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ScienceWeek
ScienceWeek - May 24, 2002 Vol. 6 Number 21
An Online Research Digest Published Weekly Since 1997
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No one of us really will ever know very much. This is why we
shall have to find comfort in the fact that taken together we
know more and more.
-- J. Robert Oppenheimer (1904-1967)
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Top Graphic: Old Houses at Krumau 1914
-- Egon Schiele (1890-1918)
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Section 1
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Contents of this Issue (Full reports in Section 2):
Basic Sciences:
1. Neurobiology: On the Influence of Dendritic Synapse Location
2. On the Critical Period Hypothesis of Language Acquisition
3. Functional Neurogenesis in the Adult Hippocampus
4. On the Evolution of Chloroplasts
5. Marine Biology: On Deep-Sea Vent and Seep Invertebrates
6. On Synthetic Gene Networks
7. On Quantum Teleportation and Entanglement Swapping
8. Warming of the Southern Ocean Since the 1950s
9. Configurational Entropy and Biochemical Cooperativity
10. Transport of Bose-Einstein Condensates with Optical Tweezers
11. On the Origin of Intergalactic and Interstellar Magnetic
Fields
12. Geophysics: On Mantle Flow
Praxis:
13. On Eradication of Poliomyelitis
14. Neurological Deficits in Cocaine-Exposed Infants
15. Brucellosis and the Brucella Genome
16. On Sickle Cell Disease
17. Detection of Specific Peptide-Protein Binding by MRI
18. On Screening Newborns for Metabolic Diseases
19. Meteorology: Annular Modes
20. On Ultrafast Cooling of Water Droplets in Microemulsions
21. Amorphization of Zirconia by Ion Radiation
22. Generation of Ultra-Intense Single-Cycle Laser Pulses
23. On Carbon-Backbone Polymers
24. Ultrapermeable Reverse-Selective Nanocomposite Membranes
Miscellany:
25. In Focus: On Junk DNA and the Plant Genome
26. ScienceWeek Notices and Subscription Information
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Section 2
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1. NEUROBIOLOGY: ON THE INFLUENCE OF DENDRITIC SYNAPSE LOCATION
Nerve cells (neurons), the cellular units of all nervous
systems, have diverse morphologies within an individual, and a
variety of unique morphologies across species. In general,
however, all neurons have two fundamental anatomic components: a
cell body (soma) and a single long filamentary extension from
the cell body, the so-called "axon" (which may branch along its
length), that propagates electrical activity from the cell body
(or from the vicinity of the cell body) to other locations
(e.g., to muscle cells or to other nerve cells). Most neurons
also possess a third anatomic component: extensions of the cell
body (few or numerous) that provide junction points for the
axons of other neurons (i.e., provide surface area for
synapses), and thus serve as loci for receiving inputs. In some
neurons, dendrites are extensively branched (arborized), the
single neuron as a whole receiving inputs from as many as
100,000 other neurons, while at the other extreme there are
neurons with only one dendrite receiving input from only one or
a few other neurons.
S.R. Williams and G.J. Stuart (Australian National University,
AU) discuss dendritic synapses, the authors making the following
points:
1) Classic views of neuronal operation describe dendrites as
funnels, guiding synaptic potentials to the axon where action
potential initiation occurs (1). This role is hampered by the
filtering effects of the membrane that progressively attenuates
and slows the time course of synaptic potentials as they spread
from dendritic site of generation to the axon (1, 2). Distal
excitatory postsynaptic potentials (EPSPs) are therefore thought
to set the background level of somatic depolarization rather
than influence the precise timing of neuronal output (1, 3).
Recent evidence, however, indicates that active dendritic
mechanisms sculpt the time course of EPSPs in cortical pyramidal
neurons to render somatic EPSP time course independent of
dendritic site of generation (4, 5). Furthermore, there is
evidence that the effects of dendritic filtering on somatic EPSP
amplitude in hippocampal pyramidal neurons is overcome by a
site-dependent scaling of synaptic conductance, as is thought to
occur in motoneurons. However, controversy over the
computational role and universality of site-dependent scaling of
synaptic conductance exits.
2) The authors report they determined the somatic impact of
excitatory postsynaptic potentials generated at known dendritic
sites in neocortical pyramidal neurons. As inputs became more
distal, somatic EPSP amplitude decreased, whereas use-dependent
depression increased. Despite marked attenuation (>40-fold),
when coactivated within a narrow time window (approximately 10
milliseconds), distal EPSPs could directly influence action
potential output following dendritic spike generation. The
authors suggest these findings reveal that distal EPSPs are
ineffective sources of background somatic excitation, but
through coincidence detection have a powerful transient
signaling role.
References (abridged):
1. W. Rall, in Handbook of Physiology, vol. 1, The Nervous
System, E. R. Kandel, Ed. (American Physiological Society,
Bethesada, MD, 1977), pp. 39-97.
2. I. Segev and M. London, Science 290, 744 (2000)
3. W. Rall, in The Theoretical Foundation of Dendritic Function,
I. Segev, J. Rinzel, G. Shepherd, Eds. (MIT Press, Cambridge,
MA, 1995), pp. 122-146.
4. J. C. Magee, Nature Neurosci. 2, 508 (1999)
5. S. R. Williams and G. J. Stuart, J. Neurophysiol. 83, 3177
(2000)
Science 2002 295:1907
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2. ON THE CRITICAL PERIOD HYPOTHESIS OF LANGUAGE ACQUISITION
A.D. Friederici et al (Max Planck Institute of Cognitive
Neuroscience Leipzig, DE) discuss language acquisition, the
authors making the following points:
1) The acquisition of certain basic cognitive functions seems to
depend on appropriate input during so-called critical periods
(1, 2). Rare cases of children who grew up without language
input during their first years demonstrate that perfect mastery
of a language cannot be acquired in later periods (3). It has
been suggested that second language learning is subject to
similar restrictions (4, 5). Although the age of exposure during
language acquisition seems to have a dramatic impact on the
subsequent real-time processing of sentences, the mechanisms
underlying this critical-period effect remain unclear. Although
some researchers assume that the critical period effect can be
explained by the earlier-is-better hypothesis (e.g., refs. 1 and
3), which holds that maturational constraints determine language
learning, others assume the less-is-more hypothesis, which
claims that differences in early and late language learning are
a by-product of children's processing capacity limitations,
providing a more focused approach to the language input.
2) The employment of brain imaging and electrophysiological
techniques has shed light on the neural bases of the well
established behavioral differences between first (L1) and second
language (L2) acquisition. Lexical-semantic processing of word
meanings is relatively similar for native speakers and L2
learners. In contrast, syntactic real-time analyses of a
sentence's grammatical information seem to differ considerably
for late learners versus native speakers. In native speakers,
severe syntactic violations elicit a characteristic biphasic
response in the event-related brain potential consisting of an
early negativity and a late positivity. This early negativity
was found often with a left anterior maximum but sometimes with
a more bilateral frontocentral distribution. The factor
determining this variation, however, has not yet been identified.
3) The authors report that by using event-related brain
potentials, we demonstrate that adults who learned a miniature
artificial language display a similar real-time pattern of brain
activation when processing this language as native speakers do
when processing natural languages. Participants trained in the
artificial language showed two event-related brain potential
components taken to reflect early automatic and late controlled
syntactic processes, whereas untrained participants did not. The
authors suggest this result challenges the common view that late
second language learners process language in a principally
different way from native speakers. The suggest their findings
demonstrate that a small system of grammatical rules can be
syntactically instantiated by the adult speaker in a way that
strongly resembles native-speaker sentence processing.
References (abridged):
1. Lenneberg, E. H. (1976) Biological Foundations of Language
(Wiley, New York).
2. Hubel, D. H. & Wiesel, T. N. (1977) Proc. R. Soc. London 198,
1-59
3. Curtiss, S. (1977) Genie: A Psycholinguistic Study of a
Modern-Day Wild Child (Academic, New York).
4. Johnson, J. S. & Newport, E. L. (1989) Cognit. Psychol. 21,
60-99
5. Harley, B. & Wang, W. (1997) in Tutorials in Bilingualism:
Psycholinguistic Perspectives, eds. de Groot, A. M. B. & Kroll,
J. F. (Lawrence Erlbaum Associates, Mahwah, NJ), pp. 19-51
Proc. Nat. Acad. Sci. 2002 99:529
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3. FUNCTIONAL NEUROGENESIS IN THE ADULT HIPPOCAMPUS
The term "hippocampus" refers to a region of the cerebral cortex
in the medial part of the temporal lobe. In humans, among other
functions, the hippocampus is apparently involved in short-term
memory, and analysis of the neurological correlates of learning
behavior in animals indicates that the hippocampus is also
involved in memory in other species.
The term "neurogenesis" refers to the generation of new nerve
cells. Until a few years ago, neurogenesis was considered to be
totally absent in the adult mammalian brain, but neurogenesis
has now been identified in certain regions of the brains of
several mammalian species, and there is currently much research
devoted to relating brain neurogenesis to various brain
pathologies.
H. van Praag et al (Salk Institute, US) discuss neurogenesis in
the hippocampus, the authors making the following points:
1) Hippocampal neurogenesis has been observed in adult animals
from birds to humans(1-5). The newly generated cells may have a
function in cognition and brain repair. For example,
manipulations that increase neurogenesis, such as an enriched
environment and exercise, are associated with improved memory
function and enhanced synaptic plasticity. In addition, new
neurons are generated in the hippocampus after stroke and
seizures, and in the cortex after a selective lesion, suggesting
that they may be involved in recovery from injury. Stress, on
the other hand, has been related to decreased cell proliferation
and memory impairment. However, the conclusions from all of
these studies are based on correlations. It is still unclear
whether newly generated cells that express neuronal markers
become functional neurons.
2) Embryonic hippocampal progenitors can develop neuronal
electrophysiological properties and are able to form functional
synapses in culture. Furthermore, adult hippocampal progenitors
display voltage-dependent currents in vitro. It remains unknown
whether newly generated cells become functional neurons in vivo,
mainly owing to the methods currently available to study
neurogenesis.
3) The authors report a study that uses a retroviral vector
expressing green fluorescent protein that only labels dividing
cells, and that can be visualized in live hippocampal slices.
The authors report that newly generated cells in the adult mouse
hippocampus have neuronal morphology and can display passive
membrane properties, action potentials and functional synaptic
inputs similar to those found in mature dentate granule cells.
Our findings demonstrate that newly generated cells mature into
functional neurons in the adult mammalian brain.
References (abridged):
1. Altman, J. & Das, G. D. Autoradiographic and histological
evidence of postnatal neurogenesis in rats. J. Comp. Neurol.
124, 319-335 (1965).
