1,663 research outputs found
Minimal Bending Energies of Bilayer Polyhedra
Motivated by recent experiments on bilayer polyhedra composed of amphiphilic
molecules, we study the elastic bending energies of bilayer vesicles forming
polyhedral shapes. Allowing for segregation of excess amphiphiles along the
ridges of polyhedra, we find that bilayer polyhedra can indeed have lower
bending energies than spherical bilayer vesicles. However, our analysis also
implies that, contrary to what has been suggested on the basis of experiments,
the snub dodecahedron, rather than the icosahedron, generally represents the
energetically favorable shape of bilayer polyhedra
Non-local fluctuation correlations in active gels
Many active materials and biological systems are driven far from equilibrium
by embedded agents that spontaneously generate forces and distort the
surrounding material. Probing and characterizing these athermal fluctuations is
essential for understanding the properties and behaviors of such systems. Here
we present a mathematical procedure to estimate the local action of
force-generating agents from the observed fluctuating displacement fields. The
active agents are modeled as oriented force dipoles or isotropic compression
foci, and the matrix on which they act is assumed to be either a compressible
elastic continuum or a coupled network-solvent system. Correlations at a single
point and between points separated by an arbitrary distance are obtained,
giving a total of three independent fluctuation modes that can be tested with
microrheology experiments. Since oriented dipoles and isotropic compression
foci give different contributions to these fluctuation modes, ratiometric
analysis allows us characterize the force generators. We also predict and
experimentally find a high-frequency ballistic regime, arising from individual
force generating events in the form of the slow build-up of stress followed by
rapid but finite decay. Finally, we provide a quantitative statistical model to
estimate the mean filament tension from these athermal fluctuations, which
leads to stiffening of active networks.Comment: 12 pages, 7 figures; some clarifications and ammended figure
notation
INCORPORATION OF QUANTUM STATISTICAL FEATURES IN MOLECULAR DYNAMICS
We formulate a method for incorporating quantum fluctuations into molecular-
dynamics simulations of many-body systems, such as those employed for energetic
nuclear collision processes. Based on Fermi's Golden Rule, we allow spontaneous
transitions to occur between the wave packets which are not energy eigenstates.
The ensuing diffusive evolution in the space of the wave packet parameters
exhibits appealing physical properties, including relaxation towards quantum-
statistical equilibrium.Comment: 8 latex pages + 1 uuencoded ps figur
Soft swimming: Exploiting deformable interfaces for low-Reynolds number locomotion
Reciprocal movement cannot be used for locomotion at low-Reynolds number in
an infinite fluid or near a rigid surface. Here we show that this limitation is
relaxed for a body performing reciprocal motions near a deformable interface.
Using physical arguments and scaling relationships, we show that the
nonlinearities arising from reciprocal flow-induced interfacial deformation
rectify the periodic motion of the swimmer, leading to locomotion. Such a
strategy can be used to move toward, away from, and parallel to any deformable
interface as long as the length scales involved are smaller than intrinsic
scales, which we identify. A macro-scale experiment of flapping motion near a
free surface illustrates this new result
Direct Electrochemistry of Endonuclease III in the Presence and Absence of DNA
The electrochemistry of the base excision repair enzyme Endonuclease III (Endo III) in the presence and absence of DNA has been examined on highly oriented pyrolytic graphite (HOPG). At the surface modified with pyrenated DNA, a reversible signal is observed at 20 mV versus NHE for the [4Fe−4S]^(3+/2+) couple of Endo III, similar to Au. Without DNA modification, oxidative and reductive signals for the [4Fe−4S] cluster of Endo III are found on bare HOPG, allowing a direct comparison between DNA-bound and free redox potentials. These data indicate a shift of approximately −200 mV in the 3+/2+ couple upon binding of Endo III to DNA. This potential shift reflects a difference in affinity for DNA of more than 3 orders of magnitude between the oxidized 3+ and reduced 2+ protein and provides quantitative support for our model utilizing DNA-mediated charge transport to redistribute base excision repair enzymes in the vicinity of damaged DNA
Imaging of Sources in Heavy-Ion Reactions
Imaging of sources from data within the intensity interferometry is
discussed. In the two-pion case, the relative pion source function may be
determined through the Fourier transformation of the correlation function. In
the proton-proton case, the discretized source function may be fitted to the
correlation data.Comment: 12 pages, 3 postscript figures, accepted Physics Letters
Effect of symmetry energy on two-nucleon correlation functions in heavy-ion collisions induced by neutron-rich nuclei
Using an isospin-dependent transport model, we study the effects of nuclear
symmetry energy on two-nucleon correlation functions in heavy ion collisions
induced by neutron-rich nuclei. We find that the density dependence of the
nuclear symmetry energy affects significantly the nucleon emission times in
these collisions, leading to larger values of two-nucleon correlation functions
for a symmetry energy that has a stronger density dependence. Two-nucleon
correlation functions are thus useful tools for extracting information about
the nuclear symmetry energy from heavy ion collisions.Comment: Revised version, to appear in Phys. Rev. Let
Intermittency and Exotic Channels
It is pointed out that accurate measurements of short-range two-particle
correlations in like-charge and in channels should be
very helpful in determining the origin of the \lq\lq intermittency\rq\rq\
phenomenon observed recently for the like-charge pion pairs.Comment: 5 p., plain tex, preprint T94/078(Saclay), LPTHE 94/58(Orsay
Protein-DNA charge transport: Redox activation of a DNA repair protein by guanine radical
DNA charge transport (CT) chemistry provides a route to carry out oxidative DNA damage from a distance in a reaction that is sensitive to DNA mismatches and lesions. Here, DNA-mediated CT also leads to oxidation of a DNA-bound base excision repair enzyme, MutY. DNA-bound Ru(III), generated through a flash/quench technique, is found to promote oxidation of the [4Fe-4S](2+) cluster of MutY to [4Fe-4S](3+) and its decomposition product [3Fe-4S](1+). Flash/quench experiments monitored by EPR spectroscopy reveal spectra with g = 2.08, 2.06, and 2.02, characteristic of the oxidized clusters. Transient absorption spectra of poly(dGC) and [Ru(phen)(2)dppz](3+) (dppz = dipyridophenazine), generated in situ, show an absorption characteristic of the guanine radical that is depleted in the presence of MutY with formation instead of a long-lived species with an absorption at 405 nm; we attribute this absorption also to formation of the oxidized [4Fe-4S](3+) and [3Fe4S](1+) clusters. In ruthenium-tethered DNA assemblies, oxidative damage to the 5'-G of a 5'-GG-3' doublet is generated from a distance but this irreversible damage is inhibited by MutY and instead EPR experiments reveal cluster oxidation. With ruthenium-tethered assemblies containing duplex versus single-stranded regions, MutY oxidation is found to be mediated by the DNA duplex, with guanine radical as an intermediate oxidant; guanine radical formation facilitates MutY oxidation. A model is proposed for the redox activation of DNA repair proteins through DNA CT, with guanine radicals, the first product under oxidative stress, in oxidizing the DNA-bound repair proteins, providing the signal to stimulate DNA repair
Ductus arteriosus aneurysm presenting as pulmonary artery obstruction: Diagnosis and management
The occurrence of pulmonary artery obstruction in an 8 day old infant as a complication of an aneurysm of a nonpatent ductus arteriosus is reported, together with the echocardiographic and angiographic findings. To relieve the obstruction, the aneurysm and an intrapulmonary thrombus were successfully removed with the use of cardiopulmonary bypass when the infant was 3 months old
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