Electrostatic and hydrophobic models of solvation

Department of Biochemistry and Molecular Biophysics, Columbia University, 630 West 168th St., New York, NY 10032, USA.

Abstract

Recent progress on the use of continuum electrostatics to treat solute-solvent interactions will be discussed. The new developments are in the spirit of traditional dielectric models in that the solute, be it an ion, small molecule, or protein, is treated explicitely while the solvent is treated as a dielectric continuum. However, in contrast to earlier work, numerical methods now make it possible to describe the solute in atomic detail and, consequently, to calculate solvation energies as well as charge-charge interactions. Molecular polarizabilities can also be incorporated conveniently into the treatment. Comparisons with experiment and with the results of microscopic simulations suggest that the continuum treatment offers comparable accuracy with orders of magnitude less computer time. This has opened up the possibility of a simple, fast and reliable treatment of solvent effects on pKs, partition coefficients, conformational energies and binding energies.

The hydrophobic effect is probably the major driving force in biological systems. The magnitude of the hydrophobic effect as measured from the surface area dependence of the solubilities of hydrocarbons in water is generally though to be about 25 ca/mole.A2. However, surface tension at a hydrocarbon / water interface, which is a macroscopic measure of the hydrophobic effect, is about 72 cal/mole.A2. However, in evaluating solubility data, concentrations are usually evaluated in mole fraction units as suggested in most text books. This standard prescription does not account for size difference between molecules. A theory which accounts for size yields estimates for the microscopic hydrophobic effect of 47 cal/A2, about twice the magnitude usually assumed. Given this value, a simple model which accounts for curvature now makes it possible to reconcile the magnitudes of the macroscopic and microscopic effects. The new microscopic value successfully accounts for the effects of hydrophobic mutations on protein stability. Moreover, curvature effects suggest that concave enzyme binding sites and the surface of macromolecules can have hydrophobicities that exceed the macroscopic value.