2. Kaplan, M. S. & Hinds, J. W. Neurogenesis in the adult rat:
electron microscopic analysis of light radioautographs. Science
197, 1092-1094 (1977).
3. Kuhn, H. G., Dickinson-Anson, H. & Gage, F. H. Neurogenesis
in the dentate gyrus of the adult rat: age-related decrease of
neuronal progenitor proliferation. J. Neurosci. 16, 2027-2033
(1996).
4. Kornack, D. R. & Rakic, P. Continuation of neurogenesis in
the hippocampus of the adult macaque monkey. Proc. Natl Acad.
Sci. USA 96, 5768-5773 (1999).
5. Eriksson, P. S. et al. Neurogenesis in the adult human
hippocampus. Nature Med. 4, 1313-1317 (1998).
Nature 2002 415:1030
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4. ON THE EVOLUTION OF CHLOROPLASTS
Chloroplasts, which contain several photosynthetic pigments
(chlorophylls), are found in all photosynthetic plant cells, but
not in photosynthetic prokaryotes (i.e., not in cells without
membrane-bound organelles). The typical higher plant chloroplast
is lens-shaped, approximately 5 microns across the larger
dimension, and the number of chloroplasts per cell can vary from
1 to 100 depending on the type of cell. A mature chloroplast is
typically bounded by two membranes, an inner membrane and an
outer membrane, the membranes possessing significantly different
chemical constituents. In addition to a number of enzymes
involved in photosynthesis, chloroplasts also contain in their
interior a circular DNA molecule and protein synthetic machinery
typical of prokaryotes. The current consensus is that
chloroplasts may have originated from cyanobacteria that became
endosymbionts.
Cyanobacteria comprise a phylum of bacteria characterized by
blue-green (cyan) photosynthetic pigments, abundant in a variety
of habitats, particularly in fresh water and soil. Cyanobacteria
are responsible for generating a large portion of the free
oxygen in the Earth's atmosphere. They apparently produced
stromatolite limestone deposits, as well as the bulk of modern
petroleum deposits. (Stromatolites are laminated calcareous
microbial fossil deposits formed principally by cyanobacteria
and algae.)
T. Cavalier-Smith (University of Oxford, UK) discusses the
evolution of chloroplasts, the authors making the following
points:
1) During or following the global warming that thawed the last
"snowball earth" glaciation approximately 580 million years ago
[1] , chloroplasts originated from a cyanobacterial symbiont in
a biciliate protozoan [2,3] . The resulting cellular chimera was
so successful that it rapidly diversified into two primary
lineages of eukaryotic algae: the now rare glaucophytes like
Cyanophora, which retained the cyanobacterial peptidoglycan wall
within their chloroplast envelope, and the green plant/red algal
lineage, which lost the peptidoglycan. The latter split into red
algae, which retained the cyanobacterial phycobilisome pigments,
and green algae, which replaced them by chlorophyll b to adapt
to different light frequencies. Soon afterwards, a red alga and
two different green algal cells were implanted into yet other
biciliate hosts to form three further groups of eukaryotic
algae: a process called secondary symbiogenesis.
2) New light on these early events in eukaryotic evolution comes
from host genes encoding proteins that were secondarily imported
into the acquired plastids [4] and, as reported by Andersson and
Roger [5], from symbiont genes that were apparently retained in
the host nucleus after the symbiont was lost. Secondary
symbiogenesis has also been greatly clarified by the complete
sequence of a cryptomonad nucleomorph, an evolutionarily
miniaturised relic of the red algal nucleus that was enslaved
over 500 million years ago.
3) Several well-established eukaryote groups comprise a mixture
of photosynthetic algae and non-photosynthetic heterotrophs,
notably dinoflagellates, heterokonts, cryptophytes and
Euglenozoa. We now know that all these acquired chloroplasts
secondarily from other eukaryotes (red or green algae). Early in
the twentieth century, however, it was thought that such groups
with mixed nutritional properties were ancestrally
photosynthetic and their non-photosynthetic members evolved by
losing chloroplasts. After the 1960s revival of Mereschkowsky's
symbiogenetic theory of chloroplast origins, the alternative
dogma arose that such groups were ancestrally heterotrophic and
acquired plastids by numerous independent symbioses. However,
each symbiogenetic origin of an organelle is evolutionarily
complex, requiring novel organelle-specific protein-targeting
machinery and the acquisition by over a thousand genes of
appropriate targeting signals. The author suggests that
symbiogenesis is very rare and chloroplast loss distinctly
commoner.
4) In summary: The author suggests that chloroplasts originated
from cyanobacteria only once, but have been laterally
transferred to other lineages by symbiogenetic cell mergers.
Such secondary symbiogenesis is rarer and chloroplast losses
commoner than often assumed.
References (abridged):
1. Hoffman P.F., Kaufman A.J., Halverson G.P. and Schrag D.P.
(1998) A neoproterozoic snowball earth. Science, 281:1342-1346.
2. Cavalier-Smith T. (1982) The origins of plastids. Biol. J.
Linn. Soc., 17:289-306.
3. Cavalier-Smith T. (2000) Membrane heredity and early
chloroplast evolution. Trends Plant Sci., 5:174-182.
4. Fast N.M., Kissinger J.C., Roos D.S. and Keeling P.J. (2001)
Nuclear encoded, plastid-targeted genes suggest a single common
origin for apicomplexan and dinoflagellates plastids. Mol. Biol.
Evol., 18:418-426.
5. Andersson, J.A., Roger, A.J. (2002). A cyanobacterial gene in
non-photosynthetic protists — an early chloroplast acquisition
in eukaryotes. Curr. Biol. 2002 12:115.
Current Biology 2002 2002 12:R62
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5. MARINE BIOLOGY: ON DEEP-SEA VENT AND SEEP INVERTEBRATES
The term "hydrothermal" refers to hot solutions rising from
cooling molten rock (magma). "Hydrothermal vents" are hot
springs occurring in volcanic regions of the ocean floor. The
heavy-metal ions and hydrogen sulfide dissolved in the
overheated vent fluid precipitate as metal sulfides as soon as
contact with seawater cools the fluid. This reaction produces
the characteristic underwater black "smoke" plume, with a vent
"smoker chimney" building up from precipitated materials, mostly
gypsum and sulfides.
C.L. Van Dover et al (College of William and Mary, US) discuss
deep-sea vent and seep invertebrates, the authors making the
following points:
1) Deep-sea hydrothermal vents and their attendant invertebrate
communities were discovered in 1977 during exploration of the
Galapagos Spreading Center. Vents are now known to occur along
all active mid-ocean ridges and back-arc spreading centers and
at some seamounts. Vent fluids are geothermally heated and
enriched in energy-yielding reduced compounds during circulation
of seawater through the upper ocean crust. Deep-sea vents were
the first complex ecosystems (i.e., involving metazoans)
discovered to be based on microbial chemoautotrophic production
[reviewed in (1)], and their discovery greatly expanded our
understanding of the limits to life on Earth (2). Since 1977,
taxonomists have described more than 400 morphological species
from vents (7) and 200 more from seeps (6). This corresponds to
a species description every 2 weeks throughout the past 25 years.
2) Microorganisms that extract energy from reduced inorganic
compounds (chemoautotrophs) concentrated in vent effluents were
found to be primary producers in these deep-sea ecosystems, and
they in turn are filtered or grazed upon by invertebrates (e.g.,
barnacles and limpets). Many of the invertebrates (e.g.,
vestimentiferan tubeworms, bivalve mollusks, provannid
gastropods, and bresiliid shrimp) host chemoautotrophic
microorganisms as either epi- or endosymbionts.
3) Ecosystems similar to those of vents were subsequently found
at cold seeps (3). Seeps are areas where chemically modified
fluids derived from hydrocarbon reservoirs, methane hydrates,
pore waters in sediments, and sites of organic enrichment such
as whale skeletons are released into the ocean. Most seeps occur
along continental margins and trenches in association with
accumulated sediment. The common requirement of vent and seep
communities is the presence of a reduced compound (typically
hydrogen sulfide or methane) that can be oxidized by microbes to
release energy for the fixation of organic carbon from CO(sub2)
(or methane). A complete distinction between vent and seep
ecosystems may be inappropriate, as evidence for shared
evolutionary histories and even some shared species suggests
that there are links between the invertebrate taxa of vents and
seeps on evolutionary and ecological time scales (4,5).
4) In summary: Deep-sea hydrothermal vents and cold seeps are
submarine springs where nutrient-rich fluids emanate from the
sea floor. Vent and seep ecosystems occur in a variety of
geological settings throughout the global ocean and support food
webs based on chemoautotrophic primary production. Most vent and
seep invertebrates arrive at suitable habitats as larvae
dispersed by deep-ocean currents. The recent evolution of many
vent and seep invertebrate species (<100 million years ago)
suggests that Cenozoic tectonic history and oceanic circulation
patterns have been important in defining contemporary
biogeographic patterns.
References (abridged):
1. C. L. Van Dover, The Ecology of Deep-Sea Hydrothermal Vents
(Princeton Univ. Press, Princeton, NJ, 2000).
2. R. R. Hessler, V. A. Kaharl, in Seafloor Hydrothermal
Systems: Physical, Chemical, Biological, and Geological
Interactions, S. E. Humphris, R. A. Zierenberg, L. S.
Mullineaux, R. E. Thomson, Eds. (American Geophysical Union,
Washington, DC, 1995), pp. 72-84.
3. C. K. Paull et al., Science 226, 965 (1984).
4. Deep-sea vents and seeps are typically at depths greater than
1000 meters, well below the euphotic zone. Ambient seawater
temperature is 1 degree to 3 degrees celsius, oxygen
concentrations are near saturation, and ambient pressure exceeds
100 atmosphers. Anoxic conditions exist at the interface between
chemoautotrophic communities and the emanating flux of reduced
compounds. Thermally buoyant fluids (from a few degrees to > 60
degrees celsius above ambient) enriched with heavy metals and
radioactive elements bathe hydrothermal vent invertebrates. Vent
ecosystems are most often associated with bare basalt substrata,
whereas seeps occur in sedimented environments, where
accumulations of detrital organic material are present. See (1)
for a more complete review.
5. B. Hecker, Bull. Biol. Soc. Wash. 6, 465 (1985) .
Science 2002 295:1253
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6. ON SYNTHETIC GENE NETWORKS
The central event in the process by which genetic information in
DNA is converted into RNA (transcription) is the
RNA-polymerase-catalyzed conversion of the sequence code of the
template strand of a gene into a complementary RNA transcript.
RNA polymerases are found in all living cells, with one type
found in prokaryotes (cells without a cell nucleus and other
membrane-bound organelles), and 3 types of RNA polymerase found
in eukaryotes (cells with a cell nucleus).
