Structural Studies on C-Amidated Amino Acids and Peptides: Function of Amide Group in Molecular Association in Crystal Structures of Val–Gly–NH2, Ser–Phe–NH2, Gly–Tyr–NH2 and Pro–Tyr–NH2 Hydrochloride Salts

As part of a series of elucidation of the structural features of peptides caused by C-terminal α-amidation, the crystal structures of H–Val–Gly–NH2, H–Ser–Phe–NH2, H–Gly–Tyr–NH2, and H–Pro–Tyr–NH2 hydrochloride salts were analyzed by the X-ray diffraction method. Although respective molecules take energetically allowable torsion angles concerning the backbone and side chains, their conformations are not necessarily the same as the corresponding unamidated ones. This results from the different molecular packing requirements, rather than from different conformational features inherent in the C-amidated and -unamidated peptides. As for the molecular packing feature, each peptide tended to form a repeated structure through those hydrogen bonds in which both amide NH and O=C groups participate. The chloride ions are located between the neighboring peptides and are hydrogen-bonded to the respective amide NHs, leading to the sheet structure. The hydrogen-bonding feature of the amide group and its function in molecular packing was discussed based on the results analyzed so far.