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ScienceWeek
SCIENCEWEEK
ScienceWeek November 15, 2002 Vol. 6 Number 46
An Online Research Digest Published Weekly Since 1997
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It is our responsibility as scientists, knowing the great
progress which comes from a satisfactory philosophy of
ignorance, the great progress which is the fruit of freedom of
thought, to proclaim the value of this freedom, to teach how
doubt is not to be feared but welcomed and discussed, and to
demand this freedom as our duty to all coming generations.
-- Richard P. Feynman (1918-1988)
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Section 1
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1. ON THE NURSING SHORTAGE AND SURGICAL PATIENT MORTALITY.
There is much concern about a growing hospital nurse shortage,
and inadequate nurse staffing is apparently a major impediment
to the provision of high-quality hospital care. In hospitals
with high patient-to-nurse ratios, surgical patients experience
higher risk-adjusted 30-day mortality and failure-to-rescue
(deaths following complications) rates.
2. ON ALGAL RHODOPSINS. It has been known for over a century
that unicellular flagellate algae navigate through their
environment by photomotility responses. In the green alga
Chlamydomonas reinhardtii, the chromophore used for the
photomotility responses has been shown to be retinal, and its
light-induced isomerization is responsible for triggering
swimming behavior.
3. ON SELF-REPLICATING CHEMICAL SYSTEMS. The unique aspect of
self-replicating chemical systems is that the reaction product
has the potential to direct additional reactions. The system is
termed autocatalytic when the product is an efficient template,
and each covalent bond that is formed generates additional
template molecules that can direct further joining reactions.
4. ON THE CASIMIR FORCE. In 1948, Hendrik Casimir (1909-2000)
calculated that the quantum fluctuations of an electromagnetic
field, so-called zero-point fluctuations, give rise to an
attractive force between objects. This force is a particularly
striking consequence of the quantum theory of electrodynamics,
according to which under certain conditions the force may be
repulsive.
5. ON EUGENE WIGNER (1902-1995). Once Otto Hahn and Fritz
Strassman discovered fission in 1938, Wigner and fellow
Hungarians Leo Szilard, Edward Teller, and John von Neumann
realized that nuclear bombs were possible. Wigner and Szilard
became obsessed in their desire to push forward the development
of the atomic bomb.
6. PEPTIDES AND SKIN INFECTIONS. A number of endogenous
antimicrobial peptides have been shown to play an integral part
in innate immunity. Two major classes of peptides in mammalian
skin, beta-defensins and cathelicidins, have antimicrobial
activity against bacterial, fungal, and viral pathogens.
7. ON CHEMICAL FORCE SPECTROSCOPY. Chemical force microscopy
(CFM) exploits specifically functionalized atomic force
microscopy tips to study intermolecular interactions on a
nanometer scale. Force measurements can be conducted in a liquid
medium, so that solvent chemical effects on the interaction
between organic group assemblies can be analyzed.
8. ON LIGHT-TRAPPING OF MICRODISKS. Linearly polarized optical
tweezers rotationally trap a birefringent thin microdisk disk,
and scattered light forms a trackable streak. Near a surface the
disk exhibits a translational-orientational switchback
oscillation. Circularly polarized tweezers rotate the disk and
streak, yielding a "colloidal lighthouse".
9. SCIENCEWEEK NOTICES AND SUBSCRIPTION INFORMATION.
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Section 2
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1. ON THE NURSING SHORTAGE AND SURGICAL PATIENT MORTALITY
L.H. Aiken et al (University of Pennsylvania, US) discuss the
nursing shortage, the authors making the following points:
1) The past decade has been a turbulent time for US hospitals
and practicing nurses. News media have trumpeted urgent concerns
about hospital understaffing and a growing hospital nurse
shortage.(1-3) Nurses nationwide consistently report that
hospital nurse staffing levels are inadequate to provide safe
and effective care.(4,5) Physicians agree, citing inadequate
nurse staffing as a major impediment to the provision of
high-quality hospital care. The shortage of hospital nurses may
be linked to unrealistic nurse workloads. Forty percent of
hospital nurses have burnout levels that exceed the norms for
health care workers.(4) Job dissatisfaction among hospital
nurses is 4 times greater than the average for all US workers,
and 1 in 5 hospital nurses report that they intend to leave
their current jobs within a year.(4)
2) In 1999, California passed legislation mandating
patient-to-nurse ratios for its hospitals, which goes into
effect in July 2003. The California legislation was motivated by
an increasing hospital nursing shortage and the perception that
lower nurse retention in hospital practice was related to
burdensome workloads and high levels of job-related burnout and
job dissatisfaction. Stakeholder groups advocated widely
divergent minimum ratios. On medical and surgical units,
recommended ratios ranged from 3 to 10 patients for each nurse.
In early 2002, California's governor announced that hospitals
must have at least 1 licensed nurse for every 6 medical and
surgical patients by July 2003, a ratio that will move to 1 to 5
when the mandates are fully implemented.
3) The authors report a study to determine the association
between the patient-to-nurse ratio and patient mortality,
failure-to-rescue (deaths following complications) among
surgical patients, and factors related to nurse retention. The
study involved cross-sectional analyses of linked data from
10,184 staff nurses surveyed, 232,342 general, orthopedic, and
vascular surgery patients discharged from the hospital between
April 1, 1998, and November 30, 1999, and administrative data
from 168 nonfederal adult general hospitals in Pennsylvania. The
main outcome measures were risk-adjusted patient mortality and
failure-to-rescue within 30 days of admission, and
nurse-reported job dissatisfaction and job-related burnout.
4) Results: After adjusting for patient and hospital
characteristics (size, teaching status, and technology), each
additional patient per nurse was associated with a 7% increase
in the likelihood of dying within 30 days of admission and a 7%
increase in the odds of failure-to-rescue. After adjusting for
nurse and hospital characteristics, each additional patient per
nurse was associated with a 23% increase in the odds of burnout
and a 15% increase in the odds of job dissatisfaction.
5) The authors conclude: In hospitals with high patient-to-nurse
ratios, surgical patients experience higher risk-adjusted 30-day
mortality and failure-to-rescue rates, and nurses are more
likely to experience burnout and job dissatisfaction.
References (abridged):
1. Stolberg SG. Patient deaths tied to lack of nurses. New York
Times. August 8, 2002:A18.
2. Parker-Pope T. How to lessen impact of nursing shortage on
your hospital stay. Wall Street Journal. March 2, 2001:B1.
3. Trafford A. Second opinion: less care for patients.
Washington Post. August 20, 2002:HE01.
4. Aiken LH, Clarke SP, Sloane DM, et al. Nurses' reports on
hospital care in five countries. Health Aff (Millwood).
2001;20:43-53.
5. Henry J. Kaiser Family Foundation. Survey of physicians and
nurses. Available at: http://www.kff.org/content/1999/1503 .
Accessed March 22, 2002.
J. Am. Med. Assoc. 2002 288:1987
Related Background Brief:
WHAT SHOULD WE EXPECT FROM CALIFORNIA'S MINIMUM NURSE STAFFING
LEGISLATION? In 1999, California passed the first legislation in
the United States to establish minimum staffing levels in
hospitals for registered nurses (RNs) and licensed vocational
nurses. The author provides estimates of the increase in RN
expenditures required by this mandate, by hospital size and for
regions of California. Issues related to the implementation of
minimum ratios also are discussed. The authors suggests that
attention must be paid to other staffing regulations, special
concerns of rural hospitals, the possibility that minimum ratios
result in lower RN staffing, and the effect of the nursing
shortage on the ability of hospitals to meet requirements. J.
Spetz: J Nurs Adm 2001 31:132.
Related Background Brief:
THE CALIFORNIA EXPERIMENT: ALTERNATIVES FOR MINIMUM
NURSE-TO-PATIENT RATIOS. In making policy, it is ideal to study
a problem, test solutions, and then implement the best solution.
In California there is legislation regarding nurse staffing that
requires implementation of a solution before the problem has
been accurately stated and before alternative solutions have
been tested. Assembly Bill 394 directs the California Department
of Health Services to establish "minimum, specific, and
numerical licensed nurse-to-patient ratios by licensed nurse
classification and by hospital unit" and have these ratios in
place January 1, 2002. This law is scheduled for implementation
before there is specific evidence of optimal staffing levels for
various patient populations. The author presents a comprehensive
literature review of activities related to nurse staffing in the
United States, provides data related to existing patient
classification systems in California, and suggests an
alternative to staffing ratios to protect consumers. J.A. Seago:
J Nurs Adm 2002 32:48.
Related Background Brief:
HOSPITAL STAFFING, ORGANIZATION, AND QUALITY OF CARE:
CROSS-NATIONAL FINDINGS. The authors report a study to examine
the effects of nurse staffing and organizational support for
nursing care on nurses' dissatisfaction with their jobs, nurse
burnout, and nurse reports of quality of patient care in an
international sample of hospitals. The study involved a
multisite cross-sectional survey of adult acute-care hospitals
in the United States (Pennsylvania), Canada (Ontario and British
Columbia), England, and Scotland. 10,319 nurses working on
medical and surgical units in 303 hospitals across the five
jurisdictions were involved in the study. Results:
Dissatisfaction, burnout, and concerns about quality of care
were common among hospital nurses in all five sites.