D. McMillen et al (Boston University, US) discuss synthetic gene
networks, the authors making the following points:
1) Cellular protein levels are determined by the interplay
between the rates of gene expression and protein degradation.
Because the regulation of expression occurs mainly at the level
of DNA transcription (1, 2), cells often manipulate their
protein levels through the modification of relevant
transcription rates. Such regulation is accomplished by specific
regulatory proteins called transcription factors and can occur
in a positive or negative sense. Positive regulation refers to
an increase in transcription rate, usually accomplished by
enhancing RNA polymerase binding at a promoter site. Negative
regulation, in turn, usually refers to the inhibition of
polymerase binding at a promoter site. In both cases, expressed
proteins act to regulate their own production and/or that of
other proteins. Such regulatory feedback can lead to complex
network dynamics, and an important theme in "postgenomic"
research will be to understand these dynamics and how they
affect cellular behavior.
2) The design and construction of de novo synthetic gene
networks (3-5) provides a natural framework for reducing the
complexity of gene regulation. This approach combines tools from
nonlinear dynamics and statistical physics with the extensive
array of techniques in traditional molecular biology.
Mathematical models are utilized in the design and analysis of
the various features of the network, and, to date, the
qualitative agreement between model and experiment has supported
the notion of such an engineering-based approach (3-5). The
power of this methodology is that it can be used to study
simplified systems to gain insight into the general "themes" of
gene regulation. These themes include subnetworks that act as
switches (5) or oscillators (3), as well as networks that
utilize feedback to dampen the effects of internal noise. In
addition to the insights gleaned from the construction of small
networks, such genetic modules may have important
biotechnological applications in their own right. In this
context, synthetic gene circuits may provide a means for
controlling complex biochemical systems in much the same way
that digital and analog circuits provide a means for controlling
electronic and mechanical systems.
References (abridged):
1. Jacob, F. & Monod, J. (1961) J. Mol. Biol. 3, 318-356
2. Dickson, R. , Abelson, J. , Barnes, W. & Reznikoff, W. (1975)
Science 187, 27-35
3. Elowitz, M. B. & Leibler, S. (2000) Nature (London) 403,
335-338
4. Weiss, R. & Knight, T. F. (2000) DNA6: Sixth International
Meeting on DNA Based Computers, June 13-17, 2000, Leiden, The
Netherlands.
5. Gardner, T. S. , Cantor, C. R. & Collins, J. J. (2000) Nature
(London) 403, 339-342
Proc. Nat. Acad. Sci. 2002 99:679
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7. ON QUANTUM TELEPORTATION AND ENTANGLEMENT SWAPPING
Quantum teleportation is the transmission and reconstruction
over arbitrary distances of the state of a quantum system, an
effect first suggested by Bennett et al in 1993 (Phys. Rev.
Lett. 70:1895). The achievement of the effect depends on the
phenomenon of entanglement, an essential feature of quantum
mechanics. Entanglement is unique to quantum mechanics, and
involves a relationship (a "superposition of states") between
the possible quantum states of two entities such that when the
possible states of one entity collapse to a single state as a
result of suddenly imposed boundary conditions, a similar and
related collapse occurs in the possible states of the entangled
entity no matter where or how far away the entangled entity is
located.
In general, a "Hilbert space" is a linear vector space that can
have an infinite number of dimensions, the concept important
because in quantum mechanics the state of a system is
represented by a vector in Hilbert space. The dimension of the
Hilbert space is not related to the physical dimension of the
system. The concept is named after the mathematician David
Hilbert (1862-1943).
D.W. Berry and B.C. Sanders {Macquarie University, AU) discuss
quantum teleportation and entanglement swapping, the authors
making the following points:
1) Quantum teleportation enables disembodied transport of the
state of a system to a distant system through (i) a shared
entanglement resource, (ii) a classical communication channel
between the sender and receiver [1] and (iii) an experimentally
established isomorphism between the Hilbert spaces of the sender
and receiver [2]. Quantum teleportation is significant in
several areas, including transmission of quantum states in noisy
environments [1], sharing states in distributed quantum networks
[3] and implementation of quantum computation using resources
prepared offline [4,5]. Teleportation was initially proposed for
discrete-variable systems, where the state to be teleported has
finite-N levels [1], and a continuous-variable version has been
adapted for squeezed light experiments. The authors discuss
quantum teleportation of a quantum state in an arbitrary but
finite N-dimensional Hilbert space , realized physically as a
spin system, thereby generalizing the recent spin quantum
teleportation proposal by Kuzmich and Polzik (Phys. Rev. Lett.
2000 85:5639), which is only valid in the infinite-N limit.
2) "Entanglement swapping" is closely related to quantum
teleportation. Whereas quantum teleportation enables the state
of a system (e.g. a particle or collection of particles) to be
teleported to an independent physical system via classical
communication channels and a shared entanglement resource, the
purpose of entanglement swapping is to instill entanglement
between systems that hitherto shared no entanglement. An
entanglement resource is required for entanglement swapping to
occur; indeed the nomenclature "entanglement swapping" describes
the transfer of entanglement from a priori entangled systems to
a priori separable systems.
3) A connection between entanglement swapping and quantum
teleportation can be seen as follows. Consider quantum
teleportation of the state of one particle, which is initially
entangled with a second particle, but the state of the second
particle does not undergo quantum teleportation. In perfect
quantum teleportation, the state of the first particle is
faithfully transferred to a third particle that was initially
independent of the first two particles. Thus, subsequent to the
quantum teleportation, the second and third particles are
entangled, perfectly replacing the a priori entanglement of the
first and second particles. The entanglement resource inherent
in quantum teleportation devices enables this entanglement
swapping to occur; thus, equivalence between optimal
entanglement resources for quantum teleportation and
entanglement swapping might be expected, but the authors
demonstrate that the optimal entanglement resources differ
between quantum teleportation and entanglement swapping for
finite-N spin systems.
References (abridged):
1. Bennett C H, Brassard G, Crépeau C, Jozsa R, Peres A and
Wootters W K 1993 Phys. Rev. Lett. 70:1895
2. van Enk S J 2001 J. Mod. Opt. 48:2049
3. Cirac J I, Ekert A K, Huelga S F and Macchiavello C 1999
Phys. Rev. A 59:4249
4. Gottesman D and Chuang I L 1999 Nature 402:390
5. Knill E, Laflamme R and Milburn G J 2001 Nature 409:46
New Journal of Physics 2002 4:8
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8. WARMING OF THE SOUTHERN OCEAN SINCE THE 1950S
The term "isopycnal" refers to a line joining points of equal
density within a water mass. A 3-dimensional surface of equal
density is called an "isopycnal surface".
Sarah T. Gille (University of California San Diego, US)
discusses warming of the Southern Ocean, the author making the
following points:
1) The Southern Ocean plays a critical role in global climate.
With no continental barriers, it serves as a conduit to transmit
climatic signals between the Pacific, Atlantic, and Indian
Oceans. The predominant current of the Southern Ocean, the
fast-flowing Antarctic Circumpolar Current, is characterized by
strongly tilting isopycnals. Because water mixes preferentially
along constant density surfaces, tilting isopycnals bring
mid-depth water into contact with the ocean surface and serve as
a barrier to southward transport, possibly helping to isolate
the Antarctic continent from mid-latitude climate variability
(1).
2) Recent examinations of global ocean temperature changes have
shown substantial warming in the upper 1000 meters, averaging
approximately 0.1 degrees celsius between 1955 and 1995 (2).
Southern Hemisphere ocean warming also averages approximately
0.1 degrees celsius over the same time period, but detailed
study of the Southern Ocean has been hampered by the limited
number of shipboard observations available south of 30 degrees S
(3-5). The author reports a study that makes use of the large
number of mid-depth temperature observations collected during
the 1990s by Autonomous Lagrangian Circulation Explorer floats
to characterize modern-day temperatures in the Southern Ocean,
the study comparing these temperature measurements with historic
shipboard measurements.
3) In summary: Autonomous Lagrangian Circulation Explorer floats
recorded temperatures in depths between 700 and 1100 meters in
the Southern Ocean throughout the 1990s. The author reports
these temperature records are systematically warmer than earlier
hydrographic temperature measurements from the region,
suggesting that mid-depth Southern Ocean temperatures have risen
0.17 degrees celsius between the 1950s and the 1980s. This
warming is faster than that of the global ocean and is
concentrated within the Antarctic Circumpolar Current, where
temperature rates of change are comparable to Southern Ocean
atmospheric temperature increases. The subsurface Southern Ocean
has warmed during the past 50 years, and these changes may have
broader implications, since the water that is ventilated in the
region around Antarctica spreads gradually around the globe.
References (abridged):
1. S. R. Rintoul, C. W. Hughes, D. Olbers, in Ocean Circulation
and Climate, G. Siedler, J. Church, J. Gould, Eds. (Academic
Press, San Diego, 2001), pp. 271-302.
2. S. Levitus, J. L. Antonov, T. P. Boyer, C. Stephens, Science
287, 2225 (2000)
3. T. P. Boyer et al., World Ocean Database 1998, vol. 5,
Temporal Distribution of Ocean Station Data Temperature
Profiles, NOAA Atlas NESDIS 22, (U.S. Government Printing
Office, Washington, DC, 1998).
4. P. S. Wong, N. L. Bindoff, J. A. Church, J. Clim. 14, 1613
(2001)
5. T. P. Barnett, D. W. Pierce, R. Schnur, Science 292, 270
(2001)
Science 2002 295:1275
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9. CONFIGURATIONAL ENTROPY AND BIOCHEMICAL COOPERATIVITY
S. Jusuf et al (University of Pennsylvania, US) discuss
biochemical cooperativity, the authors making the following
points:
1) In this context, "cooperativity" is a common biochemical
phenomenon in which two or more otherwise independent processes
are thermodynamically coupled. Because cooperative processes are
usually attended by changes in molecular conformation,
thermodynamic coupling is usually attributed to an
enthalpy-driven mechanism. In the family of glycopeptide
antibiotics that includes vancomycin, however, cooperative
phenomena occur that cannot be explained by conformational
change.
