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An x-ray crystallographic analysis was carried out for Boc-(Aib-DeltaZPhe)4-Aib-OMe (1: Boc = t-butoxycarbonyl; Aib = alpha-aminoisobutyric acid; DeltaZPhe = Z-alpha,beta-didehydrophenylalanine) to provide the precise conformational parameters of the octapeptide segment -(Aib-DeltaZPhe)4-. Peptide 1 adopted a typical 3(10)-helical conformation characterized by = +/-55.8 degrees (50 degrees -65 degrees), = +/-26.7 degrees (15 degrees -45 degrees), and = +/-179.5 degrees (168 degrees -188 degrees) for the average values of the -(Aib-DeltaZPhe)4- segment (the range of the eight values). The 3(10)-helix contains 3.1 residues per turn, being close to the "perfect 3(10)-helix" characterized by 3.0 residues per turn. NMR and Fourier transform infrared (FTIR) spectroscopy revealed that the 3(10)-helical conformation at the atomic resolution is essentially maintained in solution. Energy minimization of peptide 1 by semiempirical molecular orbital calculation converged to a 3(10)-helical conformation similar to the x-ray crystallographic 3(10)-helix. The preference for a 3(10)-helix in the -(Aib-DeltaZPhe)4- segment is ascribed to strong inducers of the 3(10)-helix inherent in Aib and DeltaZPhe residues-in particular, the Aib residues tend to stabilize a 3(10)-helix more effectively. Therefore, the -(Aib-DeltaZPhe)4- segment is useful to rationally design an optically inactive 3(10)-helical backbone, which will be of great importance to provide novel insights into noncovalent and covalent chiral interactions of a helical peptide with a chiral molecule.

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An X-ray crystallographic analysis was carried out for Boc-(Aib-∆ Z Phe) 2 -Aib-OMe 1 and Boc--Pro-(Aib-∆ Z Phe) 2 -Aib-OMe 2 (Aib = α-aminoisobutyric acid; ∆ Z Phe = α,β-dehydrophenylalanine; Boc = tert-butoxycarbonyl; OMe = methoxy) to provide detailed conformational data for oligopeptides possessing an -(Aib-∆ Z Phe) 2 -segment. Both peptides adopted a typical 3 10 -helical conformation characterized by <φ> = 52.8Њ, <ψ> = 29.3Њ, and <ω> = Ϫ173.8Њ for the average values of the four residues of the -(Aib-∆ Z Phe) 2 -segment in peptide 1, and <φ> = 54Њ, <ψ> = 27Њ, and <ω> = Ϫ175Њ for those in peptide 2. The preference for a 3 10 -helix in the -(Aib-∆ Z Phe) 2 -segment is ascribed to the presence of Aib and ∆ Z Phe residues being strong inducers for the formation of a 3 10 -helix. In peptide 2, the Nterminal -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 -Pro residue leading to a left-handed 3 10 -helical segment prefers a semiextended conformation rather than a right-handed helical conformation.