al (Catholic University of Chile Santiago, CL)
discuss health and greenhouse gases. While debates about energy
choices, long-term climate change impacts, and the capacity to
adapt to those impacts continue to evolve, there is little doubt
that air pollution from current patterns of fossil-fuel use for
electricity generation, transport, industry, and housing are
already sickening or killing millions throughout the world.
Increasing power generation by conventional fossil-fuel
combustion further threatens human health and welfare by
increasing air pollution. It has been estimated that reducing
emissions from older coal-fired power plants in the US could
provide substantial benefits to public health, including the
avoidance of 18,700 deaths, 3 million lost work days, and 16
million restricted-activity days each year. By reducing
emissions from 9 older coal plants in the Midwest, approximately
300 deaths, 2000 respiratory and cardiac hospital admissions,
10,000 asthma attacks, and 400,000 person-days of respiratory
symptoms could be avoided each year. Deaths from air pollution,
including indoor and outdoor sources, have been ranked as one of
the top 10 causes of disability by the World Health Organization
(WHO). In 1995, WHO estimated that 460,000 avoidable deaths
occur annually worldwide as a result of suspended particulate
matter, largely from outdoor urban exposures. In 1997 it was
estimated that annually nearly 700,000 deaths worldwide are
related to air pollution and that approximately 8 million
avoidable deaths from air pollution will occur by 2020.
Science 2001 293:1257
Related Background:
FINE PARTICLE AIR POLLUTION AND URBAN MORTALITY
J.M. Samet et al (5 authors Johns Hopkins University, US)
assessed the effects of 5 major outdoor-air pollutants on daily
mortality rates in 20 of the largest cities and metropolitan
areas in the US from 1987 to 1994. The pollutants were
particulate matter less than 10 microns in *aerodynamic
diameter, ozone, carbon monoxide, sulfur dioxide, and nitrogen
dioxide. The authors report they found consistent evidence that
the level of particulate matter less than 10 microns in
aerodynamic diameter is associated with the rate of death from
all causes and from cardiovascular and respiratory illnesses.
Weaker evidence indicated that increases in ozone levels
increased the relative rates of death during the summer, when
ozone levels are highest, but not during the winter. Levels of
the other pollutants were not significantly related to the
mortality rate. The authors conclude: "There is consistent
evidence that the levels of fine particulate matter in the air
are associated with the risk of death from all causes and from
cardiovascular and respiratory illnesses. These findings
strengthen the rationale for controlling the levels of
respirable particles in outdoor air."
New Engl. J. Med. 2000 343:1742
Notes:
... ... *aerodynamic diameter: In this context, the term
"aerodynamic diameter" is defined as the diameter of a
unit-density sphere that has the same settling velocity in gas
as the particle of interest.
Related Background:
EARTH SCIENCES: TRANSPACIFIC AIR POLLUTION
In general, the term "air pollution" refers to the release into
the atmosphere of gases, finely divided solids, or finely
dispersed liquid aerosols at rates that exceed the capacity of
the atmosphere to dissipate them or to dispose of them through
incorporation into solid or liquid layers of the biosphere. Not
all air pollution is anthropogenic: dust storms in desert areas
and smoke from forest and grass fires contribute to chemical and
particulate pollution of the atmosphere. For example, dust blown
from the Sahara Desert in Africa has been detected in West
Indian islands. Pesticides have been discovered in Antarctica,
where they have never been used, suggesting the extent to which
aerial transport can carry pollutants from one geographic region
to another. Another example: fallout of tetraethyl lead from
urban automobile exhausts has been observed in the oceans and on
the Greenland ice sheet. Perhaps the most important natural
source of air pollution is volcanic activity, which can pour
great amounts of ash and toxic fumes into the atmosphere.
In this context, the term "aerosol" refers to a dispersion in
which a finely divided solid is suspended in air and the
particles are of colloidal dimensions. The term "colloidal
dimensions" usually refers to the range approximately 1
nanometer to 100 nanometers in diameter, although some authors
include larger diameters.
