Abstract
An X-ray crystallographic analysis was carried out for Boc-(Aib-ΔZPhe)2-Aib-OMe 1 and Boc-L-Pro-(Aib-ΔZPhe)2-Aib-OMe 2 (Aib = α-aminoisobutyric acid; ΔZPhe = α,β-dehydrophenylalanine; Boc = tert-butoxycarbonyl; OMe = methoxy) to provide detailed conformational data for oligopeptides possessing an -(Aib-ΔZPhe)2- segment. Both peptides adopted a typical 310-helical conformation characterized by = 52.8°, = 29.3°, and = −173.8° for the average values of the four residues of the -(Aib-ΔZPhe)2- segment in peptide 1, and = 54°, = 27°, and = −175° for those in peptide 2. The preference for a 310-helix in the -(Aib-ΔZPhe)2- segment is ascribed to the presence of Aib and ΔZPhe residues being strong inducers for the formation of a 310-helix. In peptide 2, the N-terminal L-Pro residue adopted a semiextended conformation, leading to a left-handed screw sense for the following achiral segment. This result was also supported by conformational energy calculation, in which the L-Pro residue leading to a left-handed 310-helical segment prefers a semiextended conformation rather than a right-handed helical conformation.
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