C-H center dot center dot center dot O hydrogen bond mediated chain reversal in a peptide containing a gamma-amino acid residue, determined directly from powder X-ray diffraction data

摘要

The finding that peptides containing -amino acid residuesudgive rise to folding patterns hitherto unobserved in -aminoudacid peptides[1] has stimulated considerable interest in theudconformational properties of peptides built from , , andud residues,[2] as the introduction of additional methyleneud(CH2) units into peptide chains provides further degrees ofudconformational freedom. Studies of the influence of introducing-amino acids into regular polypeptide structures derived from residues have demonstrated that extra methylene groups can be inserted into helical backbones and into the strand and turn segments of hairpins.[3] In regard to the influence of CH2 group insertion into the i2 position of isolated peptide turns, we are investigating the conformational properties of a series of model sequences Piv-Pro-Xxx- NHMe (defined in Scheme 1 and reference [4]).ud