... ... K.E. Wilkening et al (3 authors at 3 installations, CA
US) present a commentary on recent evidence of trans-Pacific air
pollution, the authors making the following points:
1) The authors point out that the once-pristine air above the
North Pacific Ocean is now polluted, with pollutants transported
on mid-latitude westerly winds from Eurasia to the Pacific Ocean
basin and across to North America. The authors suggest the
expected economic expansion around the Pacific Rim and in the
rest of the world will deliver even more pollution unless
preventive measures are taken. The risk of adverse effects on
wildlife, ecosystems, climate, and human health throughout the
Pacific region will increase. Even remote areas such as Arctic
and alpine environments are threatened, and ocean productivity
and the atmospheric energy budget over the North Pacific Ocean
could be altered.
2) The authors point out that two recent events have been
particularly important in focusing attention on trans-Pacific
pollutant transfer: a) in 1997, rapid transport of pollutants
from Asia to the Olympic peninsula of Washington State was
observed; and b) in April 1998, satellite remote sensing showed
aerosols being transported across the Pacific to North America
from a massive dust storm in Western China. Observational data,
computer simulations, and research on pollutant concentrations
in various media such as snow, fish, or eagles have since
provided additional evidence of a potential pan-Pacific air
quality problem.
3) The authors conclude: "Research into the dynamics of
long-range transport, deposition, and impacts of atmospheric
pollutants in the Pacific region is only beginning. The nature,
magnitude, and spatial distribution of the pollutants and their
effects are largely unknown. Greatly expanded interdisciplinary
and international research effort is required before trans-
Pacific air pollution and other environmental issues in the
Pacific region can be addressed effectively."
Science 2000 290:65
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21. ON PLANT-INSECT BIOCHEMICAL WARFARE
Jack C. Schultz (Pennsylvania State University, US) discuss
plant-insect warfare, the author making the following points:
1) Each of the world's 300,000 plant species is a target for
attack from a range of nearly 400,000 species of plant-eating
insects. Plant-eating (herbivory) is among the Earth's most
important interactions in terms of number of taxa, biomass and
mass transfer, as well as evolutionary impact on plant traits,
community structure and ecosystem function. The population
biologists Paul Ehrlich and Peter Raven have even claimed that
insect herbivory has generated much of terrestrial biodiversity.
But we are now realizing that the interactions between plants
and insects are far more dynamic than was previously thought,
and involve much shared chemistry.
2) Insects are responsible for 15 percent of the world's crop
losses. Even in natural systems, they consume 10 percent of
plant production each year. Herbivory is limited directly or
indirectly by plant chemicals, which at first glance seem to
have no role in normal plant metabolism but are highly active in
animal tissues. Plant–herbivore interactions are often described
as an ongoing biochemical warfare that occurs on an evolutionary
timescale.
3) Our perspective of plant defenses and plant–insect
interactions is now changing. Plants are not static, chemically
defended fortresses — they respond to attack with rapid,
long-lasting, variable, and often specific biochemical,
physiological and developmental changes. Plants respond
differentially to many stimuli, including various insect
species. Every class of constitutive chemical defense responds
to a physical insect attack, or to insect regurgitant or saliva,
over periods of minutes to days. The few 'stealthy' insect
species that fail to elicit any response at all are now
considered exceptional.
4) Several signaling pathways coordinate these responses.
Fatty-acid signals (for example, oxylipins, which are
synthesized from linolenic acid released from membranes by
lipases) regulate expression of defense-related genes and are
central to most wound-mediated plant responses. Peptides,
phenolics, terpenoids and classical plant hormones (such as
cytokinins and ethylene) also can help to coordinate plant
responses.
Nature 2002 416:267
References (abridged):
1. Ehrlich, P. R. & Raven, P. H. Evolution 18, 586–608 (1964).
2. Karban, R. & Baldwin, I. T. Induced Responses to Herbivory
(Univ. Chicago Press, Chicago, 1997).
3. Shiu, S-H. & Bleecker, A. B. Proc. Natl Acad. Sci. USA 98,
10763–10768 (2001).
4. Chiu, J., et al. Mol. Biol. Evol. 16, 826–838 (1999).
Related background:
VOLATILES AS PLANT DEFENSE MECHANISMS
Sophie L. Wilkinson (CEN) discusses plant defense mechanisms
involving signaling volatiles. Plants are not passive in the
face of an attack by insects, but they can marshal elegant
defenses and perhaps even inform neighboring plants of danger.
