The occurrence of C-H...O hydrogen bonds in proteins

Hydrogen bonds are a major feature of protein structure. By a generally accepted definition, they occur whenever a proton is shared by two electronegative atoms. Hence, only hydrogens bonded to nitrogen and oxygen atoms are usually considered in analyses of protein hydrogen bond networks. However, X-ray and neutron diffraction studies have shown that crystals of various organic compounds exhibit close C-H...X contacts (where X is an electronegative atom, in most cases oxygen) which show all the stereochemical hallmarks of hydrogen bonds. In this work, we describe an analysis of short C-H...O interactions in a sample of known protein structures representing different categories of tertiary folds and refined at a resolution of at least 2 A. Although our analysis is based on the calculated coordinates of hydrogen atoms, its results are statistically significant: we find strong evidence that a large percentage of short C...O contacts constitute cohesive interactions. Moreover, the stereochemical study of C-H...O = C contacts, in which the orientation of free electron orbitals on the acceptor oxygen atom can be predicted, reveals that these interactions exhibit stereochemical features typical of hydrogen bonds. Among the hydrogen atoms involved in these contacts, the most common are those bonded to alpha carbon. This is consistent with the fact that these hydrogens are more acidic than others. We describe four different categories of C-H...O = C bonds. Those found between C alpha-H groups and main chain oxygens in adjacent strands of beta sheets are the most ubiquitous. Our results call for a revision of crystallographic restrained refinement programs which treat close carbon-oxygen contacts as purely repulsive; they may also have implications for the understanding of some enzymatic reaction mechanisms.