Solid-state conformations of oligopeptides possessing an -(Aib-ΔZPhe)2- segment

Abstract

An X-ray crystallographic analysis was carried out for Boc-(Aib-Δ^ZPhe)₂-Aib-OMe 1 and Boc-L-Pro-(Aib-Δ^ZPhe)₂-Aib-OMe 2 (Aib = α-aminoisobutyric acid; Δ^ZPhe = α,β-dehydrophenylalanine; Boc = tert-butoxycarbonyl; OMe = methoxy) to provide detailed conformational data for oligopeptides possessing an -(Aib-Δ^ZPhe)₂- segment. Both peptides adopted a typical 3₁₀-helical conformation characterized by <ϕ> = 52.8°, <ψ> = 29.3°, and <ω> = −173.8° for the average values of the four residues of the -(Aib-Δ^ZPhe)₂- segment in peptide 1, and <ϕ> = 54°, <ψ> = 27°, and <ω> = −175° for those in peptide 2. The preference for a 3₁₀-helix in the -(Aib-Δ^ZPhe)₂- segment is ascribed to the presence of Aib and Δ^ZPhe 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.