1,299 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
Synopsis of the Executive Profile of Environmental Management: Mesoamerican Subregion
This executive profile summarized here identifies the great environmental challenges of the Mesoamerican region, highlights the achievements of the last decade, and points out the future course that will guide the region's environmental management advancements. There are three main issues addressed here: natural resources management, environmental impact of urban and industrial development and the relationship between the environment and competitiveness. This document was presented at the Environment Network of the Regional Policy Dialogue's 1st Hemispheric Meeting: Towards an Effective Environmental Management held on April 4th and 5th, 2002.Environment & Natural Resources :: Environmental Policy, Environment & Natural Resources :: Biodiversity, Environment & Natural Resources :: Natural Resources Management, Environmental Management
Ion Pair Potentials-of-Mean-Force in Water
Recent molecular simulation and integral equation results alkali-halide ion
pair potentials-of-mean-force in water are discussed. Dielectric model
calculations are implemented to check that these models produce that
characteristic structure of contact and solvent-separated minima for oppositely
charged ions in water under physiological thermodynamic conditions. Comparison
of the dielectric model results with the most current molecular level
information indicates that the dielectric model does not, however, provide an
accurate description of these potentials-of-mean-force. We note that linear
dielectric models correspond to modelistic implementations of second-order
thermodynamic perturbation theory for the excess chemical potential of a
distinguished solute molecule. Therefore, the molecular theory corresponding to
the dielectric models is second-order thermodynamic perturbation theory for
that excess chemical potential. The second-order, or fluctuation, term raises a
technical computational issue of treatment of long-ranged interactions similar
to the one which arises in calculation of the dielectric constant of the
solvent. It is contended that the most important step for further development
of dielectric models would be a separate assessment of the first-order
perturbative term (equivalently the {\it potential at zero charge} ) which
vanishes in the dielectric models but is generally nonzero. Parameterization of
radii and molecular volumes should then be based of the second-order
perturbative term alone. Illustrative initial calculations are presented and
discussed.Comment: 37 pages and 8 figures. LA-UR-93-420
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