2) Glycopeptide antibiotics form homodimeric complexes in which
each monomer binds a polypeptide ligand terminating in
—D-Ala-D-Ala. The processes of ligand binding and dimerization
mutually enhance each other, i.e., they exhibit positive
cooperativity.(1) However, high-resolution x-ray
crystallographic studies of vancomycin have demonstrated that
neither process significantly alters the structural framework
that is common to glycopeptide antibiotics.(2) It has been
suggested that cooperativity arises from the tightening of the
dimeric interface as ligands bind to an antibiotic dimer.(3)
However, crystallographic studies also show that the H-bonds
across the dimeric interface are not shortened. The average
H-bond length across the dimeric interface of a vancomycin dimer
with a single bound acetate ion is 2.17 angstroms, while it is
slightly longer at 2.19 angstroms for a vancomycin dimer with
two D-Ala residues.(2)
3) The authors report that the consequences of ligand binding
and dimerization on the dynamic behavior of glycopeptide
antibiotic complexes were assessed by conducting molecular
dynamics simulations of vancomycin, chloroeremomycin (CEM), and
their various complexes with the ligand Ac-D-Ala-D-Ala.
Vancomycin and CEM were chosen because they exhibit similar
ligand-binding affinities, yet CEM has a much higher
dimerization constant and it exhibits greater cooperativity in
binding.(3) Prior studies of vancomycin using molecular dynamics
simulation and published parameter sets have demonstrated that
this approach can yield accurate thermodynamic predictions.(4)
The authors report that cooperativity in these systems can arise
solely from changes in vibrational activity.
References (abridged):
1. Loll, P. J.; Axelsen, P. H. Annu. Rev. Biophvs. Biophys.
Struct. 2000, 29, 265.
2. (a) Loll, P. J.; Bevivino, A. E.; Korty, B. D.; Axelsen, P.
H. J. Am. Chem. Soc. 1997, 119, 1516. (b) Loll, P. J.; Miller,
R.; Weeks, C. M.; Axelsen, P. H. Chem. Viol. 1998,5, 293. (c)
Loll, P. J.; Kaplan, J.; Selinsky, B. S.; Axelsen, P. H. J.
Meet. Chem. 1999, 42, 4714.
3. (a) Williams, D. H.; Maguire, A. J.; Tsuzuki, W.; Westwell,
M. S. Science 1998, 280, 711. (b) Mackay, J. P.; Gerhard, U.;
Beauregard, D. A.; Westwell, M. S.; Searle, M. S.; Williams, D.
H. /. Am. Chem. Soc. 1994, 776, 4581. (c) Mackay, J. P.;
Gerhard, U.; Beauregard, D. A.; Maplestone, R. A.; Williams, D.
H. J. Am. Chem. Soc. 1994, 776, 4573.
4. (a) Li, D.; Sreenivasan, U.; Juranic, N.; Macura, S.; Puga,
F. J., II; Prohnert, P. M.; Axelsen, P. H. /. Mol. Recognit.
1997, 70, 73. (b) Axelsen, P. H.; Li, D. Bioorg. Med. Chem.
1998, 6, 877. (c) Axelsen, P. H.; Li, D. J. Comput. Chem. 1998,
19, 1278.
J. Am. Chem. Soc. 2002 124:3490
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10. TRANSPORT OF BOSE-EINSTEIN CONDENSATES WITH OPTICAL TWEEZERS
According to current physics, all particles in nature are either
fermions or bosons, with fermions (always elementary particles)
having half-integer spin (spin-states characterized by
half-integer multiples of Planck's constant divided by 2 pi), and
bosons (all other particles) having integer spin (spin-states
characterized by integer multiples of Planck's constant divided
by 2 pi). In general, bosons are particles that obey
*Bose-Einstein statistics, and they include photons, *pi mesons,
all nuclei having an even number of particles, and all particles
with integer or zero spin.
Bose-Einstein statistics is the statistical mechanics of a
system of indistinguishable particles for which there is no
restriction on the number of particles that may simultaneously
exist in the same quantum energy state. Particles that obey
Bose-Einstein statistics are called "bosons".
In general, "Bose-Einstein condensation" is a phenomenon
occurring in a macroscopic system consisting of a relatively
large number of bosons at a sufficiently low temperature
(microkelvins down to nanokelvins) in which a significant
fraction of the particles occupy a single quantum state of
lowest energy (the ground state). In an atomic Bose-Einstein
condensate, several thousand atoms essentially become a single
atom, a "superatom", and this effect has been observed
experimentally with atoms of rubidium and lithium, the atoms
trapped and cooled by special methods.
T.L. Gustavson et al (Massachusetts Institute of Technology, US)
discuss transport of Bose-Einstein condensates, the authors
making the following points:
1) Since the achievement of Bose-Einstein condensation in dilute
gases of alkali atoms in 1995, intensive experimental and
theoretical efforts have yielded a great deal of progress in
understanding many aspects of Bose-Einstein condensation [1,2].
Bose-Einstein condensates are well-controlled ensembles of atoms
useful for studying novel aspects of quantum optics, many-body
physics, and superfluidity. Condensates are now used in
scientific studies of increasing complexity requiring multiple
optical and magnetic fields as well as proximity to surfaces.
2) Conventional condensate production techniques severely limit
optical and mechanical access to experiments due to the many
laser beams and magnetic coils needed to create the condensates.
This conflict between cooling infrastructure and accessibility
to manipulate and study condensates has been a major restriction
to previous experiments. So far, most experiments are carried
out within a few millimeters of where the condensate was
created. What is highly desirable is a condensate "beam line"
that delivers condensates to a variety of experimental
platforms. Transport of charged particles and energetic neutral
particles between vacuum chambers is standard, whereas it is a
challenge to avoid excessive heating for ultracold atoms. Thus
far, transport of large clouds of atoms has only been
accomplished with laser-cooled atoms at microkelvin temperatures
[3,4]. Condensates are typically a few orders of magnitude
colder and hence much more sensitive to heating during the
transfer.
3) The authors report a demonstration of an application of
optical tweezers that can transfer Bose condensates over
distances of at least 44 centimeters (limited by the vacuum
chamber) with a precision of a few micrometers. This separates
the region of condensate production from that used for
scientific studies. The "science chamber" has excellent optical
and mechanical access, and the vacuum requirements in this
region may well be less stringent than those necessary for
production of condensates. The authors suggest this technique is
ideally suited to deliver condensates close to surfaces, e.g.,
to microscopic waveguides and into electromagnetic cavities. The
authors report they have used this technique to transfer
condensates into a macroscopic wiretrap located 36 centimeters
away from the point where the condensates were produced.
References (abridged):
1. W. Ketterle, D. Durfee, and D. Stamper-Kum, in Proceedings of
the International School of Physics—Enrico Fermi, edited by M.
Inguscio, S. Stringari, and C. Wieman (IOS Press, Tokyo, 1999),
p. 67.
2. F. Dalfovo, S. Giorgini, L. P. Pitaevskii, and S. Stringari,
Rev. Mod. Phys. 71, 463 (1999).
3. M. Greiner, I. Bloch, T. W. Hansch, and T. Esslinger, Phys.
Rev. A 63, 031401 (2001).
4. T. Kishimoto, P. Schwindt, Y.-J. Wang, W. Jhe, D. Anderson,
and E. Comell, DAMOP/DAMP Poster (London, Ontario, Canada, 2001).
xPhys. Rev. Lett. 2002 88:020401
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11. ON THE ORIGIN OF INTERGALACTIC AND INTERSTELLAR MAGNETIC
FIELDS
Ellen G. Zweibel (University of Colorado, US) discusses cosmic
magnetic fields, the author making the following points:
1) Magnetic fields are found in galaxies and clusters of
galaxies throughout the Universe. There is even some evidence
for magnetized plasma (ionized gases) bridging the space between
two galaxy clusters. But the origin of these magnetic fields and
their influence on the intergalactic environment remains largely
a mystery(1). Theories of how these fields are generated follow
either the top-down model, in which magnetization occurred on a
cosmic scale when the Universe was much younger, or the
bottom-up idea, in which magnetic fields are generated on small
scales and then merge into larger systems.
2) Strong radio sources, such as quasars, generate their radio
emission by synchrotron radiation from electrons moving at
relativistic speeds (close to the speed of light) in a magnetic
field. So where there is strong radio emission there are almost
always magnetic fields. The idea of radio galaxies polluting the
intergalactic medium with magnetic fields is not entirely
new(4), but better theory(2) and observations(3) now allow for
increasingly detailed calculations of their contribution to the
magnetization of intergalactic space. At these large scales,
such orphan magnetic fields could have a substantial influence
on the formation and evolution of galaxies.
3) Quasars are the most luminous objects we know at optical, and
in some cases radio, wavelengths. They are extremely distant
active galactic nuclei that formed when the Universe was very
young. According to the standard model, quasars shine so
brightly because energy is efficiently radiated by material
accreting from an orbiting disk onto a massive black hole in the
nucleus(5). Roughly 10 to 20 percent of quasars eject plasma and
magnetic fields into their surroundings in the form of powerful
jets and winds moving at relativistic speeds. The jets feed into
diffuse 'lobes', which are clouds of radio-emitting material
often found on either side of the compact core, forming a
double-lobed structure. The radio lobes can be several million
light years in radius, so together they span a region perhaps
100 times larger than the parent galaxy. The magnetic fields are
believed to be generated within the black-hole accretion disks,
which are about the size of the Solar System (although the
fields may be amplified further within the jets).
References (abridged):
1. Zweibel, E. G. & Heiles, C. Nature 385, 131-136 (1997).
2. Furlanetto, S. R. & Loeb, A. Astrophys. J. 556, 619-634
(2001).
3. Kronberg, P. P., Dufton, Q. W., Li, H. & Colgate, S. A.
Astrophys. J. 560, 178-186 (2001).
4. Rees, M. J. & Setti, G. Nature 219, 127-128 (1968).
5. Begelman, M. C., Blandford, R. D. & Rees, M. J. Rev. Mod.
Phys. 56, 255-351 (1984).
Nature 2002 415:31
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12. GEOPHYSICS: ON MANTLE FLOW
The term "lithosphere" refers to the outer layer of the Earth,
comprising the crust and upper mantle, and extending to a depth
of 50 to 70 kilometers. The traditional view of tectonics
(changes in the structure of the Earth's crust) is that the
lithosphere consists of a strong brittle layer overlying a weak
ductile layer. "Plate tectonics" is the current consensus theory
that the Earth's lithosphere is broken into fairly rigid plates,
seven or eight major plates and many smaller plates, and that
convection within the underlying less rigid "asthenosphere"
causes the plates (and the associated continents and crust) to
move relative to each other.
In this context, the term "subduction" refers to the process of
underthrusting of the edge of a tectonic plate into the mantle
underlying an adjacent plate.
