Studies on intermediate water structure and application of Ab initio SCF calculation for hydration

Abstract

Part A Intermediate water structures in a aqueous solution of $N^{-\alpha}$-acetyl-N-methylphenylalaninamide, Phe $(amid)_2$, were investigated by the energy minimization method. To determined the configurations of water molecules around Phe $(amide)_2$, the spherical interaction energy map was introduced, and this method was effective to solve the multi-dimensional minimization problem. The results show that ninetyeight water molecules hydrate Phe$(amide)_2$ and several kinds of cyclic structures are observed. The water molecules show the two kins of characteristic water, one of which is the shell water and the other is bulk one. The water molecules in the shell are tightly bound to the hydrophilic groups in Phe$(amide)_2$, and those in the bulk form the clathrate-lie structures. The average binding energy of the shell water is -22.86 kcal/mole and that of bulk one is -17.36 kcal/mole. The distribution of water molecules around Phe$(amide)_2$ agrees with the results of molecular dynamics and Monte Carlo simulation studies. The distribution of the cyclic structures shows that the six-membered ones are distributed mainly at the region between the first and second hydration shell. Part B The determination of analytic intermolecular potential function from ab initio SCF calculation is described and applied to methane + water, methanol + water, acetic acid + water, adn acetate anion + water systems. The derived potential functions are the inverse powers forms of interatomic distances. The comparison of the calculations of the potential functions with ab initio SCF ones shows the good agreement, although the forms of the potential functions are very simple. Isoenergy contour maps of orientationally optimized geometries are reported and discussed, where the energy surfaces are varied depend on the size of the basis sets. Also, the various optimized structures and dimerization energies of solute + water systems are examined. For methanol + water, methanol is a proton ...