PEPTIDE–WATER ASSOCIATION IN PEPTIDE CRYSTALS

Abstract

The structures of 37 peptide crystals, containing 78 water-peptide hydrogen bonds and 77 other hydrogen bonds involving water, were surveyed to identify the geometry of peptide backbone hydration. In the sample, hydration of peptide carbonyl occurred more frequently than hydration of peptide N–H. The most probable value of the C'=0. O water angle was near 138°, considerably greater than the 120° to the axis of a lone electron pair on the carbonyl oxygen. Associated water oxygens tended to be in the plane of the peptide bond, but not restricted to it. Cyclic association with water, to form 10-membered rings using N_i–H and C_{i + 1} =0 atoms, was common in glycine-containing cyclic hexapeptides. The distribution of angles between two hydrogen bonds at a single water molecule, as defined by the three nonhydrogen atoms involved, was centered near the tetrahedral angle.