Plant security measures are of two types: a) Direct defense
involves the expression of defense genes that lead to production
of chemicals, such as nicotine or protease inhibitors, that are
unpalatable or harmful to insects. b) Alternatively, a plant
under attack can rely on indirect defense, which involves the
emission of volatile chemicals such as terpenes that attract
predatory or parasitic insects. Insect protectors recruited by
volatiles, for example, might be predatory wasps that lay eggs
in plant-eating caterpillars; when the eggs hatch, the wasp
larvae eat the caterpillars. Researchers have been unraveling
such complex interactions between plants and insects since the
1980s, when Marcel Dicke (Wageningen University, NL)
demonstrated that plants communicate with the enemies of their
enemies. Apparently, terpenes and methyl salicylate are
involved. In many plant species, the hormone methylsalicylate is
emitted only when the plant is attacked by insects but not when
other types of damage occur. Plants apparently recognize
chemicals in herbivore oral secretions and thus can discriminate
between mechanical damage and an attacking herbivore insect.
Chem. Engin. News 2001 30 Jul
Related Background:
DEFENSIVE FUNCTION OF HERBIVORE-INDUCED PLANT VOLATILE EMISSIONS
"Ecology" (environmental biology) is the study of the
relationship between organisms and their environment, with the
term "environment" including both other organisms and the
physical surroundings. The word "ecology" was introduced by the
zoologist Ernst Haeckel (1834-1919), who applied the term
"oekologie" to "the relation of the animal both to its organic
as well as its inorganic environment."
Of major importance in ecology is the categorization of
organisms as "autotrophs" or "heterotrophs". All biological
communities have a basic structure of interaction that forms a
"trophic pyramid". The trophic pyramid consists of trophic
levels, with food energy passed from one level to the next along
the food chain. The base of the pyramid is composed of
autotrophs, the primary producers of the ecosystem, those
biological organisms (e.g., plants) that do not obtain energy
and nutrients by eating other organisms. Instead, autotrophs
harness solar energy by photosynthesis (photoautotrophs) or,
more rarely, they harness chemical energy by oxidation
(chemoautotrophs), both cases involving the production of
organic substances from inorganic substances. All other
organisms in the ecosystem are consumers called "heterotrophs",
which either directly or indirectly depend on the autotrophs for
food energy.
In many land-based ecosystems, the major autotrophs are plants,
and the major heterotrophs feeding on plants are insects. One of
the more intriguing ecological relationships in nature is that
of the "trophic triangle", an often remarkable instance of
co-evolution among different species. One example is the insect-
plant-insect trophic triangle, in which a species of plant, prey
to a predatory insect species that destructively feeds on it,
evolves a defense mechanism that sends out a signal that
attracts another species of insect that feeds on the first
species of insect. Such co-evolutionary triangles, delicate
minuets of prey and predation, have become an important focus of
experimental ecology.
... ... A. Kessler and I.T. Baldwin (Max Planck Institute for
Chemical Ecology Jena, DE) present a report on an insect-plant-
insect trophic triangle, the authors making the following points:
1) The authors point out that plants defend themselves against
plant-eating animals (herbivores) with chemical and physical
defenses that directly influence herbivore performance and
indirectly influence such performance through traits that
attract the natural enemies of herbivores. One such indirect
defense, the release of volatile organic compounds specifically
after herbivory, is known to attract parasitoids and predators
to actively feeding larvae in the laboratory, and evidence from
agricultural systems suggests a role for herbivore-induced
volatile organic compounds in increasing predation evolutionary
selection pressure. But exclusive evidence has been lacking, and
it is not even known whether plants growing in natural
populations increase volatile organic compound emissions after
herbivore attack.
