The concept of CH...O hydrogen bonds has recently gained much interest, with a number of reports indicating the significance of these non-classical hydrogen bonds in stabilizing nucleic acid and protein structures. Here, we analyze the CH...O interactions in the protein-DNA interface, based on 43 crystal structures of protein-DNA complexes. Surprisingly, we find that the number of close intermolecular CH...O contacts involving the thymine methyl group and position C5 of cytosine is comparable to the number of protein-DNA hydrogen bonds involving nitrogen and oxygen atoms as donors and acceptors. A comprehensive analysis of the geometries of these close contacts shows that they are similar to other CH...O interactions found in proteins and small molecules, as well as to classical NH...O hydrogen bonds. Thus, we suggest that C5 of cytosine and C5-Met of thymine form relatively weak CH...O hydrogen bonds with Asp, Asn, Glu, Gln, Ser, and Thr, contributing to the specificity of recognition. Including these interactions, in addition to the classical protein-DNA hydrogen bonds, enables the extraction of simple structural principles for amino acid-base recognition consistent with electrostatic considerations.
Copyright 1998 Academic Press Limited.