C-H…O Hydrogen Bond Mediated Chain Reversal in a Peptide Containing a gamma-Amino Acid Residue, Determined Directly from Powder X-ray Diffraction Data

Abstract

The finding that peptides containing beta-amino acid residues give rise to folding patterns hitherto unobserved in alpha-amino acid peptides[1] has stimulated considerable interest in the conformational properties of peptides built from beta, gamma, and delta residues,[2] as the introduction of additional methylene (CH2) units into peptide chains provides further degrees of conformational freedom. Studies of the influence of introducing omega-amino acids into regular polypeptide structures derived from alpha residues have demonstrated that extra methylene groups can be inserted into helical backbones and into the strand and turn segments of beta hairpins.[3] In regard to the influence of CH2 group insertion into the i +2 position of isolated peptide beta turns, we are investigating the conformational properties of a series of model sequences Piv-Pro-xxx-NHMe (defined in Scheme 1 and reference [4])