HYDRATION PHENOMENA, CLASSICAL ELECTROSTATICS, AND THE BOUNDARY ELEMENT METHOD

A formalism for a computational treatment of the polarization of a solvent containing an electrolyte and of polar solutes immersed in it is presented. The solvent is modeled as a continuum dielectric. Polarization effects are represented by a polarization charge density at the dielectric boundaries, by induced dipoles at the polarizable atoms of the solute, and by the Boltzmann distribution of ions around the solute. All these are found with a combination of the boundary element and finite element methods as a numerical solution of a system of algebraic equations. Applications of this formalism to systems of different complexity and detail of description are presented. Excellent agreement is found between the calculated and experimental hydration enthalpies of a variety of polar and charged molecules. Experimental values of the solution dipole moment and of the hydration enthalpy of the polarizable molecule of water are well reproduced in calculations starting with point charges fitted to its vacuum dipole moment. Interionic potentials of mean force for alkali-metal halide ion pairs with similar positions and magnitudes of two minima and the barrier between them are found in our calculations and in calculations based on the microscopic representation of the solvent. pK differences within classes of structurally similar compounds can be predicted with an accuracy of one pK unit. Our method that does not account for a significant part of the solvation entropy generally leads to the results similar to the results of the molecular free energy perturbation methods in applications where entropy changes are not expected to be very large. Ways to incorporate full hydration entropy changes into our method are discussed. Calculations performed for the C-peptide of the ribonuclease suggest that the main-chain electrostatics contributes about half of the total energy driving the helix-coil transition and that interactions of charged groups with the main chain of the helix can be dominated by local effects, and not by the "helix dipole". Calculations of the ionic atmosphere around rodlike polyelectrolytes with helical distribution of charge reveal dramatic variations in the potentials and the distribution of counterions and may allow evaluation of the role of hydration and the counterions in stabilization of different forms of DNA. © 1990 American Chemical Society.