2,195 research outputs found
Solvent coarse-graining and the string method applied to the hydrophobic collapse of a hydrated chain
Using computer simulations of over 100,000 atoms, the mechanism for the
hydrophobic collapse of an idealized hydrated chain is obtained. This is done
by coarse-graining the atomistic water molecule positions over 129,000
collective variables that represent the water density field and then using the
string method in these variables to compute the minimum free energy pathway
(MFEP) for the collapsing chain. The dynamical relevance of the MFEP (i.e. its
coincidence with the mechanism of collapse) is validated a posteriori using
conventional molecular dynamics trajectories. Analysis of the MFEP provides
atomistic confirmation for the mechanism of hydrophobic collapse proposed by
ten Wolde and Chandler. In particular, it is shown that lengthscale-dependent
hydrophobic dewetting is the rate-limiting step in the hydrophobic collapse of
the considered chain.Comment: 11 pages, 7 figures, including supporting informatio
Symmetry effects and equivalences in lattice models of hydrophobic interaction
We establish the equivalence of a recently introduced discrete model of the
hydrophobic interaction, as well as its extension to continuous state
variables, with the Ising model in a magnetic field with temperature-dependent
strength. In order to capture the effect of symmetries of the solvent particles
we introduce a generalized multi-state model. We solve this model - which is
not of the Ising type - exactly in one dimension. Our findings suggest that a
small increase in symmetry decreases the amplitude of the solvent-mediated part
of the potential of mean force between solute particles and enhances the
solubility in a very simple fashion. High symmetry decreases also the range of
the attractive potential. This weakening of the hydrophobic effect observed in
the model is in agreement with the notion that the effect is entropic in
origin.Comment: 19 pages, 2 figure
Warm and Cold Denaturation in the Phase Diagram of a Protein Lattice Model
Studying the properties of the solvent around proteins, we propose a much
more sophisticated model of solvation than temperature-independent pairwise
interactions between monomers, as is used commonly in lattice representations.
We applied our model of solvation to a 16-monomer chain constrained on a
two-dimensional lattice. We compute a phase diagram function of the temperature
and a solvent parameter which is related to the pH of the solution. It exhibits
a native state in which the chain coalesces into a unique compact conformation
as well as a denatured state. Under certain solvation conditions, both warm and
cold denaturations occur between the native and the denatured states. A good
agreement is found with the data obtained from calorimetric experiments,
thereby validating the proposed model.Comment: 7 pages, 2 figure
The superheated Melting of Grain Boundary
Based on a model of the melting of Grain Boundary (GB), we discuss the
possibility of the existence of superheated GB state. A Molecular Dynamics
simulation presented here shows that the superheated GB state can realized in
the high symmetric tilt GB. Whether the sizes of liquid nuclei exceed a
critical size determined the superheating grain boundary melting or not. Our
results also indicate that the increase of melting point due to pressure is
smaller than the superheating due to nucleation mechanism.Comment: Accepted by PRB, 7 pages and 5 figure
Elasticity and metastability limit in supercooled liquids: a lattice model
We present Monte Carlo simulations on a lattice system that displays a first
order phase transition between a disordered phase (liquid) and an ordered phase
(crystal). The model is augmented by an interaction that simulates the effect
of elasticity in continuum models. The temperature range of stability of the
liquid phase is strongly increased in the presence of the elastic interaction.
We discuss the consequences of this result for the existence of a kinetic
spinodal in real systems.Comment: 8 pages, 5 figure
Kinetic glass behavior in a diffusive model
Three properties of the Edwards-Anderson model with mobile bonds are
investigated which are characteristic of kinetic glasses. First is two-time
relaxation in aged systems, where a significant difference is observed between
spin and bond autocorrelation functions. The spin subsystem does not show
two-time behavior, and the relaxation is stretched exponential. The bond
subsystem shows two-time behavior, with the first relaxation nearly exponential
and the second similar to the spin one. Second is the two-temperature behavior,
which can be tuned by bond dilution through the full range reported in the
literature. Third is the rigid-to-floppy transition, identified as a function
of bond dilution. Simple Glauber Monte Carlo evolution without extraneous
constraints reproduces the behavior of classical kinetic simulations, with the
bond (spin) degree of freedom corresponding to configurational (orientational)
disorder.Comment: 4 pages, 3 figures, minimal corrections, to appear in Phys. Rev. B
(RC
Are Protein Folds Atypical?
