Water-tryptophan interactions: lone-pair---pi or O-H---pi? Molecular dynamics simulations of beta-galactosidase suggest that both modes can co-exist.

In proteins, the indole side chain of tryptophan can interact with water molecules either in-plane, forming hydrogen bonds, or out-of-plane, with the water molecule contacting the aromatic pi face. The latter interaction can be either of the lone pair...pi (lp...pi) type or corresponds to the O-H...pi binding mode, an ambiguity that X-ray structures usually do not resolve. Here, we report molecular dynamics (MD) simulations of a solvated beta-galactosidase monomer, which illustrate how a water molecule located at the p face of an indole side chain of tryptophan can adapt to the position of proximate residues and "select" its binding mode. In one such site, the water molecule is predicted to rapidly oscillate between the O-H...pi and lp...pi binding modes, thus gaining entropic advantage. Our MD simulations provide support for the role of lp...pi interactions in the stabilization of protein structures.