Organizational/managerial support for nursing had a pronounced
effect on nurse dissatisfaction and burnout, and both
organizational support for nursing and nurse staffing were
directly, and independently, related to nurse-assessed quality
of care. Multivariate results imply that nurse reports of low
quality care were three times as likely in hospitals with low
staffing and support for nurses as in hospitals with high
staffing and support. The authors conclude: Adequate nurse
staffing and organizational/managerial support for nursing are
key to improving the quality of patient care, to diminishing
nurse job dissatisfaction and burnout and, ultimately, to
improving the nurse retention problem in hospital settings. L.H.
Aiken et al: Int J Qual Health Care 2002 14:5.
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2. ON ALGAL RHODOPSINS
Kevin D. Ridge (University of Maryland, US) discusses algal
rhodopsins, the author making the following points:
1) It has been known for over a century that unicellular
flagellate algae navigate through their environment by
photomotility responses. One response, phototaxis, guides them
towards or away from light, while a second process, termed the
photophobic response, prevents their crossing a light/dark
boundary. Early on, it was shown that these photomotility
responses are accompanied by the generation of local
photocurrents [1]. In the green alga Chlamydomonas reinhardtii,
the chromophore used for the photomotility responses has been
shown to be retinal [2], and its light-induced isomerization is
responsible for triggering swimming behavior [3–5]. It has been
postulated that the photoreceptor(s) are localized to the plasma
membrane in a region of the alga called the eyespot. For the
past several years, the most abundant protein in the eyespot was
considered to be the photoreceptor, but this protein was
ultimately excluded. New work from Sineshchekov et al (2002)
has provided compelling evidence that two newly found
structurally distinct sensory rhodopsins, designated CSRA and
CSRB, mediate phototaxis in Chlamydomonas.
2) Microbial rhodopsins are members of what has been recently
realized to be a large and diverse family of retinal-binding
proteins that participate in a variety of light responses. Over
20 years ago, pioneering studies from several laboratories
identified four different archaeal rhodopsins in the cytoplasmic
membrane of Halobacterium salinarum: the light-driven ion pumps
bacteriorhodopsin and halorhodopsin, and the phototaxis
receptors sensory rhodopsin I and sensory rhodopsin II. The
archaeal rhodopsins all share a common structural template of
seven transmembrane helices, and contain a covalently linked
all-trans retinal chromophore. In all cases, light-induced
isomerization of the chromophore to the 13-cis configuration is
coupled to conformational changes in the protein that are
required for its specialized function: proton or chloride
translocation in the case of bacteriorhodopsin and
halorhodopsin, respectively, and phototaxis signaling by sensory
rhodopsin I and sensory rhodopsin II. This same structural motif
is also conserved among the visual rhodopsins, members of the
largest class of eukaryotic membrane receptors: the G-protein
coupled receptors, which trigger intracellular signaling
cascades.
3) In summary: The discovery of two distinct Chlamydomonas
sensory receptors responsible for phototaxis reveals additional
diversity among the microbial rhodopsins. Sequence and
architecture comparisons among this growing family highlight key
components for light-responsive functions.
References (abridged):
1. Litvin F.F., Sineshchekov O.A. and Sineshchekov V.A. (1978)
Photoreceptor electric potential in the phototaxis of the alga
Haematococcus pluvialis Nature, 271:476-478.
2. Foster K.W., Saranak J., Patel N., Zarilli G., Okabe M.,
Kline T. and Nakanishi K. (1984) A rhodopsin is the functional
photoreceptor for phototaxis in the unicellular eukaryote
Chlamydomonas Nature, 311:756-759.
3. Hegemann P., Gartner W. and Uhl R. (1991) All-trans retinal
constitutes the functional chromophore in Chlamydomonas
rhodopsin. Biophys. J., 60:1477-1489.
4. Lawson M.A., Zacks D.N., Derguini F., Nakanishi K. and
Spudich J.L. (1991) Retinal analog restoration of photophobic
responses in a blind Chlamydomonas reinhardtii mutant. Evidence
for an archaebacterial like chromophore in a eukaryotic
rhodopsin. Biophys. J., 60:1490-1498.
5. Derguini F., Mazur P., Nakanishi K., Starace D.M., Saranak J.
and Foster K.W. (1991) All-trans-retinal is the chromophore
bound to the photoreceptor of the alga Chlamydomonas reinhardtii
Photochem. Photobiol., 54:1017-1021.
Current Biology 2002 12:R588
Related Background Brief:
A RHODOPSIN IS THE FUNCTIONAL PHOTORECEPTOR FOR PHOTOTAXIS IN
THE UNICELLULAR EUKARYOTE CHLAMYDOMONAS. Rhodopsin is a visual
pigment ubiquitous in multicellular animals. If visual pigments
have a common ancient origin, as is believed, then some
unicellular organisms might also use a rhodopsin photoreceptor.
The authors demonstrate that the unicellular alga Chlamydomonas
does indeed use a rhodopsin photoreceptor. The authors report
they incorporated analogues of its retinal chromophore into a
blind mutant; normal photobehavior was restored and the color of
maximum sensitivity was shifted in a manner consistent with the
nature of the retinal analogue added. The data suggest that
11-cis-retinal is the natural chromophore and that the protein
environment of this retinal is similar to that found in bovine
rhodopsin, suggesting homology with the rhodopsins of higher
organisms. This is the first demonstration of a rhodopsin
photoreceptor in an alga or eukaryotic protist and also the
first report of behavioral spectral shifts caused by exogenous
synthetic retinals in a eukaryote. A survey of the morphology
and action spectra of other protists suggests that rhodopsins
may be common photoreceptors of chlorophycean, prasinophycean
and dinophycean algae. Thus, Chlamydomonas represents a useful
new model for studying photoreceptor cells. K.W. Foster et al:
Nature 1984 311:756.
Related Background Brief:
RETINAL ANALOG RESTORATION OF PHOTOPHOBIC RESPONSES IN A BLIND
CHLAMYDOMONAS REINHARDTII MUTANT. EVIDENCE FOR AN
ARCHAEBACTERIAL LIKE CHROMOPHORE IN A EUKARYOTIC RHODOPSIN. The
strain CC-2359 of the unicellular eukaryotic alga Chlamydomonas
reinhardtii, originally described as a low pigmentation mutant,
is found to be devoid of photophobic stop responses to
photostimuli over a wide range of light intensities. Photophobic
responses of the mutant are restored by exogenous addition of
all-trans retinal. The authors report they have combined
computer-based cell-tracking and motion analysis with retinal
isomer and retinal analog reconstitution of CC-2359 to
investigate properties of the photophobic response receptor.
Most rapid and most complete reconstitution is obtained with
all-trans retinal compared to 13-cis, 11-cis, and 9-cis retinal.
An analog locked by a carbon bridge in a 6-s-trans conformation
reconstitutes whereas the corresponding 6-s-cis locked analog
does not. Retinal analogs prevented from isomerization around
the 13-14 double bond by a five-membered ring in the polyene
chain (locked in either the 13-trans or 13-cis configuration) do
not restore the response, but enter the chromophore binding
pocket as evidenced by their inhibition of all-trans retinal
regeneration of the response. Results of competition experiments
between all-trans and each of the 13-locked analogs fit a model
in which each chromophore exhibits reversible binding to the
photoreceptor apoprotein. A competitive inhibition scheme
closely fits the data and permits calculation of apparent
dissociation constants for the in vivo reconstitution process of
2.5 x 10^(-11) M, 5.2 x 10^(-10) M, and 5.4 x 10^(-9) M, for
all-trans, 13-trans-locked and 13-cis-locked analogs,
respectively. The chromophore requirement for the trans
configuration and 6-s-trans conformation, and the lack of
signaling function from analogs locked at the 13 position, are
characteristic of archaebacterial rhodopsins, rather than the
previously studied eukaryotic rhodopsins (i.e., visual
pigments). M.A. Lawson et al: Biophys J 1991 60:1490.
Related Background Brief:
ALL-TRANS-RETINAL IS THE CHROMOPHORE BOUND TO THE PHOTORECEPTOR
OF THE ALGA CHLAMYDOMONAS REINHARDTII. Rhodopsin is the general
name for a family of visual pigments that receive light and
transmit this signal to the rest of an organism. Chlamydomonas
reinhardtii is a unicellular eukaryote whose light-tracking
system consists of a single eye. Through spectral studies of
Chlamydomonas' reaction to light of different wavelengths
(action spectroscopy), it has been shown in vivo that the
photoreceptor of Chlamydomonas is functionally similar to
vertebrate rhodopsin. The authors sought to characterize the
photoreceptor further by identifying the molecule that is
incorporated into the rhodopsin of Chlamydomonas forming the
chromophore. High performance liquid chromatography analysis of
organic extracts of retinaloximes from membrane fractions
enriched in eye-spots and in cells virtually free of interfering
carotenoids identified syn-all-trans as the existing
retinaloxime isomer. The authors conclude that all-trans-retinal
is the native molecule that is available to be incorporated into
the rhodopsin of Chlamydomonas and therefore forms the
functioning chromophore on binding. F. Derguini et al: Photochem
Photobiol 1991 54:1017.