Protein structures are a very special class among all possible structures. It
was suggested that a ``designability principle'' plays a crucial role in
nature's selection of protein sequences and structures. Here we provide a
theoretical base for such a selection principle, using a novel formulation of
the protein folding problem based on hydrophobic interactions. A structure is
reduced to a string of 0's and 1's which represent the surface and core sites,
respectively, as the backbone is traced. Each structure is therefore associated
with one point in a high dimensional space. Sequences are represented by
strings of their hydrophobicities and thus can be mapped into the same space. A
sequence which lies closer to a particular structure in this space than to any
other structures will have that structure as its ground state. Atypical
structures, namely those far away from other structures in the high dimensional
space, have more sequences which fold into them, and are thermodynamically more
stable. We argue that the most common folds of proteins are the most atypical
in the space of possible structures.Comment: 15 pages, 5 figure
Inquiries into the Nature of Free Energy and Entropy in Respect to Biochemical Thermodynamics
Free energy and entropy are examined in detail from the standpoint of
classical thermodynamics. The approach is logically based on the fact that
thermodynamic work is mediated by thermal energy through the tendency for
nonthermal energy to convert spontaneously into thermal energy and for thermal
energy to distribute spontaneously and uniformly within the accessible space.
The fact that free energy is a Second-Law, expendable energy that makes it
possible for thermodynamic work to be done at finite rates is emphasized.
Entropy, as originally defined, is pointed out to be the capacity factor for
thermal energy that is hidden with respect to temperature; it serves to
evaluate the practical quality of thermal energy and to account for changes in
the amounts of latent thermal energies in systems maintained at constant
temperature. A major objective was to clarify the means by which free energy is
transferred and conserved in sequences of biological reactions coupled by
freely diffusible intermediates. In achieving this objective it was found
necessary to distinguish between a 'characteristic free energy' possessed by
all First-Law energies in amounts equivalent to the amounts of the energies
themselves and a 'free energy of concentration' that is intrinsically
mechanical and relatively elusive in that it can appear to be free of First-Law
energy. The findings in this regard serve to clarify the fact that the transfer
of chemical potential energy from one repository to another along sequences of
biological reactions of the above sort occurs through transfer of the First-Law
energy as thermal energy and transfer of the Second-Law energy as free energy
of concentration.Comment: 18-page PDF; major correction in APPENDIX; minor corrections
elsewher
Dynamical heterogeneity in a glass forming ideal gas
We conduct a numerical study of the dynamical behavior of a system of
three-dimensional crosses, particles that consist of three mutually
perpendicular line segments rigidly joined at their midpoints. In an earlier
study [W. van Ketel et al., Phys. Rev. Lett. 94, 135703 (2005)] we showed that
this model has the structural properties of an ideal gas, yet the dynamical
properties of a strong glass former. In the present paper we report an
extensive study of the dynamical heterogeneities that appear in this system in
the regime where glassy behavior sets in. On the one hand, we find that the
propensity of a particle to diffuse is determined by the structure of its local
environment. The local density around mobile particles is significantly less
than the average density, but there is little clustering of mobile particles,
and the clusters observed tend to be small. On the other hand, dynamical
susceptibility results indicate that a large dynamical length scale develops
even at moderate densities. This suggests that propensity and other mobility
measures are an incomplete measure of dynamical length scales in this system.Comment: 11 pages, 7 figure
- …