Abstract: | The synthesis, physical and analytical characterization, and crystal-state structural analysis by X-ray diffraction of three analogues of the Nα-acylated tripeptide amide tail of oxytocin, each containing a cyclic Cα, α- disubstituted glycine at position 2, have been performed. The peptides arc Boc-L-Pro-Ac3c-Gly-NH2, Z-L-Pro-Ac5c-Gly-NH2 and Z-L-Pro-Ac5c-Gly-NH2. While the former is folded in a type-II β-turn conformation at the -L-Pro-Ac3c- sequence, the two latter tripeptides form two consecutive (type-II, type-I′) β-turns. The Ac5c- and Ac6c-tripeptides are the first examples of such a highly folded structural combination in a position-2 analogue of the Nα-acylated -L-Pro-L-Leu-GIy-NH2 sequence. |