Crystallographic evidence for C alpha-H...O=C hydrogen bonds in a collagen triple helix

The crystal structure of the collagen triple-helical peptide (Pro-Hyp-Gly)4-Pro-Hyp-Ala-(Pro-Hyp-Gly)5 shows evidence for the existence of interchain contacts between alpha-carbon hydrogens from Gly and Hyp residues, and carbonyl groups from Gly and Pro residues on neighboring chains. The geometrical disposition of these contacts makes it reasonable to describe them as C alpha-H...O=C hydrogen bonds. Two repetitive patterns can be identified, and one of them is identical to a similar type of interaction reported recently for beta-sheets in globular proteins, which suggests a more universal character for C-H...O hydrogen bonds in building protein secondary structure elements. They are presumably much weaker in energy than the interchain N-H...O=C hydrogen bonds responsible for the alignment of the three chains in the collagen triple helix, and therefore their contribution will be a small but cooperative decrease on the total interchain hydrogen bonding energy.