Solid-state conformations of oligopeptides possessing an -(Aib-Delta(Z)Phe)(2)- segment

An X-ray crystallographic analysis was carried out for Boc-(Aib-Delta (Z)Ph e)(2)-Aib-OMe 1 and Boc-L-Pro-(Aib-Delta (Z)Phe)(2)-Aib-OMe 2 (Aib=alpha -a minoisobutyric acid; Delta (Z)Phe=alpha,beta -dehydrophenylalanine; Boc=ter t-butoxycarbonyl; OMe=methoxy) to provide detailed conformational data for oligopeptides possessing an -(Aib-Delta (Z)Phe)(2)- segment. Both peptides adopted a typical 3(10)-helical conformation characterized by < phi > =52.8 degrees, < psi > =29.3 degrees, and < omega >=-173.8 degrees for the avera ge values of the four residues of the -(Aib-Delta (Z)Phe)(2)-segment in pep tide 1, and < phi > =54 degrees, < psi > =27 degrees, and < omega >=-175 de grees for those in peptide 2. The preference for a 3(10)-helix in the -(Aib -Delta (Z)Phe)(2)- segment is ascribed to the presence of Aib and Delta (Z) Phe residues being strong inducers for the formation of a 3(10)-helix. In p eptide 2, the N-terminal L-Pro residue adopted a semiextended conformation, leading to a left-handed screw sense for the following achiral segment. Th is result was also supported by conformational energy calculation, in which the L-Pro residue leading to a left-handed 3(10)-helical segment prefers a semiextended conformation rather than a right-handed helical conformation.