5,901 research outputs found
Modeling of solvent flow effects in enzyme catalysis under physiological conditions
A stochastic model for the dynamics of enzymatic catalysis in explicit,
effective solvents under physiological conditions is presented.
Analytically-computed first passage time densities of a diffusing particle in a
spherical shell with absorbing boundaries are combined with densities obtained
from explicit simulation to obtain the overall probability density for the
total reaction cycle time of the enzymatic system. The method is used to
investigate the catalytic transfer of a phosphoryl group in a phosphoglycerate
kinase-ADP-bis phosphoglycerate system, one of the steps of glycolysis. The
direct simulation of the enzyme-substrate binding and reaction is carried out
using an elastic network model for the protein, and the solvent motions are
described by multiparticle collision dynamics, which incorporates hydrodynamic
flow effects. Systems where solvent-enzyme coupling occurs through explicit
intermolecular interactions, as well as systems where this coupling is taken
into account by including the protein and substrate in the multiparticle
collision step, are investigated and compared with simulations where
hydrodynamic coupling is absent. It is demonstrated that the flow of solvent
particles around the enzyme facilitates the large-scale hinge motion of the
enzyme with bound substrates, and has a significant impact on the shape of the
probability densities and average time scales of substrate binding for
substrates near the enzyme, the closure of the enzyme after binding, and the
overall time of completion of the cycle.Comment: 15 pages in double column forma
The statistics of particle velocities in dense granular flows
We present measurements of the particle velocity distribution in the flow of
granular material through vertical channels. Our study is confined to dense,
slow flows where the material shears like a fluid only in thin layers adjacent
to the walls, while a large core moves without continuous deformation, like a
solid. We find the velocity distribution to be non-Gaussian, anisotropic, and
to follow a power law at large velocities. Remarkably, the distribution is
identical in the fluid-like and solid-like regions. The velocity variance is
maximum at the core, defying predictions of hydrodynamic theories. We show
evidence of spatially correlated motion, and propose a mechanism for the
generation of fluctuational motion in the absence of shear.Comment: Submitted to Phys. Rev. Let
Breakdown of weak-field magnetotransport at a metallic quantum critical point
We show how the collapse of an energy scale in a quantum critical metal can
lead to physics beyond the weak-field limit usually used to compute transport
quantities. For a density-wave transition we show that the presence of a finite
magnetic field at the critical point leads to discontinuities in the transport
coefficients as temperature tends to zero. The origin of these discontinuities
lies in the breakdown of the weak field Jones-Zener expansion which has
previously been used to argue that magneto-transport coefficients are
continuous at simple quantum critical points. The presence of potential
scattering and magnetic breakdown rounds the discontinuities over a window
determined by tau Delta < 1 where Delta is the order parameter and tau is the
quasiparticle elastic lifetime.Comment: 4 pages, 3 figures RevTeX forma
Multiple-Point and Multiple-Time Correlations Functions in a Hard-Sphere Fluid
A recent mode coupling theory of higher-order correlation functions is tested
on a simple hard-sphere fluid system at intermediate densities. Multi-point and
multi-time correlation functions of the densities of conserved variables are
calculated in the hydrodynamic limit and compared to results obtained from
event-based molecular dynamics simulations. It is demonstrated that the mode
coupling theory results are in excellent agreement with the simulation results
provided that dissipative couplings are included in the vertices appearing in
the theory. In contrast, simplified mode coupling theories in which the
densities obey Gaussian statistics neglect important contributions to both the
multi-point and multi-time correlation functions on all time scales.Comment: Second one in a sequence of two (in the first, the formalism was
developed). 12 pages REVTeX. 5 figures (eps). Submitted to Phys.Rev.
