Helical structures of l-Leu-based peptides having chiral six-membered ring amino acids

Abstract

l-Leu-based peptides having chiral six-membered ring amino acids: Boc-{l-Leu-l-Leu-[(1R,3R)- or (1S,3R)-Ac6c3M]}n-OMe [n=1 (2), 2 (4), 3 (5)], were synthesized. A conformational analysis using FTIR absorption and NOESY NMR spectra revealed that these hexa- and nonapeptides having (1R,3R)-Ac6c3M or (1S,3R)-Ac6c3M formed helical structures in CDCl3 solution, while that using CD spectra suggested that the nonapeptide 5b having (1S,3R)-Ac6c3M become more right-handed (P) α-helical than the nonapeptide 5a having (1R,3R)-Ac6c3M in TFE solution. An X-ray crystallographic analysis showed that the hexapeptide 4b having (1S,3R)-Ac6c3M formed a partially distorted right-handed (P) 310-/α-helical structure in the crystal state, while the nonapeptides 5a and 5b having (1R,3R)-Ac6c3M or (1S,3R)-Ac6c3M formed similar right-handed (P) α-helical backbone structures.