699 research outputs found
Scaled-Particle Theory and the Length-scales Involved in Hydrophobic Hydration of Aqueous Biomolecular Assemblies
Hydrophobic hydration plays a crucial role in self-assembly processes over
multiple length-scales, but the extrapolation of molecular-scale models to
larger length-scale hydration phenomena is sometimes not warranted.
Scaled-particle theories are based upon an interpolative view of that issue. We
revisit the scaled-particle theory proposed thirty years ago by Stillinger,
adopt a practical generalization, and consider the implications for hydrophobic
hydration in light of our current understanding. The generalization is based
upon identifying a molecular length, implicit in previous applications of
scaled-particle models, that provides an effective radius for joining
microscopic and macroscopic descriptions. We demonstrate that the generalized
theory correctly reproduces many of the anomalous thermodynamic properties of
hydrophobic hydration for molecularly sized solutes, including solubility
minima and entropy convergence, successfully interpolates between the
microscopic and macroscopic extremes, and provides new insights into the
underlying molecular mechanisms. The results are discussed in terms of
length-scales associated with component phenomena; in particular we first
discuss the micro-macroscopic joining radius identified by the theory, then we
discuss in turn the Tolman length that leads to an analogous length describing
curvature corrections of a surface area model of hydrophobic hydration free
energies, and the length-scales on which entropy convergence of hydration free
energies are expected.Comment: 19 pages, 14 figures, one figure added, submitted to Rev. Mod. Phy
Quasi-Chemical Theory and Implicit Solvent Models for Simulations
A statistical thermodynamic development is given of a new implicit solvent
model that avoids the traditional system size limitations of computer
simulation of macromolecular solutions with periodic boundary conditions. This
implicit solvent model is based upon the quasi-chemical approach, distinct from
the common integral equation trunk of the theory of liquid solutions. The
physical content of this theory is the hypothesis that a small set of solvent
molecules are decisive for these solvation problems. A detailed derivation of
the quasi-chemical theory escorts the development of this proposal. The
numerical application of the quasi-chemical treatment to Li ion hydration
in liquid water is used to motivate and exemplify the quasi-chemical theory.
Those results underscore the fact that the quasi-chemical approach refines the
path for utilization of ion-water cluster results for the statistical
thermodynamics of solutions.Comment: 30 pages, contribution to Santa Fe Workshop on Treatment of
Electrostatic Interactions in Computer Simulation of Condensed Medi
Ion Sizes and Finite-Size Corrections for Ionic-Solvation Free Energies
Free energies of ionic solvation calculated from computer simulations exhibit
a strong system size dependence. We perform a finite-size analysis based on a
dielectric-continuum model with periodic boundary conditions. That analysis
results in an estimate of the Born ion size. Remarkably, the finite-size
correction applies to systems with only eight water molecules hydrating a
sodium ion and results in an estimate of the Born radius of sodium that agrees
with the experimental value.Comment: 2 EPS figure
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