The authors report a study in which they quantified volatile
emissions from Nicotania attenuata plants growing in natural
populations during attack by 3 species of leaf-feeding
herbivores (the caterpillars of Manduca quinquemaculata
[Lepidoptera]; the leaf bug Dicyphus minimus [Heteroptera]; the
flea beetle Epitrix hirtipennis [Coleoptera]). In the
experiment, the authors mimicked the individual release of 5
commonly emitted volatiles. Three compounds (cis-3-hexen-1-ol;
linalool; cis-alpha- bergamotene) increased egg predation rates
by a generalist predator (Geocoris pallens [Heteroptera; a
bug]). Linalool and the complete blend decreased lepidopteran
egg-deposition rates. As a consequence, a plant could reduce the
number of herbivores by more than 90 percent by releasing
volatiles. The authors suggest these results confirm that
indirect defenses can operate in nature.
Science 2001 291:2141
Related Background:
INITIAL CHEMICAL SIGNAL IN INSECT-PLANT-INSECT TROPHIC TRIANGLES
Both corn and cotton plants, when attacked by plant-eating
insects, release a volatile substance that specifically attracts
other insects that are the natural predators of the plant-eating
insects. A group led by J. H. Tumlinson (U.S. Department of
Agriculture), studying the trophic triangle of the beet armyworm
caterpillar (Spodoptera exigua Hubner), corn seedlings (Zea mays
L.), parasitic wasp (Cotesia marginiventris), have isolated and
synthesized the chemical substance responsible for the initial
signal. They have named the substance volicitin. It is present
in the oral secretions of the caterpillar, and it induces the
damaged corn seedlings to release a volatile blend of terpenoids
and indole, which calls in the parasitic female wasps that are
the natural enemies of the caterpillars. The wasps lay eggs in
the caterpillars, and the hatched larvae destroy the
caterpillars by eating them. Mechanically damaged plants exposed
to synthetic volicitin, in the absence of caterpillar attack,
release the usual volatiles that attract the wasps. Plants
mechanically damaged but not exposed to volicitin do not release
the volatiles.
Science 1997 9 May
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22. NMR SPECTROSCOPY AND STRUCTURAL PROTEOMICS
A. Yee et al (University of Toronto, CA) discuss NMR and
proteomics, the authors making the following point:
1) Structural proteomics, which aims to determine the
3-dimensional structures of all proteins, has become a major
initiative within the biomedical community, and the large number
of protein structures expected from these projects will yield
valuable clues to the rules for predicting protein folding and
to the understanding of biochemical function. In these early
stages of the structural proteomics effort, one of the main
goals is to identify the best technologies and the most
efficient processes to convert gene sequence into 3-dimensional
structural information. One of the decisions will be to
determine the optimal use of x-ray crystallography and NMR
spectroscopy, which are the two techniques that will provide the
majority of experimental data for these initiatives.
2) X-ray crystallography currently is perceived as the potential
workhorse for structural proteomics, because if provided with a
well-diffracting crystal, it is possible to determine a
3-dimensional structure in hours. However, the throughput of
structure determination using x-ray crystallography remains
unclear, because the rate-determining step continues to be the
production of well-diffracting crystals, a process that is
unpredictable and can take between hours and months.
3) NMR structure determination is limited currently by size
constraints and lengthy data collection and analysis times
(often months), and the method is best applied to proteins
smaller than 250 amino acids. On the other hand, NMR experiments
do not require crystals, and samples appropriate for structure
determination can be identified within minutes of the protein
being purified. In summary, x-ray crystallography and NMR
spectroscopy seem to have complementary deficiencies, and the
relative success of these methods in structural proteomics
remains to be determined.
4) The authors describe an approach to structural proteomics of
small proteins using NMR spectroscopy. Over 500 small proteins
from several organisms were cloned, expressed, purified, and
evaluated by NMR spectroscopy. Although there was variability
among proteomes, overall 20 percent of these proteins were found
to be readily amenable to NMR structure determination. NMR
sample preparation was centralized in one facility, and a
distributive approach was used for NMR data collection and
analysis.
