51 research outputs found
Distribution functions of a heteronuclear hard dumbbell at a hard wall. Pure fluid and mixtures with hard spheres
Structure of ternary additive hard-sphere fluid mixtures
Monte Carlo simulations on the structural properties of ternary fluid
mixtures of additive hard spheres are reported. The results are compared with
those obtained from a recent analytical approximation [S. B. Yuste, A. Santos,
and M. Lopez de Haro, J. Chem. Phys. 108, 3683 (1998)] to the radial
distribution functions of hard-sphere mixtures and with the results derived
from the solution of the Ornstein-Zernike integral equation with both the
Martynov-Sarkisov and the Percus-Yevick closures. Very good agreement between
the results of the first two approaches and simulation is observed, with a
noticeable improvement over the Percus-Yevick predictions especially near
contact.Comment: 11 pages, including 8 figures; A minor change; accepted for
publication in PR
The polydisperse cell model: Non-linear screening and charge renormalization in colloidal mixtures
We propose a model for the calculation of renormalized charges and osmotic
properties of mixtures of highly charged colloidal particles. The model is a
generalization of the cell model and the notion of charge renormalization as
introduced by Alexander and his collaborators (J. Chem. Phys. 80, 5776 (1984)).
The total solution is partitioned into as many different cells as components in
the mixture. The radii of these cells are determined self-consistently for a
given set of parameters from the solution of the non-linear Poisson-Boltzmann
equation with appropriate boundary conditions. This generalizes Alexanders's
model where the (unique) Wigner-Seitz cell radius is fixed solely by the
colloids packing fraction. We illustrate the technique by considering a binary
mixture of colloids with the same sign of charge. The present model can be used
to calculate thermodynamic properties of highly charged colloidal mixtures at
the level of linear theories, while taking the effect of non-linear screening
into account
A cluster theory for a Janus fluid
Recent Monte Carlo simulations on the Kern and Frenkel model of a Janus fluid
have revealed that in the vapour phase there is the formation of preferred
clusters made up of a well-defined number of particles: the micelles and the
vesicles. A cluster theory is developed to approximate the exact clustering
properties stemming from the simulations. It is shown that the theory is able
to reproduce the micellisation phenomenon.Comment: 27 pages, 8 figures, 6 table
Entropic Tension in Crowded Membranes
Unlike their model membrane counterparts, biological membranes are richly
decorated with a heterogeneous assembly of membrane proteins. These proteins
are so tightly packed that their excluded area interactions can alter the free
energy landscape controlling the conformational transitions suffered by such
proteins. For membrane channels, this effect can alter the critical membrane
tension at which they undergo a transition from a closed to an open state, and
therefore influence protein function \emph{in vivo}. Despite their obvious
importance, crowding phenomena in membranes are much less well studied than in
the cytoplasm.
Using statistical mechanics results for hard disk liquids, we show that
crowding induces an entropic tension in the membrane, which influences
transitions that alter the projected area and circumference of a membrane
protein. As a specific case study in this effect, we consider the impact of
crowding on the gating properties of bacterial mechanosensitive membrane
channels, which are thought to confer osmoprotection when these cells are
subjected to osmotic shock. We find that crowding can alter the gating energies
by more than in physiological conditions, a substantial fraction of
the total gating energies in some cases.
Given the ubiquity of membrane crowding, the nonspecific nature of excluded
volume interactions, and the fact that the function of many membrane proteins
involve significant conformational changes, this specific case study highlights
a general aspect in the function of membrane proteins.Comment: 20 pages (inclduing supporting information), 4 figures, to appear in
PLoS Comp. Bio
Modeling the high-pressure behavior of binary mixtures of carbon dioxide+alkanols using an excess free energy mixing rule
A Didactic Model of Macromolecular Crowding Effects on Protein Folding
A didactic model is presented to illustrate how the effect of macromolecular crowding on protein folding and association is modeled using current analytical theory and discrete molecular dynamics. While analytical treatments of crowding may consider the effect as a potential of average force acting to compress a polypeptide chain into a compact state, the use of simulations enables the presence of crowding reagents to be treated explicitly. Using an analytically solvable toy model for protein folding, an approximate statistical thermodynamic method is directly compared to simulation in order to gauge the effectiveness of current analytical crowding descriptions. Both methodologies are in quantitative agreement under most conditions, indication that both current theory and simulation methods are capable of recapitulating aspects of protein folding even by utilizing a simplistic protein model
An estimate of limiting values of relative volatility with help of the theorem of corresponding states. II. Binary systems of tetrachloromethane, benzene, and cyclohexane
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