P.G. Silver and W. E. Holt (Carnegie Institution of Washington,
US) discuss mantle flow, the authors making the following points:
1) The authors suggest it is surprising that after more than
three decades into the plate tectonic revolution, we have so
little direct observation of the mantle flow field that
accompanies tectonic plate motion. The most straightforward
measure of mantle flow is provided by the trajectory of
subducted slabs whose seismicity and high seismic velocities
provide tracers of the flow. Yet even in subduction zone
environments there is evidence for complex three-dimensional
flow both above and below the slab (1-4). Far from slabs, even
less information is available to delineate the mantle flow
field. Various approaches have been used to predict this flow
field theoretically. One approach (5) calculates the mantle flow
field that would result if the motions of the plates are imposed
as boundary conditions, in addition to considering the trenches
and ridges as sources and sinks of mass. This flow field is
dominated by plate-entrained flow and a corresponding
counterflow.
2) More recently, several groups have calculated the
instantaneous field arising from density anomalies in the mantle
inferred from either seismic tomography or the history of
subduction. The plates are again taken as boundary conditions on
this flow field, and a plate velocity is chosen such that the
integrated torque on each plate vanishes. Both approaches
adequately predict plate velocities, although the accompanying
mantle flow fields and driving forces are different. The major
difference in these approaches has to do with the role of
density anomalies that are not directly attached to currently
subducting plates, but are either inferred from global seismic
tomography or from the long-term history of subduction. One way
of testing these models is to measure the flow field beneath a
plate that is not attached to a slab, but that has a mantle
density anomaly beneath it and therefore different mantle flow
fields predicted by the various models. The North American plate
has these characteristics.
3) The authors report they have combined observations of surface
deformation and upper mantle seismic anisotropy to estimate the
horizontal mantle flow field for western North America. They
report that the mantle velocity is 5.5 ± 1.5 centimeters per
year due east in a hot spot reference frame, nearly opposite to
the direction of North American plate motion (west-southwest).
The flow is only weakly coupled to the motion of the surface
plate, producing a small drag force. This flow field is probably
due to heterogeneity in mantle density associated with the
former Farallon oceanic plate beneath North America.
References (abridged):
1. R. M. Russo and P. G. Silver, Science 263, 1105 (1994)
2. J. Polet, et al., J. Geophys. Res. 105, 6287 (2000)
3. V. Peyton, et al., Geophys. Res. Lett. 28, 379 (2001)
4. G. P. Smith, et al., Science 292, 713 (2001)
5. C. G. Chase, Geophys. J. R. Astron. Soc. 56, 1 (1979)
Science 2002 295:1054
Related Background:
GEOPHYSICS: MODELS OF MANTLE CONVECTION
Don L. Anderson (California Institute of Technology, US)
discusses mantle convection, the author making the following
points:
1) There are two competing models for mantle convection. In the
first model, the mantle is stratified into two or more separate
convecting regions. In the second model, the whole mantle
convects as a single unit. Recent progress in plate tectonics,
seismology, solid-state physics, and mantle convection is
providing strong support for the stratified convection model.
The results may also help explain how plate tectonics relate to
mantle convection: upper mantle convection may be driven by
plate tectonics, whereas the deep mantle may convect in a
completely different style.
2) Evidence for whole mantle convection comes primarily from
seismology, and involves high-velocity seismic anomalies that
appear to be slabs traversing the mantle. The evidence for
occasional slab penetration below 650 kilometers is usually
considered sufficient evidence for whole mantle convection.
Whole mantle convection is also the reigning paradigm among
geodynamic modelers because of the seismic evidence and the
similarity between the geoid (the surface of constant
gravitational potential that would represent the sea surface if
the oceans were not in motion) and deep mantle seismic
tomography (which works much like medical x-ray tomography
except that seismic velocities are imaged). Whole mantle
convection simulations are also easier to do.
3) Arguments for stratified convection are more complex and more
difficult to understand. Pressure suppresses the effect of
temperature on density, making it more difficult for the deep
mantle to convect. Pressure also suppresses the effect of
temperature on seismic velocities, which are used by
seismologists to map temperature variations. Ab initio
calculations of mantle minerals indicate that subtle differences
in seismic gradients and velocities may be compositional: even
small changes in chemistry can stratify mantle convection.
Science 2001 293:2106
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13. ON ERADICATION OF POLIOMYELITIS
Poliovirus is an RNA virus that is the causative agent of the
human disease paralytic poliomyelitis. The majority of
poliovirus infections remain asymptomatic, but 1 to 2 percent of
such infections result in neurological complications within the
spinal cord and brainstem, the complications producing a
characteristic clinical syndrome dominated by flaccid paralysis.
Selective targeting of neurons that innervate muscle (motor
neurons) in the spinal cord by poliovirus apparently involves a
specific motor neuron cell-surface receptor (CD155), with a
contribution of certain favorable intracellular conditions.
The Morbidity and Mortality Weekly Report (Centers for Disease
Control and Prevention, US) discusses the global eradication of
poliomyelitis, the report making the following points:
1) From the initiation of the global poliomyelitis eradication
initiative in 1988 through 2001, the number of countries where
polio is endemic decreased from 125 to 10, and the number of
reported polio cases decreased by >99% from an estimated 350,000
to <1000. Wild type 2 poliovirus has not been detected worldwide
since October 1999. The American and Western Pacific Regions of
the World Health Organization (WHO) have been certified free of
indigenous wild poliovirus. Current challenges to global polio
eradication efforts include ongoing intense transmission in
northern India, continued importations of wild poliovirus into
polio-free areas, and the detection of circulating
vaccine-derived poliovirus.
2) In 2000, reported global vaccination coverage with 3 doses of
oral poliovirus vaccine among children aged <12 months was 82%.
Routine coverage varies across WHO regions. The African Region
reported the lowest oral polio vaccine coverage (55% in 2000).
In most countries or areas where polio remained endemic in 2001,
oral polio vaccine coverage in 2000 was <50%.
3) All countries where polio is endemic and many countries where
polio was recently endemic continued to conduct supplemental
immunization activities during 2001. Approximately 575 million
children in 94 countries received an estimated 2 billion doses
of oral polio vaccine in 2001. All countries used house-to-house
vaccination in part or all of the target areas as the primary
means to reach the highest possible coverage of children aged <5
years.
4) In 2001, 537 confirmed polio cases (as of March 10, 2002)
were reported; 473 (88%) were laboratory-confirmed. In 2000, a
total of 2,971 cases were reported, of which 719 (24%) were
laboratory-confirmed.
J. Am. Med. Assoc. 2002 287:1931
Morbidity and Mortality Weekly Report 2002 51:253
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14. ON NEUROLOGICAL DEFICITS IN COCAINE-EXPOSED INFANTS
Cocaine is an alkaloid extracted from leaves of the plant
Erythroxylan coca, native to South America. Once absorbed,
cocaine is rapidly broken down by esterases (including
cholinesterases) released simultaneously with dopamine in the
brain. The drug binds with high affinity to dopamine,
norepinephrine, and serotonin uptake sites, preventing the
reuptake of these neurotransmitters from the synaptic cleft and
leading to their increased concentration in the synapse.
L.T. Singer et al (Case Western Reserve University, US) discuss
deficits in cocaine-exposed infants, the authors making the
following points:
1) Maternal use of cocaine during pregnancy remains a
significant and enduring public health problem, particularly in
urban areas of the United States and among women of low
socioeconomic status.(1) An estimated 1 million children have
been born after fetal cocaine exposure since the mid-1980s, when
the so-called crack epidemic emerged with the availability of a
cheap, potent, smokable form of cocaine.(2) Cocaine has effects
on monoaminergic neurotransmitter systems important for the
development of neuronal circuitry and human learning.(3-5) A
growing body of research documents relationships between
prenatal cocaine exposure and prematurity, low birth weight,
microcephaly, and newborn behavioral abnormalities, which has
raised concerns regarding long-term cognitive and developmental
outcomes.
2) There are few longitudinal studies of cocaine-exposed
infants, however, and the findings of these studies are
contradictory. While some studies have found generalized
developmental delays in cocaine-exposed infants, others have not
demonstrated differences. Still other studies, including a
recent meta-analysis, show only subtle cognitive effects or find
deficits only when more specific areas of functioning are
measured. These studies are inconclusive for a number of
reasons. Most had high rates of attrition ranging from 30% to
55%, often combined with small sample sizes. High attrition and
small sample sizes are especially problematic in assessing
teratogenic effects of fetal cocaine exposure on cognitive
outcomes. Cocaine-exposed infants experience a large number of
negative environmental factors known to be related to poorer
child developmental outcomes that are also likely to
differentially affect subject recruitment and retention. These
confounding variables include minority race, low socioeconomic
status, poor prenatal care, low maternal education and IQ,
greater maternal psychological distress, a less stimulating home
environment, larger family, higher risk for out-of-home
placement, and maternal use of drugs in addition to cocaine,
especially alcohol, marijuana, and tobacco. The effects of such
factors must be considered before poorer child outcomes are
attributed to fetal cocaine exposure; thus, adequate sample
sizes must be used.
3) The authors report a study involving 415 consecutively
enrolled infants (218 cocaine-exposed and 197 unexposed)
identified from a high-risk, low–socioeconomic status, primarily
black (80%) population screened through clinical interview and
urine and meconium samples for drug use. The retention rate was
94% at 2 years of age. The authors report that cocaine-exposed
children had significant cognitive deficits and a doubling of
the rate of developmental delay during the first 2 years of
life. Because 2-year outcomes are predictive of later cognitive
outcomes, the authors suggest it is possible that these children
will continue to have learning difficulties at school age.
References (abridged):
1. Kandel DB, Warner LA, Kessler RC. Epidemiology of drug use
and abuse among women. In: Wetherington CL, Roman AB, eds. Drug
Addiction Research and the Health of Women. Rockville, Md:
National Institutes of Health; 1998:105-130.
2. National Institute on Drug Abuse. National Pregnancy and
Health Survey: Drug Use Among Women Delivering Live Births.
Rockville, Md: National Institutes of Health; 1992. Publication
96-3819.
3. Cregler LL, Mark H. Medical complications of cocaine abuse. N
Engl J Med. 1986;315:1495-1500.
4. Mayes LC. Developing brain and in utero cocaine exposure:
effects on neural ontogeny. Dev Psychopathol. 1999;11:685-714.
5. Volpe J. Effects of cocaine on the fetus. N Engl J Med.
1992;327:399-407.
J. Am. Med. Assoc. 2002 287:1952
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15. ON BRUCELLOSIS AND THE BRUCELLA GENOME
V.G. DelVecchio et al (University of Scranton, US) discuss
brucellosis, the authors making the following points:
1) Brucellosis is a major zoonotic disease that causes abortion
in wild and domestic animals and Malta or undulant fever in
humans. Although the spread of the disease is controlled in
developed countries by livestock testing, vaccination, and
slaughter programs, brucellosis is a major problem in the
Mediterranean region and parts of Asia, Africa, and Latin
America, where it causes severe economic losses (1).