Proc. Nat. Acad. Sci. 2002 99:1825
References (abridged):
1. Smith, T. (2000) Nat. Struct. Biol. Suppl. 7, 927.
2. Christendat, D. , et al. (2000) Nat. Struct. Biol. 7, 903-909
3. Delaglio, F. , et al.(1995) J. Biomol. NMR 6, 277-293
4. Gronenborn, A. M. & Clore, G. M. (1996) Protein Sci. 5,
174-177
5. Serber, Z. , et al (2001) J. Am. Chem. Soc. 123, 2446-2447
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23. HOSPITAL VOLUME AND SURGICAL MORTALITY
J.D. Birkmeyer et al (Dartmouth-Hitchcock Medical Center, US)
discuss hospital volume and surgical mortality, the authors
making the following points:
1) Over the past three decades, numerous studies have described
higher rates of operative mortality with selected surgical
procedures at hospitals where few such procedures are performed
(low-volume hospitals). Several recent reviews suggest that
thousands of preventable surgical deaths occur each year in the
US because elective but high-risk surgery is performed in
hospitals that have inadequate experience with the surgical
procedures involved. However, the relative importance of
hospital volume in various surgical procedures is disputed.
Using information from the national Medicare claims data base
and the Nationwide Inpatient Sample, the authors examined the
mortality associated with six different types of cardiovascular
procedures and eight types of major cancer resections between
1994 and 1999 (total number of procedures, 2.5 million).
2) The authors report that mortality decreased as volume
increased for all 14 types of procedures, but the relative
importance of volume varied markedly according to the type of
procedure. Absolute differences in adjusted mortality rates
between very-low-volume hospitals and very-high-volume hospitals
ranged from over 12 percent (for pancreatic resection, 16.3
percent vs. 3.8 percent) to only 0.2 percent (for carotid
endarterectomy, 1.7 percent vs. 1.5 percent). The absolute
differences in adjusted mortality rates between very-low-volume
hospitals and very-high-volume hospitals were greater than 5
percent for esophagectomy and pneumonectomy, 2 to 5 percent for
gastrectomy, cystectomy, repair of a nonruptured abdominal
aneurysm, and replacement of an aortic or mitral valve, and less
than 2 percent for coronary-artery bypass grafting,
lower-extremity bypass, colectomy, lobectomy, and nephrectomy.
3) The authors conclude that in the absence of other information
about the quality of surgery at the hospitals near them,
Medicare patients undergoing selected cardiovascular or cancer
procedures can significantly reduce their risk of operative
death by selecting a high-volume hospital.
New Engl. J. Med. 2002 346:1128
References (abridged):
1. Luft HS, Bunker JP, Enthoven AC. Should operations be
regionalized? The empirical relation between surgical volume and
mortality. N Engl J Med 1979;301:1364-1369.
2. Hannan EL, O'Donnell JF, Kilburn H Jr, Bernard HR, Yazici A.
Investigation of the relationship between volume and mortality
for surgical procedures performed in New York State hospitals.
JAMA 1989;262:503-510.
3. Begg CB, Cramer LD, Hoskins WJ, Brennan MF. Impact of
hospital volume on operative mortality for major cancer surgery.
JAMA 1998;280:1747-1751.
4. Flood AB, Scott WR, Ewy W. Does practice make perfect? I. The
relation between hospital volume and outcomes for selected
diagnostic categories. Med Care 1984;22:98-114.
5. Dudley RA, Johansen KL, Brand R, Rennie DJ, Milstein A.
Selective referral to high-volume hospitals: estimating
potentially avoidable deaths. JAMA 2000;283:1159-1166.
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24. ON CELL BARRIERS TO DRUG AND GENE DELIVERY
D.P. McIntosh et al (Harvard University, US) discuss cell
barriers in drug and gene delivery, the authors making the
following points:
1) Molecular medicine has discovered many new therapeutic
modalities by using state-of-the-art techniques in molecular
biology. High through-put in vitro assays that screen for
pharmacological actions on the desired cell type are frequently
used to design new drugs. Although the development of such
agents is certainly justified by their success in vitro, they
frequently perform much less effectively in vivo, where the
agent must reach its target cells in a tissue in sufficient
quantities to be potent while sparing bystander organs.
