Solvent interactions with pi ring systems in proteins.

The interaction of water molecules with apolar amino acids is an important aspect of the hydrophobic effect and hence of protein folding. Our distributed multiple electrostatic model for water interacting with phenylalanine dipeptides shows that minimum energy sites exist above the aromatic ring such that a solvent molecule can interact with the pi electrons, but only when this site is not blocked by main-chain atoms or disturbed by main-chain polar atoms. This is consistent with the experimental evidence of others that water can hydrogen bond to aromatic pi electrons. In contrast, our analysis of solvent interactions with phenylalanine residues based on 48 high-resolution, well-refined protein structures shows that the dominant interaction of solvent molecules is with the edge of the ring and not with the pi elections. As the faces of phenylalanine rings tend to be buried, and solvent interactions with neighbouring polar atoms are more favourable, the interaction of water molecules with the faces of aromatic pi rings appears not to occur frequently in proteins.