2) The disease is transmitted to humans by consumption of
nonpasteurized milk and milk products or by direct contact with
infected animals or carcasses (2, 3). On contact, Brucellae
penetrate the skin or mucosal membranes and enter the lymph
nodes, which become hemorrhagic, resulting in bacteremia, which
facilitates dissemination throughout the body. During the early
phase of infection, Brucellae invade macrophages, adapt to the
acidic environment, and multiply in the vacuolar compartments
(4). Like Salmonella, Mycobacterium, and Legionella, Brucella
prevents phagosome/lysosome fusion (5). The infection involves
many tissue types and organs. Symptoms, which may include fever,
chills, headache, pain, fatigue, dementia, and arthritis, are
nonspecific. The infection can be treated with combinations of
antibiotics such as doxycycline and streptomycin or doxycycline
and rifampin. Vaccines against Brucellae have varying degrees of
success in controlling the disease; however, human vaccines are
not available, and the animal vaccines currently in use are
pathogenic to humans.
3) The authors report that the genome of B. melitensis (strain
16M) was sequenced and found to contain 3,294,935 base pairs
distributed over two circular chromosomes of 2,117,144 bp and
1,177,787 base pairs encoding 3,197 open reading frames.
4) In a commentary on this work, E. Moreno and I. Moriyon
(National University Heredia, CR) make the following points:
a) On September 23, 1905, a cargo carrying 60 goats from Malta
arrived in New York. The herd was kept in quarantine because of
several deaths that occurred during the journey. Crewmen, an
agent from the U.S. Bureau of Animal Industry, which was
responsible for the shipment, and a woman who drank milk that
"escaped" from the quarantine station displayed the
characteristic symptoms of "Mediterranean fever." Lieutenant
Colonel David Bruce, a physician of the Royal Army, who
discovered "Micrococcus melitensis " in 1887 in infected British
soldiers residing in Malta, had forewarned the U.S. sanitary
authorities about the risk of "Mediterranean fever" by importing
goats from Malta. In November 1906, after isolation of "M.
melitensis," the goats were destroyed.
b) Almost 100 years after this episode, the genome sequence of
Brucella melitensis (renamed after David Bruce) has been
resolved by DelVecchio et al. (2002), bringing new light to the
understanding of the biology of this pathogen. The disease,
known as brucellosis, is found in all continents, affecting
mainly low-income countries; in addition, it constitutes a
contemporary concern because Brucella strains are potential
agents of biological warfare.
References (abridged):
1. Corbel, M. J. (1997) Emerg. Infect. Dis. 3, 213-221
2. Young, E. J. (1983) Rev. Infect. Dis. 5, 821-842
3. Young, E. J. (1995) Clin. Infect. Dis. 21, 283-289
4. Porte, F. , Liautard, J. P. & Kohler, S. (1999) Infect.
Immun. 67, 4041-4047
5. Baldwin, C. L. & Winter, A. J. (1994) Immunol. Ser. 60,
363-380
Proc. Nat. Acad. Sci. 2002 99:1; 99:443
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16. ON SICKLE CELL DISEASE
Sickle cell disease is a chronic hemolytic anemia occurring
almost exclusively in the black population and is characterized
by sickle-shaped red blood cells due to homozygous inheritance
of a variant of hemoglobin called Hb-S. There is a milder
heterozygous form which produces some sickling of red blood
cells, but without the debilitating anemia. At present there are
approximately 75,000 individuals in the U.S. with the homozygous
form of the disease. There is no cure, and individuals usually
die at about the age of 40 due to the effects of the disease.
Jack R. Lancaster Jr. (Louisiana State University, US) discusses
sickle cell disease, the author making the following points:
1) Sickle cell disease (SCD) is the most common genetic disease
among African Americans, with an 8% incidence of the trait among
this population. This autosomal recessive disorder involves a
single amino acid substitution in the beta subunit of
hemoglobin, forming an abnormal protein (hemoglobin S) that
after deoxygenation results in polymerization and the consequent
characteristic sickle-like shape of the erythrocytes. The
pathogenesis of this disease has classically been attributed to
the effects of passage of the rigid malformed cells through the
vasculature, resulting in abnormal blood flow caused by physical
trapping or increased adhesion of the sickled cell to the
micro-vascular endothelium, the process commonly referred to as
"vaso-occlusive crisis".
2) Perfusion abnormalities are well documented in the clinical
literature, and there is good evidence that a complex array of
inflammatory events are also involved, including increased
levels of circulating cytokines, up-regulation of cellular
adhesion-related receptors, and appearance of activated
macrophages and leukocytes, which results in increased
production of reactive oxygen species (1). Recognition of the
importance of these events has led to the recent inclusion of
sickle cell disease as a chronic inflammatory disease (2). It
has been speculated that an important consequence of this
sequence of events is a defect in the actions of nitric oxide
(nitrogen monoxide, NO), which will further exacerbate the
syndrome by means of an array of effects (1-3). Among the most
important of the multiple biological actions of nitric oxide in
the cardiovascular system are the stimulation of vasodilation
and the inhibition of vascular cell adhesion and aggregation.
Thus, under conditions of attenuated endothelial nitric oxide
production, blood flow will be restricted because of
constriction and also vascular coagulation.
References (abridged):
1. Asian, M., Thornlcy-Brown, D. & Freeman, B. A. (2000) Ann.
N.Y. Acad. Sci. 899, 375-391.
2. Stuart, M. J. & Setty, B. N. (2001) Curr. Opin. Hematol. 8,
111-122.
3. Gladwin, M. T. & Rodgers, G. P. (2000) Lancet 355,1476-1478.
Proc. Nat. Acad. Sci. 2002 99:552
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17. DETECTION OF SPECIFIC PEPTIDE-PROTEIN BINDING BY MRI
Magnetic resonance imaging (MRI) is essentially a technique for
examining morphology (as opposed to _functional_ magnetic
resonance imaging, which is a technique for examining anatomical
correlates of function). In general, in its conventional use,
MRI involves magnetic coils producing a static magnetic field
parallel to the long axis of the patient or subject, combined
with inner concentric magnetic coils producing a static magnetic
field perpendicular to the long axis. A radio-frequency coil
specifically designed for the head perturbs the static fields to
generate a magnetic resonance image. The interaction physics in
this technique is that between the magnetic fields and atomic
nuclei in brain tissue. "Sliced" views can be obtained from any
angle, and the resolution is quite high and on the order of
millimeters for magnetic field strengths of 1.5 tesla.
L.M. De Leon-Rodriguez et al (University of Texas, US) discuss a
new application of magnetic resonance imaging, the authors
making the following points:
1) Contrast agents have become important diagnostic tools for
clinical MRI studies. These paramagnetic metal complexes
function by accelerating the relaxation rate of bulk water
protons. Any image contrast produced by the agent typically
reflects nonuniform distribution of the complex in different
tissues, a crude physical process. A major step forward in the
development of contrast agents would be to devise molecules
whose ability to relax water protons is triggered or enhanced
greatly by recognition of a particular biomolecule. This would
open up the possibility of developing MRI tests specific for
biomarkers indicative of particular disease states.
2) Meade and coworkers (1) have reported a novel strategy toward
this end in which the paramagnetic ion is encased within a
restrictive cavity that can be cleaved by a particular enzyme.
In the absence of enzyme, water coordination to the metal is
restricted while in its presence the coordination site is
unblocked and the relaxation rate is enhanced. Another strategy
is to take advantage of the increase in water relaxivity that
occurs upon slowing molecular rotation of a small paramagnetic
complex, by either binding to a macromolecule(2-3) or
polymerization of the agent itself.(4) One common targeting
protein is human serum albumin because it displays a rather wide
range of binding capabilities. Typically, interactions between
low molecular weight gadolinium (Gd3+) complexes and albumin are
rather weak.(3,5)
3) One approach to increase specificity and binding interactions
is to design a Gd3+ chelate that binds at the active site of an
enzyme as an inhibitor.(2) The authors report a somewhat
different approach that has potential for screening a variety of
biomolecules by MRI, namely a peptide-based contrast agent that
is activated upon binding to a specific target protein.
References (abridged):
1. Moats, R. A.; Fraser, S. E.; Meade, T. J. Angew. Chem., Int.
Ed. Enel. 1997, 36, 726.
2. Anelli, P. L.; Bertini, I.; Fragai, M.; Lattuada, L.;
Luchinat, C.; Parigi, G. Eur. J. Inorg. Chem. 2000, 625-630.
3. Nivorozhkin, A. L.; Kolodziej, A. P.; Caravan, P.;
Greenfield, M. T.; Lauft'er, R. B.; McMurry, T. J. Angew. Chem.,
Int. Ed. 2001, 40, 2903-2906.
4. Bogdanov, A.; Matuszewski, L.; Bremer, C.; Petrivsky, A.;
Weissleder, R. Mol. Imag. 2002, /, 16-23.
5. Muller, R. N.; Radiichel, B.; Laurent, S.; Platzek, J.;
Pierart, C.; Mareski, P.; Elst, L. V. Eur. J. Inorg. Chem. 1999,
1949-1955.
J. Am. Chem. Soc. 2002 124:3514
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18. ON SCREENING NEWBORNS FOR METABOLIC DISEASES
David S. Millington (Duke University, US) discusses metabolic
disease screening, the author making the following points:
1) Metabolic diseases arise from inherited defects in enzymes
involved in the production of energy, a process that is
essential for the well-being of cells in every organ of a
healthy person. Most of these "inborn errors of metabolism"
occur when a child inherits two copies of a defective gene, one
from each parent. The parents are normally unaware that they
carry defective genes because these diseases are recessive:
Functional copies of these genes mask the effects of defective
copies. When both parents carry a defective copy of the same
gene, each of the children they conceive carries a l-in-4 risk
of being affected with the metabolic disease. Individual
metabolic diseases are rare — ranging in frequency from about 1
in 10,000 to less than 1 in 1,000,000. But hundreds if not
thousands of enzymes are involved in normal metabolism, so,
collectively, these diseases account for a significant fraction
of chronic illness and death in infancy. It is estimated that
metabolic disorders occur in at least 1 in 3,500 newborns.
2) The symptoms of metabolic diseases are easily confused with
much more common conditions. Consequently, their diagnosis is
challenging even to specialists. But correct diagnosis is
essential for appropriate treatment, without which tragic
outcomes are all too common. Public awareness of metabolic
diseases was all but unknown in the United States until 1964,
when widespread neonatal testing began for phenyiketonuria, or
PKU, a disease that causes profound and irreversible mental
retardation when not treated. Over the decades since then, most
states have expanded screening to cover but a handful of
additional diseases. Newborn screening involves pricking a
newborn's heel and collecting blood from the small wound on
cotton fiber paper. The paper carrying the dried blood spots —
commonly known as a "PKU card" or a "Guthrie card," after its
inventor — then goes to a state laboratory, which typically
screens for PKU and a handful of other diseases.