Depending on the route of administration, the endothelium and/or
epithelium form significant barriers that greatly limit the in
vivo accessibility of many drugs, antibodies, and gene vectors
to their intended target sites of pharmacological action,
namely, the cells comprising the tissue. For example, poor
tissue penetration has hindered many monoclonal antibodies from
reaching their cell-specific antigens to achieve effective
tissue- or cell-directed pharmaco-delivery in vivo. Moving the
target from the tissue cell surface to the surface of vascular
endothelium has theoretical advantages in tissue-specific
delivery. This vascular targeting strategy still requires the
identification of a tissue-specific target on endothelium,
validation of expected delivery in vivo and, to be useful for
many therapies, a means by which to enter and even cross the
vascular wall for access to underlying tissue cells.
2) The microvascular endothelium in most organs acts as a
significant barrier to the free passage of blood-borne molecules
and cells to the underlying interstitium and tissue cells.
Specific transport mechanisms are expected to exist for the
transendothelial transport of essential circulating blood
macromolecules to the subendothelial space to meet the metabolic
needs of the surrounding tissue cells. Continuous endothelium
contains distinct flask-shaped invaginations in the plasma
membrane called "caveolae" that are open to the luminal blood
vessel space where circulating molecules may enter them. These
caveolae may provide a trafficking pathway for macromolecules
into and possibly across cells. Based on morphological studies
showing few plasmalemmal vesicles existing free and unattached
to other membranes inside the cell, some investigators have
concluded that caveolae are not dynamic but rather static
structures. However, caveolae can bud from the plasma membrane
and they contain key functional docking and fusion proteins.
Whether caveolae can traffic their cargo across cells
(transcytosis) remains unproven, primarily because comparative
analysis has not been possible by using probes capable of
targeting caveolae with high affinity and specificity in vivo
vs. physically identical nontargeting control probes. The
utility of caveolae in overcoming cell barriers to facilitate
efficient pharmacodelivery in vivo is unknown. The molecular
composition of caveolae, including possible tissue-specific
differences, is also unknown.
Proc. Nat. Acad. Sci. 2002 99:1996
References (abridged):
1. Jain, R. K. (1998) Nat. Med. 4, 655-657
2. Miller, N. & Vile, R. (1995) FASEB J. 9, 190-199
3. Thrush, G. R. , Lark, L. A. , Clinchy, B. C. & Vitetta, E. S.
(1996) Annu. Rev. Immunol. 14, 49-71
4. Tomlinson, E. (1987) Adv. Drug Delivery Rev. 1, 87-198.
5. Dvorak, H. F. , Nagy, J. A. & Dvorak, A. M. (1991) Cancer
Cells 3, 77-85
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25. IN FOCUS: SUPRAMOLECULAR POLYMERS
"With the introduction of supramolecular polymers, which are
polymers based on monomeric units held together with directional
and reversible secondary interactions, the playground for
polymer scientists has broadened and is not restricted to
macromolecular species, in which the repetition of monomeric
units is mainly governed by covalent bonding. The importance of
supramolecular interactions within polymer science is beyond
discussion and dates back to the first synthesis of synthetic
polymers; the materials properties of, e.g., nylons, are mainly
the result of cooperative hydrogen bonding. More recently, many
exciting examples of programmed structure formation of polymeric
architectures based on the combination of a variety of secondary
supramolecular interactions have been disclosed. When the
covalent bonds that hold together the monomeric units in a
macromolecule are replaced by highly directional noncovalent
interactions, supramolecular polymers are obtained. In recent
years, a large number of concepts have been disclosed that make
use of these noncovalent interactions. Although most of the
structures disclosed keep their polymeric properties in
solution, it was only after the careful design of
multiple-hydrogen-bonded supramolecular polymers that systems
were obtained that show true polymer materials properties, both
in solution and in the solid state. Polymers based on this
concept hold promise as a unique class of novel materials
because they combine many of the attractive features of
conventional polymers with properties that result from the
reversibility of the bonds between monomeric units.
Architectural and dynamic parameters that determine polymer
properties, such as degree of polymerization, lifetime of the
chain, and its conformation, are a function of the strength of
the noncovalent interaction, which can reversibly be adjusted.
This results in materials that are able to respond to external
stimuli in a way that is not possible for traditional
macromolecules."
L. Brunsveld et al: Chem. Rev. 2001 101:4071
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