Related Background Brief:
CHLAMYRHODOPSIN REPRESENTS A NEW TYPE OF SENSORY PHOTORECEPTOR.
In order to find optimal light conditions for photosynthetic
growth, the green alga Chlamydomonas uses a visual system. An
optical device, a rhodopsin photoreceptor and an electrical
signal transduction chain that mediates between photoreceptor
and flagella, comprise this system. The authors present an
improved strategy for the preparation of eyespot membranes.
These membranes contain a retinal binding protein, which has
been proposed to be the apoprotein of the phototaxis receptor.
The retinal binding protein, which the authors named
"chlamyopsin", was purified and opsin-specific antibodies were
raised. Using these antibodies, the opsin was localized in the
eyespot region of whole cells during growth and cell division.
The opsin cDNA was purified and sequenced. The sequence reveals
that chlamyopsin is not a typical seven helix receptor. It shows
some homology to invertebrate opsins but not to opsins from
halobacteria. It contains many polar and charged residues and
might function as a light-gated ion channel complex. The authors
suggest it is likely that this lower plant rhodopsin diverged
from animal opsins early in opsin evolution. W. Deininger et al:
EMBO J 1995 14:5849.
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3. ON SELF-REPLICATING CHEMICAL SYSTEMS
N. Paul and G.F. Joyce (Scripps Research Institute, US) discuss
self-replicating systems, the authors making the following
points:
1) In living systems, replicative processes transfer genetic
information from template nucleic acid molecules to newly
synthesized, complementary products. Several nonenzymatic
template-dependent ligation systems have been devised to study
the role of a template in binding and positioning complementary
substrates for covalent bond formation (1-5). These have
included simple self-replicating systems of the form A + B -->
T, where A and B are substrates that bind to a complementary
template, T, and become joined to form a product molecule that
is identical to the template. The unique aspect of
self-replicating systems is that the reaction product has the
potential to direct additional reactions. The system is termed
autocatalytic when the product is an efficient template, and
each covalent bond that is formed generates additional template
molecules that can direct further joining reactions. The
realization of autocatalytic behavior in a self-replicating
system implies that sustained exponential growth may be possible.
2) The self-replicating systems that have been studied to date
use template molecules composed of nucleic acids, peptides, or
small organic compounds. The nucleic acid-based systems are the
most straightforward and rely on simple Watson-Crick pairing
interactions between a short oligonucleotide template and two
complementary oligonucleotide substrates. The substrates are
bound at adjacent positions along the template and are joined
through a reaction involving chemical groups at their opposed
ends. Peptide-based self-replicating systems are similar, except
that the components are oligopeptides that have the capacity to
form alpha-helices. The template is the hydrophobic face of an
alpha-helix that interacts with the corresponding face of two
peptide substrates. Unlike nucleic acid systems, where the
templating interactions involve all of the nucleotide subunits,
peptide replication systems involve only a few amino acid
residues in the helix-helix interactions. The remaining residues
are responsible for maintaining the overall fold of the helix,
which demonstrates that the templating properties of a
self-replicating system do not necessarily involve interactions
with every residue in the polymer. Self-replicating systems
based on organic compounds further generalize the notion of a
template, as these systems are not based on a polymer, but
rather on a low molecular weight compound that presents a
templating surface to bind the two substrates.
3) In summary: A self-replicating molecule directs the covalent
assembly of component molecules to form a product that is of
identical composition to the parent. When the newly formed
product also is able to direct the assembly of product
molecules, the self-replicating system can be termed
autocatalytic. The authors report that a self-replicating system
was developed based on a ribozyme that catalyzes the assembly of
additional copies of itself through an RNA-catalyzed RNA
ligation reaction. The R3C ligase ribozyme was redesigned so
that it would ligate two substrates to generate an exact copy of
itself, which then would behave in a similar manner. This
self-replicating system depends on the catalytic nature of the
RNA for the generation of copies. A linear dependence was
observed between the initial rate of formation of new copies and
the starting concentration of ribozyme, consistent with
exponential growth. The autocatalytic rate constant was 0.011
min^(-1), whereas the initial rate of reaction in the absence of
pre-existing ribozyme was only 3.3 × 10^(-11) (M)min^(-1).
Exponential growth was limited, however, because newly formed
ribozyme molecules had greater difficulty forming a productive
complex with the two substrates. Further optimization of the
system may lead to the sustained exponential growth of ribozymes
that undergo self-replication.
References (abridged):
1. Naylor, R. & Gilham, P. T. (1966) Biochemistry 5, 2722-2728.
2. Lohrmann, R. & Orgel, L. E. (1979) J. Mol. Evol. 12, 237-257.
3. Inoue, T. & Orgel, L. E. (1983) Science 219, 859-862.
4. Rohatgi, R. , Bartel, D. P. & Szostak, J. W. (1996) J. Am.
Chem. Soc. 118, 3332-3339.
5. Zhan, Z. J. & Lynn, D. G. (1997) J. Am. Chem. Soc. 119,
12420-12421.
Proc. Nat. Acad. Sci. 2002 99:12733
Related Backgrround Brief:
KINETIC AND MECHANISTIC ANALYSIS OF NONENZYMATIC,
TEMPLATE-DIRECTED OLIGORIBONUCLEOTIDE LIGATION. The role of
divalent cations in the mechanism of pyrophosphate-activated,
template-directed oligoribonucleotide ligation has been
investigated. The dependence of the reaction rate on Mg2+
concentration suggests a kinetic scheme in which a Mg2+ ion must
bind before ligation can proceed. Mn2+, Ca2+, Sr2+, and Ba2+ can
also catalyze the reaction. Although Pb2+ and Zn2+ do not
catalyze the reaction in the absence of other divalent ions,
they significantly modulate the reaction rate when added in the
presence of Mg2+, with Pb2+ stimulating the reaction (up to
65-fold) and Zn2+ inhibiting the reaction. The logarithm of the
ligation rate increases linearly, with slope of 0.95, as a
function of pH, indicating that the reaction involves a single
critical deprotonation step. The ligation rates observed with
the different divalent metal ion catalysts (Mn2+ > Mg2+ > Ca2+ >
Sr2+ = Ba2+) vary inversely with the pKa values of their bound
water molecules. The pH profile and these relative ligation
rates suggest a mechanism in which a metal-bound hydroxide ion
located near the ligation junction promotes catalysis, most
likely by deprotonation of the hydroxl nucleophile. The effects
of changing either the leaving group or the attacking hydroxyl,
together with the large delta S(++) value for oligonucleotide
ligation (about -20 eu), are consistent with an associative
transition state. R. Rohatgi et al: J Am Chem Soc 1996 118:3332.
Related Background Brief:
MINIMAL SELF-REPLICATING SYSTEMS. Examples of chemical systems
capable of templating and catalyzing their own synthesis --
self-replicating systems -- have begun to appear in the chemical
literature over the last 15 years. For the biologist, these
systems represent a link with the origin of life -- their study
can shed light on prebiotic chemical evolution. However, for the
synthetic chemist, they represent the ultimate synthetic
machine, capable of templating the production of a large number
of perfect copies of themselves from a single original molecule.
The authors describe the design and synthesis of synthetic
minimal replicating systems and provide a general overview and
critique of the field. A. Robertson et al: Chem Soc Rev 2000
29:141.
Related Background Brief:
A SELF-REPLICATING PEPTIDE. The production of amino acids and
their condensation to polypeptides under plausibly prebiotic
conditions have long been known. But despite the central
importance of molecular self-replication in the origin of life,
the feasibility of peptide self-replication has not been
established experimentally. The authors report an example of a
self-replicating peptide. The authors demonstrate that a
32-residue alpha-helical peptide based on the leucine-zipper
domain of the yeast transcription factor GCN4 can act
autocatalytically in templating its own synthesis by
accelerating the thioester-promoted amide-bond condensation of
15- and 17-residue fragments in neutral, dilute aqueous
solutions. The self-replication process displays parabolic
growth pattern with the initial rates of product formation
correlating with the square-foot of initial template
concentration. D.H Lee et al: Nature 1996 382:525.
