1,299 research outputs found

    Scaled-Particle Theory and the Length-scales Involved in Hydrophobic Hydration of Aqueous Biomolecular Assemblies

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    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

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    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

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    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

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    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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