CALCULATION OF SOLVATION FREE-ENERGY USING RISM THEORY FOR PEPTIDE INSALT SOLUTION

We developed a robust, highly efficient algorithm for solving the full reference interaction site model (RISM) equations for salt solutions near a solute molecule with many atomic sites. It was obtained as an e xtension of our previously reported algorithm for pure water near the solute molecule. The algorithm is a judicious hybrid of the Newton-Rap hson and Picard methods. The most striking advantage is that the Jacob ian matrix is just part of the input data and need not be recalculated at all. To illustrate the algorithm, we solved the full RISM equation s for a dipeptide (NH2-CHCH3-CONH-CHCH3-COOH) in a 1 M NaCl solution. The extended simple point charge (SPC/E) model was employed for water molecules. Two different conformations of the dipeptide were considere d. It was assumed for each conformation that the dipeptide was present either as an un-ionized form or as a zwitterion. The structure of the salt solution near the dipeptide and salt effects on the solvation fr ee energy were also discussed. (C) 1998 John Wiley & Sons, Inc.