Related Background Brief:
EMERGENCE OF SYMBIOSIS IN PEPTIDE SELF-REPLICATION THROUGH A
HYPERCYCLIC NETWORK. Symbiosis is an association between
different organisms that leads to a reciprocal enhancement of
their ability to survive. Similar mutually beneficial
relationships can operate at the molecular level in the form of
a hypercycle, a collective of two or more self-replicating
species interlinked through a cyclic catalytic network. The
superposition of cross-catalysis onto autocatalytic replication
integrates the members of the hypercycle into a single system
that reproduces through a second-order (or higher) form of
nonlinear autocatalysis. The hypercycle population as a whole is
therefore able to compete more efficiently for existing
resources than any one member on its own. In addition, the
effects of beneficial mutations of any one member are spread
over the entire population. The formation of hypercycles has
been suggested as an important step in the transition from
inanimate to living chemistry, and a large number of hypercycles
are expected to be embedded within the complex networks of
living systems. But only one naturally occurring hypercycle has
been well documented, while two autocatalytic chemical systems
may contain vestiges of hypercyclic organization. The authors
report a chemical system that constitutes a clear example of a
minimal hypercyclic network in which two otherwise competitive
self-replicating peptides symbiotically catalyse the production
of each other. D.H. Lee et al: Nature 1997 390:591.
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4. ON THE CASIMIR FORCE
E. Buks and M.L. Roukes (Technion, IL) discuss the Casimir
force, the authors making the following points:
1) In 1948, Hendrik Casimir (1909-2000) calculated that the
quantum fluctuations of an electromagnetic field, so-called
zero-point fluctuations, give rise to an attractive force
between objects(1). This force is a particularly striking
consequence of the quantum theory of electrodynamics(2).
Casimir's calculations were idealized -- he considered two
perfectly conducting parallel plates at absolute-zero
temperature -- but there are implications for more realistic
objects. For example, Kenneth et al.(3) have extended these
considerations to real-world materials. Their work follows that
of Boyer in 1974, who also studied the case of parallel plates
but with one plate perfectly conducting and the other having
infinite magnetic permeability (permeability is a measure of the
material's response to an applied magnetic field). For this
special case Boyer found that quantum fluctuations induce a
force with the opposite sign, causing the plates to repel each
other(4). Kenneth et al (3) extend understanding of the Casimir
force phenomenon to the more general situation of realistic
"dielectric" materials that are characterized by both their
electrical permittivity (a measure of the material's response to
an applied electric field) and their magnetic permeability.
Their numerical results show that repulsive forces can arise in
the general class of materials with high magnetic permeability.
2) Although the Casimir effect is deeply rooted in the quantum
theory of electrodynamics, there are analogous effects in
classical physics. A striking example was discussed in 1836, in
P. C. Caussée's L'Album du Marin (The Album of the Mariner)(5).
Caussée reported a mysteriously strong attractive force that can
arise between two ships floating side by side -- a force that
can lead to disastrous consequences. A physical explanation for
this force was offered only recently by Boersma (1996), who
suggested that it originates in the radiation pressure of water
waves acting differently on the opposite sides of the ships. His
argument goes as follows: the spectrum of possible wave modes
around the two ships forms a continuum (any arbitrary
wave-vector is allowed); but between the vessels their opposing
sides impose boundary conditions on the wave modes, restricting
the allowed values of the component of the wave-vector that is
normal to the ships' surfaces. This discreteness created in the
spectrum of wave modes results in a local redistribution of
modes in the region between the ships, with the consequence that
there is a smaller radiation pressure between the ships than
outside them.
References (abridged):
1. Casimir, H. B. G. Proc. Kon. Ned. Akad. 51, 793-795 (1948).
2. Bordag, M., Mohideen, U. & Mostepanenko, V. M. Phys. Rep.
353, 1-205 (2001).
3. Kenneth, O., Klich, I., Mann, A. & Rezen, M. Phys. Rev. Lett.
89, 033001 (2002).
4. Boyer, T. H. Phys. Rev. A 9, 2078-2084 (1974).
5. Caussée, P. C. L'Album du Marin (Mantes, Charpentier, 1836).
Nature 2002 419:119
Related Background:
On Repulsive Casimir Forces
O. Kenneth et al (Technion, IL) discuss Casimir forces, the
authors making the following points:
1) It is well known that the fluctuations of electromagnetic
fields in vacuum or in material media depend on the boundary
conditions imposed on the fields. This dependence gives rise to
forces which are known as Casimir forces, acting on the
boundaries. The best known example for such forces is the
attractive force experienced by parallel conducting plates in
vacuum [1]. Casimir forces between similar, disjoint objects
such as two conducting or dielectric bodies are known in most
cases to be attractive [2] and are sometimes viewed as the
macroscopic consequence of van der Waals and Casimir-Polder
attraction between molecules. In view of the dominance of the
Casimir forces at the nanometer scale, where the attractive
force could lead to restrictive limits on nanodevices [3,4], the
study of repulsive Casimir forces is of increasing interest.
2) Repulsive van der Waals forces are known to be possible if
the properties of the intermediating medium are intermediate
between the properties of two polarizable molecules [5]. In such
cases the Hamaker constant becomes negative, a property which
was successfully employed to explain the wetting properties of
liquid helium. How can one get a repulsive behavior when the
intermediate substance is vacuum? A partial answer can be
obtained from the observation that a purely magnetically
polarizable particle repels a purely electrically polarizable
particle. Motivated by this result, Boyer, following Casimir's
suggestion, studied interplane Casimir force with one plate a
perfect conductor while the other is infinitely permeable. He
showed that in this case the plates repel.
3) In summary: The authors discuss repulsive Casimir forces
between dielectric materials with nontrivial magnetic
susceptibility. The authors demonstrate that considerations
based on the naive pairwise summation of van der Waals and
Casimir-Polder forces may not only give an incorrect estimate of
the magnitude of the total Casimir force but even the wrong sign
of the force when materials with high dielectric and magnetic
responses are involved. Indeed repulsive Casimir forces may be
found in a large range of parameters, and the authors suggest
that the effect may be realized in known materials. The authors
suggest the phenomenon of repulsive Casimir forces may be of
importance both for experimental study and for nanomachinery
applications.
References (abridged):
1. H.B.G. Casimir, Proc. Koninkl. Ned. Akad. Wet. 51, 793 (1948)
2. 0. Kenneth and S. Nussinov, Phys. Rev. D 65, 085014 (2002)
3. J M. Bordag, U. Mohideen, and V. M. Mostepanenko, Phys. Rep.
353, 1 (2001)
4. E. Buks and M.L. Roukes, Phys. Rev. B 63, 033402 (2001)
5. Jacob N. Israelachvili, Intermolecular and Surface Forces
(Academic Press, London, 1992)
Phys. Rev. Lett. 2002 89:033001
Related Background:
EXPERIMENTAL PHYSICS: ON THE CASIMIR FORCE IN
MICROELECTROMECHANICAL SYSTEMS
In general, in physics, the term "zero-point energy" refers to
the energy associated with a particle or system (in addition to
its mass energy) at the absolute zero of temperature. The
zero-point energy cannot be precisely zero because of quantum
uncertainty. Similarly, the lowest energy state of a field (its
"ground state") also cannot be precisely zero.
The "Casimir effect", first predicted by Hendrik B.G. Casimir
[1909-2000] in the late 1940s, is in general a quantum force
that pulls two parallel electrically conducting plates, placed a
short distance apart in a vacuum, towards one another. The basis
of the force is that the zero-point energy between the plates is
reduced compared to the zero-point energy outside the plates.
The essential reason for the zero-point energy reduction between
the plates is quantum mechanical: in quantum theory,
electromagnetic fields fluctuate and therefore can never be
exactly zero, and the plates act as boundary conditions on the
vibration mode frequencies of the vacuum electromagnetic field
between the plates. The plates as boundaries, in effect, force
certain vibration modes to drop out, producing a net reduction
of vacuum energy in the volume between the plates compared to
the vacuum electromagnetic field elsewhere. This results in an
effective pressure pushing the plates together. From a classical
perspective the effect seems bizarre, since it involves the
difference between two apparently nonexistent electromagnetic
fields producing an attractive force. All of this is related to
the so-called "active vacuum" of quantum physics, which refers
to the idea that the vacuum state in quantum mechanics has a
zero-point energy (minimum energy) which gives rise to vacuum
fluctuations, so the vacuum state does not mean a state of
nothing, but is instead an active state. This idea is now of
considerable importance in certain cosmological theories.
The Casimir force was first detected experimentally by M.J.
Sparnaay in 1958. Both the sign and magnitude of the effect
depend critically on the geometry of the surfaces. For two
ideally smooth parallel plates, the net attractive force per
unit area between the plates is given by F = k/d^(4), where (k)
is a constant involving only Planck's constant and the speed of
light, and (d) is the distance between the plates. If one
surface is spherical and the other surface a plane, the force
becomes F = k'R/d^(3), where (k') is a different constant
involving only Planck's constant and the speed of light, (R) is
the radius of the spherical surface, and (d) is the shortest
distance between the spherical surface and the plane surface.
