The strength with which a peptide group can form a hydrogen bond varies with the internal conformation of the polypeptide chain

The strength of the H-bond formed between a dipeptide and a proton acceptor molecule is assessed by correlated ab initio quantum calculations for a broad range of different conformations of the dipeptide. The H-bond energy is very sensitive to the internal (phi, Psi) angles, even when the geometry of the H-bond does not vary significantly from one conformation to another. This result indicates that the peptide NH is a much less potent proton donor in certain conformations than in others. In particular, extended conformations of a polypeptide are capable of only weak H-bonds. Thus, the interstrand NH center dot center dot center dot O H-bonds in parallel and antiparallel beta-sheets are expected to be significantly weaker than those found in other conformations, such as helices, ribbons, and beta-bends, even if the H-bond geometries are similar.