3) The case of PKU screening exemplifies the benefits of early
diagnosis of a metabolic disease. The case is particularly
strong because of the decades of experience showing its benefits
all over the world. PKU arises through the deficiency of an
enzyme that converts the amino acid phenylalanine to the amino
acid tyrosine. The original test was a simple and inexpensive
assay that detected the presence of excess phenylalanine when it
inhibits the growth of a species of bacterium; many newborn
screening laboratories still use this method. When a baby found
to have excess phenylalanine immediately goes on a diet that
restricts phenylalanine intake, the child grows essentially
normally.
References (abridged):
1. Chace, D. H., D. S. Millington, N. Terada, S. G. Kahler, C.
R. Roe and L. F. Hofman. 1993. Rapid diagnosis of
phenylketonuria by quantitative analysis of phenylalanine and
tyrosine in neonatal blood spots using tandem mass spectrometry.
Clinical Chemistry 39:66-71.
2. Millington, D. S., D. H. Chace, S. L. Hillman, N. Kodo and N.
Terada. 1994. Diagnosis of metabolic disease. In Biological Mass
Spectrometry: Present and future, ed. T. Matsuo, R. Caprioli, M.
L. Gross and Y. Seyama. Chichester, UK: Wiley & Sons Ltd.
3. Millington, D. S., N. Kodo, D. L. Norwood and C. R. Roe.
1990. Tandem mass spectrometry: A new method for acylcarnitine
profiling with potential for neonatal screening for inborn
errors of metabolism. Journal of Inherited Metabolic Disease
13:321-324.
4. Links to Internet resources:
http://www.americanscientist.org/articles/02articles/millington.ht
ml
American Scientist 2002 90:40
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19. METEOROLOGY: ANNULAR MODES
El Nino is an aperiodic intermittent (2 to 10 years) flow of
unusually warm surface water along the western coast of South
America, the flow capable of causing abnormally high rainfall in
usually dry areas and severe local ecosystem dislocations --
what is termed an El Nino "event". El Ninos are regional
phenomena, but they have global consequences. The name "El Nino"
("The Child") arose because the phenomenon usually occurs around
Christmas.
J.M. Wallace and D.W. Thompson (University of Washington
Seattle, US) discuss annular modes, the authors making the
following points:
1) When meteorologists look at the monthly or annual averages of
pressure, wind speed, and temperature taken at observation
stations located worldwide, and then subtract the local
long-term mean values, they see certain recurrent spatial
patterns. These patterns, called modes, are believed to be the
signatures of distinctive dynamical interactions. Modes are
generally favored relative to other spatial patterns because
they are reinforced by positive feedback. A familiar example is
the El Nino-southern oscillation, the signature of the
interactions between surface winds and ocean currents in the
equatorial Pacific. In that case, abnormally warm, equatorial
sea surface temperatures favor weak trade winds, which, in turn,
favor warm sea surface temperatures. Notwithstanding that El
Nino is a complicated, global pattern describing the deviations
of sea surface temperature from their average values, it is well
described by an index formed simply by averaging sea surface
temperature deviations over the equatorial Pacific: Intervals of
above normal temperatures are called El Nino events.
2) The west-to-east component of the surface wind averaged
around 55 degrees N latitude is a good index of the primary mode
of sea-level pressure deviations: the Northern Hemisphere
annular mode (NAM). Both the NAM, and the Southern Hemisphere
annular mode (SAM), which is well indexed by the strength of the
westerlies at 55 degrees S, are signatures of a symbiotic
relationship involving the meridional (north-south) profile of
the westerlies in the respective hemispheres and the wavelike
perturbations that are superimposed on them. The profile of the
westerlies influences the shape of the embedded waves. The
embedded waves, in turn, feed back on the profile of the
westerlies through wave-induced meridional transports of
westerly momentum.
3) Modes, reinforced by positive-feedback mechanisms, make a
conspicuously large contribution to maps that describe the
deviations from seasonally adjusted normals in climatic
variables averaged over monthly or yearly time scales. So, for
example, one may expand the monthly deviations in the global
sea-level pressure field in terms of a complete set of
empirically determined orthogonal functions.(1) Two such
functions are the Northern and Southern Hemisphere annular
modes, which typically make much larger contributions in their
respective hemispheres than any other function in the expansion.
By definition, the expansion coefficient of the NAM, suitably
normalized, is the NAM's index. The index of the SAM is
similarly defined and, as mentioned previously, both are well
correlated with the strength of the westerlies at their
respective 55 degrees latitudes: A positive index means the
westerlies are relatively strong.
References (abridged):
1. H. Von Storch, F. W. Zwiers, Statistical Analysis in Climate
Research, Cambridge U. Press, New York (1999).
Physics Today 2002 February
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20. ON ULTRAFAST COOLING OF WATER DROPLETS IN MICROEMULSIONS
G. Seifert et al (Martin Luther University Halle, DE) discuss
ultrafast cooling of water droplets, the authors making the
following points:
1) In a typical water-in-oil microemulsion, water is dispersed
in a nonpolar solvent in the form of nanometer-sized spherical
droplets coated by a monolayer of surfactant molecules. These
so-called reverse micelles have raised considerable interest in
the past few years, not least because of their attractivity for
a variety of technical applications such as chemical catalysis,
drug delivery, or nanocluster synthesis. A large number of
experimental and theoretical investigations revealed information
about, e.g., the essential role of the surfactant layer for
structure and phase behavior [1-5], or dealt with questions such
as solvation dynamics both in the water pool and close to the
surfactant layer. While the structure and dynamics of water in
the reverse micelles are strongly modified directly at the
interface, the basic properties of bulk water reemerge within a
few molecular layers. So, in appropriate ternary mixtures with
droplet sizes of at least several nanometers, most of the water
in such a microemulsion is found in its bulk configuration.
2) Recent femtosecond infrared laser experiments have revealed
that energy dissipation after vibrational excitation in the OH
stretching region in neat water happens on a subpicosecond time
scale. So time-resolved infrared spectroscopy with an
appropriate pulse length can be used to selectively heat the
water inside reverse micelles within a few picoseconds, and
trace the cooling process by monitoring the temperature
dependent changes of the OH stretching band.
3) The authors report a series of such experiments on
octane-AOT-water ternary mixtures (where AOT denotes sodium
di-2-ethylhexyl sulfosuccinate). The presented analysis
demonstrates a new method to determine the size of the water
droplets in microemulsions, and gives evidence that the
amphiphilic AOT interface allows for a very efficient transfer
of heat from water into the nonpolar solvent. The reverse
micelles are found to be stable against quite high transient
temperatures, which is of interest for applications, e.g., as
nanoreactors.
References (abridged):
1. H. Mays and G. Ilgenfritz, J. Chem. Soc. Faraday Trans. 92,
3145 (1996).
2. B. Farago et al., Phys. Rev. Lett. 65, 3348 (1990).
3. T. Hellweg et al., Phys. Rev. E 57, 6825 (1998).
4. L. Foret and A. Wurger, Phys. Rev. Lett. 86, 5930 (2001).
5. Y.N. Cao et al., J. Phys. Chem. B 101, 3005 (1997).
Phys. Rev. Lett. 2002 88:147402
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21. AMORPHIZATION OF ZIRCONIA BY ION RADIATION
In this context, the term "amorphous" refers to a solid that is
not crystalline, i.e., has no long-range order in its lattice.
Many powders described as "amorphous" are in fact microscopic
crystals, as can be demonstrated by x-ray diffraction. Glasses,
however, are examples of true amorphous solids. In this context,
the term "amorphization" refers to the transformation of a solid
into an amorphous state.
A. Meldrum et al (University of Alberta, CA) discuss
amorphization of zirconia, the authors making the following
points:
1) Recent investigations have established that the thermodynamic
properties of nanocrystalline solids are particle-size
dependent. Size-related effects such as melting-point
depressions and thermodynamic stability-field shifts have been
reported [1-5]. The irradiation-induced crystalline-to-amorphous
transition is also an important type of structural
transformation, but despite the 50 years of ion-irradiation data
now accumulated on a wide variety of bulk materials, the
crystalline-to-amorphous transition in nanocrystalline solids
has not been systematically studied.
2) The authors report they investigated the effects of ion
irradiation on embedded nanometer-scale precipitates of
ZrO(sub2). Zirconia is employed in superplastic structural
ceramics that demonstrate superb strength and fracture
toughness, and is also used as an oxygen sensor and a fast ion
conductor. ZrO(sub2) has been the subject of numerous
irradiation experiments to determine its stability in
high-radiation environments, due to its applications as an inert
matrix nuclear fuel and for the immobilization and "burnup" of
plutonium from dismantled nuclear weapons. Bulk ZrO(sub2)
exhibits no evidence of irradiation-induced amorphization at
doses as high as 110 displacements per atom. In no case has the
material demonstrated a tendency toward amorphization, even in
the most extreme irradiation conditions. Only by implanting
large quantities of impurity atoms, and thereby dramatically
modifying its composition, can zirconia be amorphized. Zirconia
is, therefore, one of the most radiation-resistant ceramics
currently known.
3) The authors report that at the smallest particle sizes,
radiation damage effects can be so strongly enhanced that under
the right conditions, materials that have never been made
amorphous can become highly susceptible to irradiation-induced
amorphization. The authors suggest that because light-weight,
high-strength nanocomposites are potential materials for
spacecraft shielding and sensor systems, these fundamental
results have significant implications for the design and
selection of materials to be used in environments where a large
ion flux will be encountered.
References (abridged):
1. S. H. Tolbert and A. .P. Alivisatos, Science 265, 373 (1994).
2. J.M. McHale et al., Science 277, 788 (1997).
3. S.H. Tolbert et al., Phys. Rev. Lett. 76, 4384 (1996).
4. J. Z. Jiang et al., Europhys. Lett. 50, 48 (2000).
5. A.N. Goldstein et al., Science 256, 1425 (1992).
Phys. Rev. Lett. 2002 88:025503
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22. GENERATION OF ULTRA-INTENSE SINGLE-CYCLE LASER PULSES
F.S. Tsung et al (University of California Los Angeles, US)
discuss ultra-intense laser pulses, the authors making the
following points:
1) The quest for attosecond laser pulses is at the forefront of
research in laser physics (1-3). Pulses in the attosecond range
may give rise to the development of attoelectronics, making it
possible to study the dynamics and to control electronic
processes in biology, chemistry, and solid-state physics, in the
same way femtosecond laser technology led to femtochemistry (1).