H.B. Chan et al (5 authors at Bell Laboratories, US) report a
study of actuation of microelectromechanical systems by the
Casimir force, the authors making the following points:
1) The authors point out that microelectromechanical systems are
movable structures fabricated on a semiconductor wafer through
the use of integrated circuit technology, and these systems have
become a key technology in the production of sensors and
actuators. So far, the smallest separations between surfaces of
micromachined components are typically on the order of microns,
and the operation of microelectromechanical systems has been
well described by classical mechanics. But as further
miniaturization occurs, quantum effects may become significant
in device design and operation.
2) The authors report a demonstration of the Casimir effect in
microelectromechanical systems using a micromachined torsional
device. Attraction between a polysilicon plate and a spherical
metallic surface results in a torque that rotates the plate
about two thin torsional rods. The dependence of the rotation
angle on the separation between the surfaces is in agreement
with calculations of the Casimir force. The authors suggest
their results demonstrate that quantum electrodynamical effects
play a significant role in such microelectromechanical systems
when the separation between the components is in the nanometer
range. The authors conclude: "This could open up new
possibilities for novel actuation schemes in
microelectromechanical systems based on the Casimir force and
may be important in the design of nanoelectromechanical systems."
Science 2001 291:1941
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5. ON EUGENE WIGNER (1902-1995)
Alvin M. Weinberg (Oak Ridge National Laboratory, US) discuss
Eugene Wigner, the author making the following points:
1) Eugene Wigner (1902-95), the founder of nuclear engineering,
led the group that designed the first very high-powered nuclear
reactors, which were built at Hanford, Washington, for the
production of the isotope plutonium-239. When fission was
discovered late in 1938, Wigner was the completely prepared
mind. In 1936 he had codiscovered the Breit-Wigner formula for
the shape of neutron resonances in nuclei. And, through his
friendship with Leo Szilard (1898-1964), he had been thinking
since 1934 about the possibility of releasing nuclear energy.
Szilard had postulated that a chain reaction based on neutrons
would be possible, and he took out a British admiralty patent on
the concept. Wigner was impressed with Szilard's general idea.
He says in his memoirs that he thought long and hard to see
whether some kind of law of nature -- somewhat like the law of
conservation of energy -- would preclude the possibility of a
nuclear chain reaction.(1) He decided as early as 1936 that no
such law existed, and in his unpublished speech that year to the
Gamma Alpha Society at the University of Wisconsin, he said that
within five years scientists would figure out how to release
nuclear energy. He later wrote that he had no reason for
specifying five years, but he was confident that the nuclear
community would discover a neutron-initiated nuclear reaction
that would produce both energy and neutrons.(2)
2) Once Otto Hahn (1879-1968) and Fritz Strassman (1902-1980)
discovered fission in 1938, Wigner and fellow Hungarians
Szilard, Edward Teller, and John von Neumann (1903-1957)
realized that nuclear bombs were possible. Wigner and Szilard,
whose European-Hungarian background taught them what happens
when a country is run by dictators such as Adolf Hitler, became
obsessed in their desire to push forward the development of the
atomic bomb. By early 1939, war in Europe was imminent. Wigner
realized that humanity was in a struggle with the forces of
evil, and that whoever made the bomb first would rule the world.
Therefore, he, along with Szilard and Teller, decided that they
would have to enlist the US government in the project to develop
the atomic bomb. Wigner suggested that he and Szilard persuade
Albert Einstein to write a letter apprising President Franklin
Roosevelt of the grim possibility that was opened up by the
discovery of fission. Once the letter was written in August
1939, they enlisted an economist from New York City, Alexander
Sachs, to convey it to Roosevelt.(3-5)
References (abridged):
1. Wigner kept a diary, beginning in 1941, which is published as
"Memoir of the Uranium Project" in A. M. Weinberg, ed., The
Collected Works of Eugene Paul Wigner, vol. 5, Springer-Verlag,
New York (1992). The memoir is an excellent history of the
origins of Hanford.
2. See p. 24 in A. M. Weinberg, ed., The Collected Works of
Eugene Paul Wigner, vol. 5, Springer-Verlag, New York (1992).
3. See, for example, M. Bundy, Danger and Survival: Choices
about the Bomb in the First Fifty Years, Random House, New York
(1988), p. 35.
4. M. G. I. Boissevan et al., in ref. 2, p. 297.
5. E. C. Creutz et al., in ref. 2, p. 631.
Physics Today 2002 October
Related Background:
ON LEO SZILARD (1898-1964), INVENTIONS, AND CRITICISM OF THE
RECENT NOBEL PRIZE IN PHYSICS
A number of physicists have apparently disparaged the recent
Nobel Prize in Physics as a reward for "an engineering feat, not
a scientific one" and as having "little to do with fundamental
science." The Nobel Prize in Physics, however, was chartered by
Alfred Nobel to be awarded to "the person who shall have made
the most important discovery or invention within the field of
physics", so the awarding of the prize for an invention is
certainly appropriate. In addition, any intelligent analysis of
the history of science demonstrates the intimate link between
invention/technology on the one hand and science on the other
hand, the two mutually feeding each other in a progressive
spiral of accomplishment. The apparently disparaging remarks of
certain physicists concerning the recent Nobel Prize physics
award suggest that perhaps there are too many professional
physicists with only an amateur understanding of the history of
physics. In a letter to the New York Times of 16 October 2000
concerning this criticism of the recent Nobel Prize awards as
awards for invention rather than fundamental science, James W.
Cronin, who was awarded the Nobel Prize in Physics in 1980,
states: "My colleagues' comments do not well serve those of us
who pursue fundamental physics. In most cases, it is the
development of technology that permits the very experiments that
advance fundamental science." An excellent example of a
personal linkage between fundamental science and invention can
be found in the story of the physicist Leo Szilard (1898-1964),
who conceived the idea of a nuclear chain reaction in 1934, and
who, to anyone who knew him, was an ambulatory intellectual tour
de force.
Valentine L. Telegdi (CERN, CH) presents an essay on Leo Szilard
as an inventor, the author making the following points:
1) After World War I, Szilard moved from Budapest to Berlin to
study engineering, but he soon switched to physics. During his
Berlin period (1920-1933), Szilard was granted 31 patents. In
contrast, his published output as a physicist during this period
consisted of two theoretical papers and two experimental papers.
Hardly any of Szilard's inventions were realized in practice,
and he evidently reaped no financial rewards from his German
patents or their foreign equivalents. One exception was a patent
on a "Discharge Tube to Be Used as an Electron Source", which he
assigned to the Siemens-Schuckert Company in Berlin.
2) During Szilard's Berlin period, he became interested in
improving refrigerators for household use. The commercially
available refrigerators of that time were noisy and generally
unreliable, and Szilard attempted to propose devices without
moving parts. He obtained 16 patents involving refrigerators, 5
of which were filed jointly with Albert Einstein (1879-1955) (a
former patent clerk).
3) The most famous of Szilard's patents was his 1934 patent for
the idea of a nuclear chain reaction, although Szilard focused
on beryllium (which turned out not to be practical). By this
time Szilard was already a Jewish refugee in London, having fled
Germany when Hitler came to power in 1933. Szilard fully
realized the military potential of this patent, and in order to
keep the patent secret, he assigned the patent to the British
Admiralty.
[Editor's note: Szilard emigrated to the US in 1937, and when
uranium fission was discovered by Otto Hahn (1879-1968) and
announced by Lise Meitner (1878-1968) in 1939, Szilard realized
uranium fission would make his chain reaction idea practical.
With Eugene Wigner (1902-1995) and Edward Teller (1908- ), all
Hungarian refugee physicists who had fled the Nazis, Szilard
persuaded Einstein to send his famous letter to President
Roosevelt, following which the Manhattan Project was set in
motion to prepare an atomic bomb. Einstein's famous letter was
actually written by Leo Szilard.]
Physics Today 2000 October
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6. PEPTIDES AND SKIN INFECTIONS
P.Y. Ong et al (National Jewish Medical and Research Center
Denver, US) discuss skin infections, the authors making the
following points:
1) The skin's first line of defense against invasion by
microbial agents is the stratum corneum, a nonviable, desiccated
layer of the epidermis.(1) However, this physical barrier is
susceptible to injuries that allow the entry of opportunistic
microbial agents into the skin. The innate immune system can
immediately respond to this intrusion by helping to prevent
further invasion. This immune response includes phagocytosis by
neutrophils and macrophages and their production of reactive
oxygen intermediates that kill microbial agents.(2)
2) A number of endogenous antimicrobial peptides have been shown
to play an integral part in innate immunity.(3) Two major
classes of peptides in mammalian skin, beta-defensins(4,5) and
cathelicidins, have antimicrobial activity against bacterial,
fungal, and viral pathogens.(4) These peptides, which are
produced by keratinocytes in the skin,(4) disrupt the membrane
of the target microbe or penetrate the microbial membrane,
interfering with intracellular functions. The expression of some
of these peptides, such as human beta-defensin 1 (HBD-1), is
constitutive, whereas the expression of others, including human
beta-defensin 2 (HBD-2) and LL-37, a cathelicidin, is triggered
by injury or inflammation of the skin.(4) Animal models have
shown that the expression or activation of antimicrobial
peptides is essential for the ability of skin to resist
bacterial infection.