Mode-coupling theory for multiple-time correlation functions of tagged particle densities and dynamical filters designed for glassy systems
The theoretical framework for higher-order correlation functions involving
multiple times and multiple points in a classical, many-body system developed
by Van Zon and Schofield [Phys. Rev. E 65, 011106 (2002)] is extended here to
include tagged particle densities. Such densities have found an intriguing
application as proposed measures of dynamical heterogeneities in structural
glasses. The theoretical formalism is based upon projection operator techniques
which are used to isolate the slow time evolution of dynamical variables by
expanding the slowly-evolving component of arbitrary variables in an infinite
basis composed of the products of slow variables of the system. The resulting
formally exact mode-coupling expressions for multiple-point and multiple-time
correlation functions are made tractable by applying the so-called N-ordering
method. This theory is used to derive for moderate densities the leading mode
coupling expressions for indicators of relaxation type and domain relaxation,
which use dynamical filters that lead to multiple-time correlations of a tagged
particle density. The mode coupling expressions for higher order correlation
functions are also succesfully tested against simulations of a hard sphere
fluid at relatively low density.Comment: 15 pages, 2 figure
Drying of complex suspensions
We investigate the 3D structure and drying dynamics of complex mixtures of
emulsion droplets and colloidal particles, using confocal microscopy. Air
invades and rapidly collapses large emulsion droplets, forcing their contents
into the surrounding porous particle pack at a rate proportional to the square
of the droplet radius. By contrast, small droplets do not collapse, but remain
intact and are merely deformed. A simple model coupling the Laplace pressure to
Darcy's law correctly estimates both the threshold radius separating these two
behaviors, and the rate of large-droplet evacuation. Finally, we use these
systems to make novel hierarchical structures.Comment: 4 pages, 4 figure
Mutation analysis of HIF prolyl hydroxylases (PHD/EGLN) in individuals with features of phaeochromocytoma and renal cell carcinoma susceptibility
Germline mutations in the von Hippel–Lindau disease (VHL) and succinate dehydrogenase subunit B (SDHB) genes can cause inherited phaeochromocytoma and/or renal cell carcinoma(RCC). Dysregulation of the hypoxia-inducible factor (HIF) transcription factors has been linked to VHL and SDHB-related RCC; both HIF dysregulation and disordered function of a prolyl hydroxylase domain isoform 3 (PHD3/EGLN3)-related pathway of neuronal apoptosis have been linked to the development of phaeochromocytoma. The 2-oxoglutarate-dependent prolyl hydroxylase enzymes PHD1 (EGLN2), PHD2 (EGLN1) and PHD3 (EGLN3) have a key role in regulating the stability of HIF-a subunits (and hence expression of the HIF-a transcription factors). A germline PHD2 mutation has been reported in association with congenital erythrocytosis and recurrent extra-adrenal phaeochromocytoma. We undertook mutation analysis of PHD1, PHD2 and PHD3 in two cohorts of patients with features of inherited phaeochromocytoma (nZ82) and inherited RCC (nZ64) and no evidence of germline mutations in known susceptibility genes. No confirmed pathogenic mutations were detected suggesting that mutations in these genes are not a frequent cause of inherited phaeochromocytoma or RCC
Quantum free energy differences from non-equilibrium path integrals: I. Methods and numerical application
The imaginary-time path integral representation of the canonical partition
function of a quantum system and non-equilibrium work fluctuation relations are
combined to yield methods for computing free energy differences in quantum
systems using non-equilibrium processes. The path integral representation is
isomorphic to the configurational partition function of a classical field
theory, to which a natural but fictitious Hamiltonian dynamics is associated.
It is shown that if this system is prepared in an equilibrium state, after
which a control parameter in the fictitious Hamiltonian is changed in a finite
time, then formally the Jarzynski non-equilibrium work relation and the Crooks
fluctuation relation are shown to hold, where work is defined as the change in
the energy as given by the fictitious Hamiltonian. Since the energy diverges
for the classical field theory in canonical equilibrium, two regularization
methods are introduced which limit the number of degrees of freedom to be
finite. The numerical applicability of the methods is demonstrated for a
quartic double-well potential with varying asymmetry. A general parameter-free
smoothing procedure for the work distribution functions is useful in this
context.Comment: 20 pages, 4 figures. Added clarifying remarks and fixed typo
Self-consistent Overhauser model for the pair distribution function of an electron gas at finite temperature
We present calculations of the spin-averaged pair distribution function
in a homogeneous gas of electrons moving in dimensionality D=3 or D=2 at
finite temperature. The model involves the solution of a two-electron
scattering problem via an effective potential which embodies many-body effects
through a self-consistent Hartree approximation, leading to two-body wave
functions to be averaged over a temperature-dependent distribution of relative
momentum for electron pairs. We report illustrative numerical results for
in an intermediate-coupling regime and interpret them in terms of
changes of short-range order with increasing temperature.Comment: 6 pages, 5 figures, submitted to Solid State Communication
Thermal tides in the Martian middle atmosphere as seen by the Mars Climate Sounder
The first systematic observations of the middle atmosphere of Mars (35–80km) with the Mars Climate Sounder (MCS) show dramatic patterns of diurnal thermal variation, evident in retrievals of temperature and water ice opacity. At the time of writing, the data set of MCS limb retrievals is sufficient for spectral analysis within a limited range of latitudes and seasons. This analysis shows that these thermal variations are almost exclusively associated with a diurnal thermal tide. Using a Martian general circulation model to extend our analysis, we show that the diurnal thermal tide dominates these patterns for all latitudes and all seasons
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