On the other hand, state-of-the art ultra-intense lasers can
deliver up to 1 petawatt, with pulse durations from 500
femtoseconds down to 18 femtoseconds, at 800 nanometers to 1
micron (4). Two paths toward attosecond pulses can be
identified; the first one, associated with solid-state laser
oscillator technology (5), has pushed the limit of the shortest
laser pulse down to 4.5 femtoseconds in the near-IR to visible
domain. At these wavelengths, breaking the attosecond threshold
implies the generation of subcycle pulses. The other path is
based on the careful combination of some of the short wavelength
harmonics generated in the ionization of a rare gas by intense
femtosecond laser pulses, leading to 100-attosecond extreme UV
pulses (3). The possibility of producing even shorter
single-cycle, ultra-intense pulses opens the way to new
unexplored physics and the possibility of generating
ultra-intense attosecond pulses (3).
2) Current methods for ultra-short pulse generation and
compression already push the limits of the linear and nonlinear
optics of conventional materials (5). Further developments on
ultra-intense lasers must then be based on the nonlinear optics
of plasmas (the medium capable of handling high-power densities
and heat loads) at relativistic intensities. An example is, for
instance, the plasma equivalent of the optical parametric
amplifier recently introduced by Shvets et al (1998).
3) The authors report a scheme to generate single-cycle laser
pulses, the scheme based on photon deceleration in underdense
plasmas. The authors suggest this robust and tunable process is
ideally suited for lasers above critical power because it takes
advantage of the relativistic self-focusing of these lasers and
the nonlinear features of the plasma wake. The mechanism is
demonstrated by particle-in-cell simulations in three and 2.5
dimensions, resulting in pulse shortening up to a factor of 4,
thus making it feasible to generate few-femtosecond single-cycle
pulses in the optical to IR domain with intensities greater than
10^(20) watts per square centimeter by using present-day laser
technology.
References (abridged):
1. Corkum, P. B. (2000) Nature (London) 403, 845-846
2. Porras, M. A. , Salazar-Bloise, F. & Vazquez, L. (2001) Opt.
Lett. 26, 376-378
3. Papadogiannis, N. A. , Witzel, B. , Kalpouzos, C. &
Charalambidis, D. (1999) Phys. Rev. Lett. 83, 4289-4292
4. Backus, S. , Durfee, C. G., III , Murnane, M. M. & Kapteyn,
H. C. (1998) Rev. Sci. Instr. 69, 1207-1223
5. Steinmeyer, G. , Sutter, D. H. , Gallmann, L. , Matuschek, N.
& Keller, V. (1999) Science 286, 1507-1512
Proc. Nat. Acad. Sci. 2002 99:29
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23. ON CARBON-BACKBONE POLYMERS
B.B. Busch et al (University of California Irvine, US) discuss
carbon-backbone polymers, the authors making the following
points:
1) Carbon backbone polymers comprise one of the world's largest
sources of synthetic materials.(1) They are prepared by the
polymerization of olefins, which are derived from petroleum.
Olefin polymerization reactions build the carbon backbone two
carbon atoms at a time. Catalysts of exceptional efficiency have
been developed for these purposes, and remarkable control over
many of the key variables that influence physical properties and
polymer performance has been achieved.(2) But despite these
advances, there are still synthetic challenges that remain. For
example, many olefins, particularly the more highly substituted
derivatives, do not polymerize; their polymers are unknown. The
direct synthesis of many types of polymer architectures, for
example giant macrocyclic rings, cannot be readily achieved by
olefin polymerization. In addition, there are many functional
groups that are not compatible with existing catalysts, so new
methods for their synthesis are needed. Furthermore, despite
significant recent developments, controlled synthesis of
copolymers of the more important commodity polymers such as
poly(ethylene-b-styrene) remains a challenge.
2) Although olefins are the most obvious and practical synthetic
building blocks for the carbon backbone, alternative methods for
synthesizing carbon chains offer the potential for novel
solutions to some of the synthetic challenges. If one lifts the
restriction of the C2 carbon source, new approaches utilizing
Cl, C3, C4, or larger building blocks can be envisioned,
providing of course, suitable building blocks and catalysts can
be found. These novel approaches, employing completely different
chemistries, may not suffer the same limitations of olefin
polymerization.
References (abridged):
1. (a) Barghoom, P.; Stebani, U.; Balsam, M. Adv. Mater. 1998,
10, 635 (b) Morse, P. M. Chem. Eng. News 1999, 77, 11. (c)
Younkin, T. R.; Connor, E. F.; Henderson, J. I.; Friedrich, S.
K.; Grubbs, R. H.; Bansleben, D. A Science 2000, 287, 460. (d)
Wilson, E. Chem. Eng. News 2000, 78, 14.
2. (a) Brintzinger, H. H.; Fisher, D.; Miielhaupt, R.; Rieger,
B.; Waymouth R. M. Angew. Chem., Int. Ed. Engl. 1995, 34,1143.
(b) Fink, G., Miielhaupt, R., Brintzinger, H. H., Eds. Ziegler
Catalysts: Recent Scientific Imorvations and Technological
Improvement; Springer-Verlag: Berlin, 1995. (c) Coates, G. W.;
Waymouth, R. M. Comprehensive Organometallic Chemistry
II-Pergamon Press: 1995; Vol. 12, p 1193. (d) Britovsek, G. J.
P.; Gibson, V. C.; Wass, D. F. Angew. Chem., Int. Ed. 1999, 38,
428. (e) Coates G W. Chem. Rev. 2000, 700, 1223. (f) Ittel, S.
D.; Johnson, L. K.; Brookhart M. Chem. Rev. 2000, 700, 1169. (g)
Mark, T. J; Chen, E. Y. Chem. Rev 2000, 700, 1391. (h)
Matyjaszewski, K.; Xia, J. Chem. Rev. 2001 101 2921.
J. Am. Chem. Soc. 2002 124:3636
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24. ULTRAPERMEABLE REVERSE-SELECTIVE NANOCOMPOSITE MEMBRANES
T.C. Merkel et al (Research Triangle Institute, US) discuss
nanocomposite membranes, the authors making the following points:
1) Organic-inorganic polymer nanocomposites have attracted wide
interest, because the addition of inorganic particles to
polymers can enhance conductivity (1, 2), mechanical toughness
(3), optical activity (4, 5), and catalytic activity.
Nanocomposites may also prove to be useful for molecular
separations, a diverse field affecting processes such as
biomolecule purification, environmental remediation, seawater
desalination, and petroleum chemicals and fuel production. These
separations are typically accomplished with established
technologies such as distillation, absorption, and adsorption,
which are often extremely energy- and capital-intensive, or
recently by selective permeation through membranes. Membranes
are attractive because they are a low-cost, energy-efficient,
green technology. Their widespread use in gas separations has,
however, been limited by the difficulty of preparing membranes
with the desirable combination of high selectivity, which yields
high product purity and low operating costs, and high
permeability, which reduces membrane area and capital cost.
Unfortunately, as the selectivity of conventional polymer
membrane materials increases, permeability invariably decreases
and vice versa. Attempts to overcome this fundamental limitation
have explored the addition of micron-sized porous zeolite
particles to organic polymers in the hope of combining the
mechanical elasticity and processability of polymers with the
strong size selectivity characteristic of spatially well-defined
zeolite pores. Commercialization of this approach, however, has
been hampered by poor polymer/zeolite adhesion and inadequate
particle dispersion.
2) Conventional, or size-selective, polymer membranes
preferentially allow small molecules (such as H2) to permeate
relative to larger ones (such as methane). However, another
class of membranes, reverse-selective membranes, preferentially
allows larger species to permeate in a mixture. This
counterintuitive property is possible because molecular
transport in dense polymer membranes is governed by both
penetrant solubility and diffusivity. Since in general as
penetrant size increases, solubility often increases while the
diffusion coefficient usually decreases, it is possible to
prepare a membrane system that is more permeable to large
molecules than to small molecules.
3) The authors report they have discovered that physical
dispersion of nonporous, nanoscale, fumed silica particles in
glassy amorphous poly(4-methyl-2-pentyne) simultaneously and
surprisingly enhances both membrane permeability and selectivity
for large organic molecules over small permanent gases. These
highly unusual property enhancements, in contrast to results
obtained in conventional filled polymer systems, reflect fumed
silica-induced disruption of polymer chain packing and an
accompanying subtle increase in the size of free volume elements
through which molecular transport occurs, as discerned by
positron annihilation lifetime spectroscopy. The authors suggest
that such nanoscale hybridization represents an innovative means
to tune the separation properties of glassy polymeric media
through systematic manipulation of molecular packing.
References (abridged):
1. E. Coronado, J. R. Galan-Mascaros, C. J. Gomez-Garcia, V.
Laukhin, Nature 408, 447 (2000)
2. F. Croce, G. B. Appetecchi, L. Persi, B. Scrosati, Nature
394, 456 (1998)
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Science 2002 296:519
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25. IN FOCUS: ON JUNK DNA AND THE PLANT GENOME
"The Watson and Crick discovery marked a turning point in the
history of genetics and started what we know today as molecular
genetics or molecular biology. The Mendelian gene was no longer
a mysterious element without physical foundation; it was made of
base-paired, double-stranded DNA that could replicate and be
passed on to progeny, most of the time without change, which
explained why most of the time genetic information was stably
inherited. Once in a while, however, replication errors would
lead to the appearance of mutants. For example, the albino
allele in humans is one such mutation. Furthermore, even though
the variation in DNA makeup, as found among different organisms,
came from the permutation of only four bases, it is the sheer
length of this molecule that accounts for the tremendous genetic
variability found in nature. Simple viruses have DNA containing
a few thousand base pairs, whereas bacteria contain millions,
and more complicated unicellular organisms contain tens of
millions. The totality of human DNA (the human genome) contains
more than 3 billion base pairs. The record, however, is held by
plants; some plant species contain one hundred times more DNA
than humans. This is not because these plants are more
complicated or advanced than humans. Rather, it is often the
case that most of the DNA in complex multicellular organisms is
not genetically meaningful. For example, up to 95 percent of
human DNA does not code for genetic information. This still
leaves humans equipped with about 30,000 genes, in contrast to
13,600 for the fruit fly, 19,000 for a nematode (a very small
worm), and 6,000 for yeast. Therefore, many plants contain a
higher percentage of noncoding DNA (often called junk DNA) than
other living organisms, including humans. The function of the
large amount of junk DNA in many plant species is not
understood, but we know that this portion of their DNA does not
comprise genes."
Paul Lurquin: High Tech Harvest: Understanding Genetically
Modified Food Plants (Westview Press, Cambridge MA 2002, p.34)
(July 2002)
Source information at Amazon.Com
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