3) Atopic dermatitis, a chronic inflammatory skin disease
frequently found in families with asthma and allergic rhinitis,
is complicated by recurrent infections of skin lesions by
bacterial, viral, and fungal pathogens. About 30 percent of
patients with atopic dermatitis have bacterial or viral
infections of the skin, as compared with only 7 percent of
patients with psoriasis, even though both diseases are
characterized by a defective skin barrier.
4) The authors report they compared the expression of HBD-2 and
LL-37 in skin lesions from patients with atopic dermatitis with
their expression in psoriatic lesions and normal skin, using
immunohistochemical staining, Western and immunodot blotting,
and a quantitative, real-time
reverse-transcriptase–polymerase-chain-reaction (RT-PCR) assay.
Immunohistochemical analysis confirmed the presence of abundant
LL-37 and HBD-2 in the superficial epidermis of all patients
with psoriasis. In comparison, immunostaining for these peptides
was significantly decreased in acute and chronic lesions from
patients with atopic dermatitis From their results, the authors
conclude that a deficiency in the expression of antimicrobial
peptides may account for the susceptibility of patients with
atopic dermatitis to skin infection with the bacterium S. aureus.
References (abridged):
1. Jackson SM, Elias PM. Skin as an organ of protection. In:
Fitzpatrick TB, Eisen AZ, Wolff K, Freedberg IM, Austen KF, eds.
Dermatology in general medicine. 4th ed. Vol. 1. New York:
McGraw-Hill, 1993:241-53.
2. Fearon DT, Locksley RM. The instructive role of innate
immunity in the acquired immune response. Science 1996;272:50-53.
3. Lehrer RI, Lichtenstein AK, Ganz T. Defensins: antimicrobial
and cytotoxic peptides of mammalian cells. Annu Rev Immunol
1993;11:105-128.
4. Harder J, Bartels J, Christophers E, Schroder JM. A peptide
antibiotic from human skin. Nature 1997;387:861-861.
5. Stolzenberg ED, Anderson GM, Ackermann MR, Whitlock RH,
Zasloff M. Epithelial antibiotic induced in states of disease.
Proc Natl Acad Sci U S A 1997;94:8686-8690.
New Engl. J. Med. 2002 347:1151
Related Background Brief:
DEFENSINS: ANTIMICROBIAL AND CYTOTOXIC PEPTIDES OF MAMMALIAN
CELLS. Defensins are antimicrobial and cytotoxic peptides that
contain 29-35 amino acid residues, including six invariant
cysteines whose intramolecular disulfide bonds cyclize and
stabilize them in a complexly folded, triple-stranded beta-sheet
configuration. Generated by the proteolytic processing of 93-95
amino acid precursor peptides, they constitute > 5% of the total
cellular protein in human and rabbit neutrophils
(polymorphonucleated neutrophils -- PMN) and are also produced
by rabbit lung macrophages and by mouse and rabbit small
intestinal Paneth cells. Despite their prominence in rat PMN,
defensins are not found in murine PMN. The antimicrobial
spectrum of defensins includes gram positive and gram negative
bacteria, mycobacteria, T. pallidum, many fungi, and some
enveloped viruses. Defensins exert nonspecific cytotoxic
activity against a wide range of normal and malignant targets,
including cells resistant to TNF-alpha and NK-cytolytic factor.
They appear to kill mammalian target cells and microorganisms by
a common mechanism, which involves initial electrostatic
interactions with negatively charged target cell surface
molecules (likely the head groups of polar membrane lipids),
followed by insertion into the cell membranes which they
permeabilize, forming voltage-regulated channels. In addition to
their antimicrobial and cytotoxic properties, some defensins act
as opsonins, while others inhibit protein kinase C, bind
specifically to the ACTH receptor and block steroidogenesis or
act as selective chemoattractants for monocytes. Defensins are a
newly delineated family of effector molecules whose contribution
to host defense, inflammation, and cytotoxicity may be
considerable for humans, even though it is unlikely to be
revealed by experimentation with mice. R.I. Lehrer et al: Annu.
Rev. Immunol.1993 11:105.
Related Background Brief:
EPITHELIAL ANTIBIOTIC INDUCED IN STATES OF DISEASE. Epithelial
defensins provide an active defense against the external
microbial environment. The authors report they investigated the
distribution and expression of this class of antimicrobial
peptides in normal cattle and in animals in varying states of
disease. beta-defensin mRNA was found to be widely expressed in
numerous exposed epithelia but was found at higher levels in
tissues that are constantly exposed to and colonized by
microorganisms. The authors observed induction in ileal mucosa
during chronic infection with Mycobacterium paratuberculosis and
in bronchial epithelium after acute infection with Pasteurella
haemolytica. It has been proposed that expression of
antimicrobial peptides is an integral component of the
inflammatory response. The authors suggest their results support
this hypothesis and further suggest that epithelial defensins
provide a rapidly mobilized local defense against infectious
organisms. E.D. Stolzenberg et al: pnas 1997 94:8686.
Related Background Brief:
SYNDECANS, CELL SURFACE HEPARAN SULFATE PROTEOGLYCANS, INDUCED
BY A PROLINE-RICH ANTIMICROBIAL PEPTIDE FROM WOUNDS. Cell
surface heparan sulfate proteoglycans, such as the syndecans,
are required for cellular responses to heparin-binding growth
factors and extracellular matrix components. Expression of
syndecan-1 and -4 is induced in mesenchymal cells during wound
repair in the mouse, consistent with a role for syndecans in
regulating cell proliferation and migration in response to these
effectors. The authors demonstrate that wound fluid contains
inductive activity that mimics the in vivo induction in time of
appearance, specificity for mesenchymal cells, and selectivity
for syndecan-1 and -4. The authors report they have purified and
synthesized a 4.8-kDa proline-rich protein from wound fluid that
reproduces this induction of syndecan-1 and -4 in cultured
cells. This peptide, identical to the antibacterial peptide
PR-39, is released into the wound by the cellular infiltrate and
induces syndecan expression at the same peptide concentrations
that lyse bacteria. The authors suggest these results indicate
that wounds contain a multifunctional protein that induces
mammalian cells to express cell surface heparan sulfate
proteoglycans as part of the wound repair process and that kills
bacteria as part of a nonimmune defense mechanism. R.L. Gallo et
al: pnas 1994 91:11035.
Related Background Brief:
THE EXPRESSION OF THE GENE CODING FOR THE ANTIBACTERIAL PEPTIDE
LL-37 IS INDUCED IN HUMAN KERATINOCYTES DURING INFLAMMATORY
DISORDERS. The epithelia constitute a major barrier to the
environment and provide the first line of defense against
invading microbes. Antimicrobial peptides are emerging as
participants in the defense system of epithelial barriers in
general. Originally the authors isolated the human antimicrobial
peptide LL-37 from granulocytes. The gene (CAMP or cathelicidin
antimicrobial peptide) coding for this peptide belongs to the
cathelicidin family, whose members contain a conserved pro-part
of the cathelin type. The human genome seems to have only one
gene of this family, whereas some mammalian species have several
cathelicidin genes. In the present work the authors demonstrate
up-regulation of this human cathelicidin gene in inflammatory
skin disorders, whereas in normal skin no induction was found.
By in situ hybridization and immunohistochemistry the transcript
and the peptide were located in keratinocytes throughout the
epidermis of the inflammatory regions. In addition, the peptide
was detected in partially pure fractions derived from psoriatic
scales by immunoblotting. These fractions also exhibited
antibacterial activity. The authors propose a protective role
for LL-37, when the integrity of the skin barrier is damaged,
LL-37 participating in the first line of defense and preventing
local infection and systemic invasion of microbes. M. Frohm et
al: J. Biol. Chem. 1997 272:15258.
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7. ON CHEMICAL FORCE SPECTROSCOPY
D.V. Vezenov et al (Harvard University, US) discuss chemical
force microscopy, the authors making the following points:
1) Interfaces designed at the molecular level provide an
opportunity to study intermolecular forces between dissimilar
materials. Amorphous interfaces involving organic materials are
present in problems of both industrial and academic interest,
for example, compatibility of polymer phases, lubrication,
adsorption, biological adhesion, or susceptibility of interfaces
to changes in environmental conditions.(1,2) In many
engineering, chemical, or biological processes, the behavior of
these interfaces reflects the combined response of the system as
a whole to external stress. For example, the observed
interfacial strength will be influenced by the strength of the
intermolecular interactions at the interface, the mechanical
properties of the bulk material (plastic and viscoelastic
contributions), and the rates of stress application (temperature
and velocity dependencies). Forces and energies involved in the
breaking of adhesive joints (fracture energies) are typically
orders of magnitude higher than estimates based on the
thermodynamic work of adhesion.(3) Methods that disentangle
intermolecular forces from other factors (e.g. viscoelastic
dissipation of energy) are extremely valuable. Typically,
intermolecular surface forces are interpreted in terms of basic
interactions such as van der Waals, electron donor-acceptor
(hydrogen bonding), and electrochemical interactions.(1) Most
experimental data on the thermodynamics at the interfaces come
from studies of surface or interfacial tension and
wettability,(4,5) adsorption and immersion calorimetry,(4,5)
reversed phase chromatography, and inverse gas chromatography.
