Solvent-induced forces between two hydrophilic groups

Molecular dynamics simulations were used to calculate the force between two simple hydrophilic solutes in dilute aqueous solution. The "solutes" were two water molecules in the same relative orientation as the next-nearest neighbors in hexagonal ice I. Both the direct and solvent-induced contributions to the force were calculated as a function of separation distance. The total force between the solutes was found to be most attractive at 5.0 Å (-1.6 kcal/mol/Å). The potential of mean force had a minimum at 4.3 Å, which is 0.2 Å closer than the next-nearest-neighbor distance in ice. A parallel set of simulations were conducted with the partial charges on the "solutes" removed to examine hydrophobic analogs. In this case, the total force was most attractive at 3.5 Å (-0.9 kcal/mol/Å), and the minimum of the potential was at the contact distance of 3.2 Å. In agreement with earlier predictions, the maximum solvent-induced contribution to the potential was ca. 4 times more negative for the hydrophilic "solutes" than for the hydrophobic ones. These differences are shown to be due predominantly to a solvent water molecule which simultaneously hydrogen bonds to both hydrophilic "solutes". The results support earlier assertions that solvent-induced interactions between polar amino acid residues are more important in protein folding and stability than generally considered.