Structural studies on C-amidated amino acids and peptides: structures of hydrochloride salts of C-amidated Ile, Val, Thr, Ser, Met, Trp, Gln and Arg, and comparison with their C-unamidated counterparts.

To elucidate the structural features of amino acids caused by the C-terminal alpha-amidation, the crystal structures of HCl salts of C-terminal amidated Ile, Val, Thr, Ser, Met, Trp, Gln and Arg were analysed and compared with those of their C-terminal free acids. The bonding parameter of the amide group was little affected by the different chemical properties of the side chains. As for the molecular packing patterns, some structural differences were observed by the C-amidation. The Calpha-H...O hydrogen bonds and carbonyl-carbonyl interactions were more strengthened by the salt formation with HCl in C-amides than C-acids. Furthermore, there is a clear difference between the interaction patterns with Cl ions. In most C-amide crystals, Cl ions are bifurcately hydrogen-bonded to two neighbouring amide NH(2) groups and the parallel layers of the C-amides and Cl ions are alternatively formed. In the case of the carboxyl OH in C-acid crystals, however, the direct hydrogen bond with the Cl ion is not always observed and is largely dependent on the crystal packing environment. This suggests the superior hydrogen-bonding ability of NH...Cl(-) compared with OH...Cl(-). The difference in hydrogen-bonding ability between the amide and carboxyl groups is considered, based on the spatial dispositions of the hydrogen-bonding polar atoms/groups.