2) In the present work, the authors attempt direct experimental
measurements of the intermolecular component in the overall
strength. The authors report they use an atomic force microscope
(AFM) with sharp probes. Intermolecular interactions can then be
probed in the dynamic mode using force measurements at truly
nanoscopic scale (between tens or hundreds of molecules). Sample
requirements for these types of experiments are significantly
more relaxed than those for experiments requiring large, flat,
ordered areas of sample.
3) The development of a fundamental understanding of
intermolecular interactions at organic interfaces can be
achieved by designing the interfaces and/or intervening medium
in such a way that the strength and type of interaction can be
systematically varied. The authors attain this objective with a
systematic force microscopy study of interactions between
molecular groups based on a concept of chemical force microscopy
(CFM) that exploits specifically functionalized AFM tips to
study intermolecular interactions on a nanometer scale. With
this approach, the chemical identity and surface energy of the
probe is controlled by covalently linking a self-assembled
monolayer terminating in a distinct functional group to the tip
surface. Force measurements can be conducted in a liquid medium,
and hence solvent chemical effects on the interaction between
organic group assemblies can be analyzed. Combining these
features with unique sensitivity (sub-nanonewton) and resolution
(sub-nanometer) of the AFM, yields a technique capable of
providing information about intermolecular interactions at
nanometer scale.
References (abridged):
1. Israelachvili, J. Intermolecular and Surface Forces; Academic
Press: New York, 1992.
2. Bhushan, B. Handbook of Micro/Nano Tribology, 1st ed.; CRC
Press: Boca Raton, FL, 1995.
3. Evans, A. G.; Dalgleish, B. J. Acta Metall. Mater. 1992, 40,
S295.
4. Fowkes, F. M. Ind. Eng. Chem. 1964, 56, 40.
5. Fowkes, F. M. In Surfaces and Interfaces I. Chemical and
Physical Characteristics; Burke, J. J., Reed, N. L., Weiss, V.,
Eds.; Syracuse University Press: Syracuse, NY, 1967; p 197.
J. Am. Chem. Soc. 2002 124:10578
Related Background Brief:
FUNCTIONAL GROUP IMAGING BY CHEMICAL FORCE MICROSCOPY. Mapping
the spatial arrangement of chemical functional groups and their
interactions is of significant importance to problems ranging
from lubrication and adhesion to recognition in biological
systems. The authors report that a force microscope has been
used to measure the adhesive and friction forces between
molecularly modified probe tips and organic monolayers
terminating in a lithographically defined pattern of distinct
functional groups. The adhesive interactions between simple
CH3/CH3, CH3/COOH, and COOH/COOH functional groups correlate
directly with friction images of sample surfaces patterned with
these groups. Thus, by monitoring the friction between a
specifically functionalized tip and sample, one can produce
friction images that display predictable contrast and correspond
to the spatial distribution of functional groups on the sample
surface. Applications of this chemically sensitive imaging
technique are discussed. C. D. Frisbie et al: Science 1994
265:2071.
Related Background Brief:
CHEMICAL FORCE MICROSCOPY: EXPLOITING CHEMICALLY-MODIFIED TIPS
TO QUANTIFY ADHESION, FRICTION, AND FUNCTIONAL GROUP
DISTRIBUTIONS IN MOLECULAR ASSEMBLIES. The authors report that
chemical force microscopy (CFM) has been used to measure
adhesion and friction forces between probe tips and substrates
covalently modified with self-assembled monolayers (SAMs) that
terminate in distinct functional groups. Probe tips have been
modified with SAMs using a procedure that involves coating
commercial Si3N4 cantilever/tip assemblies with a thin layer of
polycrystalline Au followed by immersion in a solution of a
functionalized thiol. This methodology provides a reproducible
means for endowing the probe with different chemical functional
groups. The spring constants and radii of the chemically
modified cantilever/tip assemblies have been characterized to
allow for quantitative friction and adhesion measurements.
Au-coated Si and Si substrates have been treated with
functionalized thiols and silanes, respectively, to produce SAM
coated substrates terminating with different functional groups.
A force microscope has been used to characterize the adhesive
interactions between probe tips and substrates that have been
modified with SAMs which terminate with COOH, CH3, and NH2
functional groups in EtOH and H2O solvents. Force vs. distance
curves recorded under EtOH show that the interaction between
functional groups decreases as follows: COOH/COOH > CH3/CH3 >
COOH/CH3. The measured adhesive forces were found to agree well
with predictions of the Johnson, Kendall, and Roberts (JKR)
theory of adhesive contact and thus show that the observed
adhesion forces correlate with the surface free energy of the
molecular groups in EtOH. Electrostatic contributions to
adhesive forces have also been studied using a COO-/NH3+
tip/surface in aqueous solutions. Force vs distance curves
recorded as a function of ionic strength show that the observed
adhesive interaction decreases with increasing ionic strength.
These results have been interpreted in terms of contact and
noncontact contributions to the experimentally measured adhesive
force. The friction forces between tips and samples modified
with COOH and CH3 groups have also been measured as a function
of applied load. The magnitude of the friction force was found
to decrease in the following manner with different tip/sample
functionalities: COOH/COOH > CH3/CH3 > COOH/CH3. Friction forces
between different chemical functional groups thus correlate
directly with the adhesion forces between these same groups.
Specifically, high friction is observed between groups that
adhere strongly, while low friction is observed between weakly
interacting functional groups. The dependence of friction forces
on the tip and sample functionality is shown to be the basis for
chemical force microscopy in which lateral force images are
interpreted in terms of the strength of both adhesive and
frictional interactions between different functional groups. A.
Noy et al: J. Am. Chem. Soc. 1995 117:7943.
Related Background Brief:
ATOMIC FORCE MICROSCOPE FOR CHEMICAL SENSING. The authors report
that a novel atomic force microscope (AFM) having a tip on whose
surface sensor molecules were immobilized has been developed for
chemical sensing. The AFM tips chemically modified with
octadecyltrichlorosilane or perfluorotrichlorosilane were more
sensitive than the unmodified tips in detecting adhesive force
on some monolayers on the silicon substrates. Using an OTS
modified tip, the hydrocarbon chain length of chemically
adsorbed alkyltrichlorosilanes on the silicon substrates was
discriminated in an ethanol environment. The origin of the
interaction between the chemically modified tips and the sample
surfaces, which depend on the monolayers and environmental
liquids where the interactions were measured, has been
speculated to arise from the entwining of carbon chains in
monolayers on the tip and the sample. T. Nakagawa et al: J
Vacuum Sci Technol B 1994 12:2215.
xRelated Background Brief:
REAL TIME MONITORING OF THE ELECTROCHEMICAL TRANSFORMATION OF A
FERROCENE-TERMINATED ALKANETHIOLATE MONOLAYER AT GOLD VIA AN
ADHESION-BASED ATOMIC FORCE MICROSCOPIC CHARACTERIZATION. The
authors report a demonstration of the real time monitoring of
the electrochemical transformation of a surface-bound redox
species using AFM-based adhesion measurements. The measurements
were conducted by using a monolayer formed by the chemisorption
of 11-mercaptoundecyl ferrocenecarboxylate (FcT) at a Au(111)
electrode and a Au-coated probe tip modified with an
octadecanethiolate (OT) monolayer. Using this probe tip-sample
combination, the 1-electron oxidation of the ferrocenyl group of
the FcT monolayer results in a decrease in the observed force of
adhesion (Fad) at the microcontact formed by the 2 different
surfaces. Using surface tension arguments, this change is
attributed to the decrease in the effective miscibility at the
microcontact as a consequence of the oxidation of the ferrocenyl
group to the ferrocenium ion. Issues related to the underlying
changes in the interfacial structure that give rise to the
differences in Fad are discussed. J-B. Green et al: J Phys Chem
1996 100:13342.
Related Background Brief:
SOLVENT EXCLUSION AND CHEMICAL CONTRAST IN SCANNING FORCE
MICROSCOPY. The authors report a quantitative study with
scanning force microscopy of the importance of a solvent in
regulating the adhesion forces between surfaces. Both samples
and tips are coated with alkyl thiolate monolayers of type
HS(CH2)10Y and force measurements are conducted as a function of
terminal group Y (Y = CH2CH3, CH2OCH3, CO2CH3, CO(NH2), CO2H,
CH2OH) and solvent (water, EtOH, n-hexadecane). Adhesive forces
in H2O span the greatest range (0.30-12.5 nN), with hydrophobic
surfaces adhering most strongly and hydrophilic surfaces most
weakly. In EtOH, the adhesive forces are substantially smaller
and in n-hexadecane, they are negligible. In H2O, these adhesive
forces are consistent with the work required to exclude solvent
from the tip-sample interface, indicating that solvent exclusion
dominates adhesion. Such macroscopic solvent exclusion cannot
explain fully the adhesive forces in EtOH. This force data is
used to evaluate the tip-sample interfacial energies (gts) of
like CH3- and CH2OCH3-terminated surfaces and the surface-vacuum
interfacial energies (gsv) of the hydrophilic surfaces. An
effective tip radius of approximately 30 nm and contact area of
approximately 10 nm^(2) (or approximately 50 contacting
molecules) is estimated from the adhesion between methyl groups
in H2O. Since solvent exclusion regulates adhesion between these
model organic surfaces, it provides a source of chemical
contrast in force imaging. The authors explore this chemical
contrast with friction force measurements of co-block
polyethylene glycol-polyamide polymer surfaces. S.K. Sinniah et
al: J. Am. Chem. Soc. 1996 118:8925.
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8. ON LIGHT-TRAPPING OF MICRODISKS
The term "laser tweezers" refers to a laser trap used to hold
and move microscopic objects. The term "laser trap" refers to a
device for confining atoms, molecules, and neutral particles up
to 10 microns in diameter, the trap consisting of a focused
laser beam tuned to a frequency such that particles are
attracted to regions of high laser intensity.
Z. Cheng et al (ExxonMobil Co., US) discuss light-trapping, the
authors making the following points:
1) Laser tweezers [1-3] are commonly used to trap and manipulate
microspheres in three dimensions, yet their utility for trapping
and manipulating nonspherical microparticles remains incomplete
due to the additional geometrical complexity. Optical trapping
of long cylinders with their symmetry axes along the laser beam
has been previously predicted [4] and observed [5]. However, for
short coin-like cylinders, the same theoretical model predicts
that tweezers cannot trap disks having radii and thicknesses
much greater than the wavelength of light stably in 3D [4].
Likewise, light scattered by trapped microspheres has been
studied, but it has not been considered for less symmetric
microparticles that have dimensions near or below the wavelength
of visible light. The interaction between light and asymmetric
dielectric particles is of widespread importance, since many
naturally occurring particles that could be tweezed are not
spherical.
2) The authors report they approach this issue by applying laser
tweezers to coin-like wax microdisks and to lithographically
prepared polymethylmethacrylate (PMMA) microdisks having a
typical diameter-to-thickness aspect ratio of about five.
Despite a calculation that suggests that tweezed microdisks
cannot satisfy force and torque equilibria simultaneously [4],
the authors report they are able to trap thin microdisks having
a wide range of radii in 3D. These "thin" disks have thicknesses
comparable to or less than the beam waist at the focus of the
tweezer (i.e., near the wavelength of light); this thickness
regime lies below that of the calculation. When trapped, a thin
disk orients with its symmetry axis perpendicular to the beam.
This orientation causes the backscattered laser light to form a
streak parallel with the disk's faces, yielding a natural
optical lever for sensing rotation. By exploiting this optical
lever, the authors introduce a novel high-speed scattering
technique, light streak tracking (LST), for precisely measuring
rotational fluctuations of a single microdisk. The wax disks are
birefringent, whereas the PMMA disks are not. This allows one to
separate the effects due to structural and optical anisotropies.
In particular, the authors find that the PMMA disks rotate
freely in linearly polarized (LP) light, whereas the wax disks
are rotationally bound.
3) In summary: Nonspherical particles can uniquely probe soft
system dynamics. The authors demonstrate that laser tweezers
stably trap thin coin-like microdisks in 3D with an edge-on
orientation. Scattered light forms a streak that the authors
track using a fast camera to measure the disk's angular
displacement. Linearly polarized tweezers rotationally trap a
birefringent disk, and the authors measure its harmonically
bound Brownian rotation over 5 decades in time. Near a surface,
the disk exhibits a translational-orientational switchback
oscillation. Circularly polarized tweezers rotate the disk and
streak, yielding a colloidal lighthouse.
References (abridged):
1. A. Ashkin, J.M. Dziedzic, J.E. Bjorkholm, and S. Chu, Opt.
Lett. 11, 288 (1986).
2. T.T. Perkins, D.E. Smith, R. G. Larson, and S. Chu, Science
268, 83 (1995).
3. Laser Tweezers in Cell Biology, edited by M.P. Sheetz
(Academic, San Diego, 1998).
4, R.C. Gauthier, J. Opt. Soc. Am. B 14, 3323 (1997).
5. R. C. Gauthier, M. Ashman, and C. P. Grover, Appl. Opt. 38,
4861 (1999).
Phys. Rev. Lett. 2002 89:108303
Related Background Brief:
SIMULTANEOUS MICROMANIPULATION IN MULTIPLE PLANES USING A
SELF-RECONSTRUCTING LIGHT BEAM. Optical tweezers are commonly
used for manipulating microscopic particles, with applications
in cell manipulation, colloid research, manipulation of
micromachines and studies of the properties of light beams. Such
tweezers work by the transfer of momentum from a tightly focused
laser to the particle, which refracts and scatters the light and
distorts the profile of the beam. The forces produced by this
process cause the particle to be trapped near the beam focus.
Conventional tweezers use gaussian light beams, which cannot
trap particles in multiple locations more than a few micrometers
apart in the axial direction, because of beam distortion by the
particle and subsequent strong divergence from the focal plane.
Bessel beams, however, do not diverge and, furthermore, if part
of the beam is obstructed or distorted the beam reconstructs
itself after a characteristic propagation distance. The authors
demonstrate how this reconstructive property may be utilized
within optical tweezers to trap particles in multiple, spatially
separated sample cells with a single beam. Owing to the
diffractionless nature of the Bessel beam, secondary trapped
particles can reside in a second sample cell far removed
(approximately 3 mm) from the first cell. Such tweezers could be
used for the simultaneous study of identically prepared
ensembles of colloids and biological matter, and potentially
offer enhanced control of "lab-on-a-chip" and optically driven
microstructures. V. Garces-Chavez et al: Nature 2002 419:125.
STRESS RESPONSE IN CAENORHABDITIS ELEGANS CAUSED BY OPTICAL
TWEEZERS: WAVELENGTH, POWER, AND TIME DEPENDENCE. Optical
tweezers have emerged as a powerful technique for
micromanipulation of living cells. Although the technique often
has been claimed to be nonintrusive, evidence has appeared that
this is not always the case. The authors present evidence that
near-infrared continuous-wave laser light from optical tweezers
can produce stress in Caenorhabditis elegans. A transgenic
strain of C. elegans, carrying an integrated
heat-shock-responsive reporter gene, was exposed to laser light
under a variety of illumination conditions. It was found that
gene expression was most often induced by light of 760 nm, and
least by 810 nm. The stress response increased with laser power
and irradiation time. At 810 nm, significant gene expression
could be observed at 360 mW of illumination, which is more than
one order of magnitude above that normally used in optical
tweezers. In the 700-760-nm range, the results show that the
stress response is caused by photochemical processes, whereas at
810 nm, it mainly has a photothermal origin. The authors suggest
these results give further evidence that the 700-760-nm
wavelength region is unsuitable for optical tweezers and suggest
that work at 810 nm at normal laser powers does not cause stress
at the cellular level. G. Leitz et al: Biophys J 2002 82:2224.
Related Background Brief:
USING OPTICS TO MEASURE BIOLOGICAL FORCES AND MECHANICS.
Spanning all size levels, regulating biological forces and
transport are fundamental life processes. Used by various
investigators over the last dozen years, optical techniques
offer unique advantages for studying biological forces. The most
mature of these techniques, optical tweezers, or the single-beam
optical trap, is commercially available and is used by numerous
investigators. Although technical innovations have improved the
versatility of optical tweezers, simple optical tweezers
continue to provide insights into cell biology. Two new,
promising optical technologies, laser-tracking microrheology and
the optical stretcher, allow mechanical measurements that are
not possible with optical tweezers. The author reviews these
various optical technologies and their roles in understanding
mechanical forces in cell biology. S.C. Kuo: Traffic 2001 2:757.
Related Background Brief:
MICROMANIPULATION OF STATOLITHS IN GRAVITY-SENSING CHARA
RHIZOIDS BY OPTICAL TWEEZERS. The authors report that infrared
laser traps (optical tweezers) were used to micromanipulate
statoliths in gravity-sensing rhizoids of the green alga Chara
vulgaris Vail. The authors report they were able to hold and
move statoliths with high accuracy and to observe directly the
effects of statolith position on cell growth in horizontally
positioned rhizoids. The first step in gravitropism, namely the
physical action of gravity on statoliths, can be simulated by
optical tweezers. The direct laser microirradiation of the
rhizoid apex did not cause any visible damage to the cells.
Through lateral positioning of statoliths a differential growth
of the opposite flank of the cell wall could be induced,
corresponding to bending growth in gravitropism... G. Leitz et
al: Planta 1995 197:278.
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