Crystal structure of achiral nonapeptide Boc–(Aib–Δ z Phe) 4 –Aib–OMe at atomic resolution: Evidence for a 3 10 ‐helix

An x‐ray crystallographic analysis was carried out for Boc–(Aib–Δ Z Phe) 4 –Aib–OMe ( 1 : Boc = t ‐butoxycarbonyl; Aib = α‐aminoisobutyric acid; Δ Z Phe = Z ‐α,β‐didehydrophenylalanine) to provide the precise conformational parameters of the octapeptide segment –(Aib–Δ Z Phe) 4 –. Peptide 1 adopted a typical 3 10 ‐helical conformation characterized by 〈ϕ〉 = ±55.8° (50°–65°), 〈ψ〉 = ±26.7° (15°–45°), and 〈ω〉 = ±179.5° (168°–188°) for the average values of the –(Aib–Δ Z Phe) 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–Δ Z Phe) 4 – segment is ascribed to strong inducers of the 3 10 ‐helix inherent in Aib and Δ Z Phe residues—in particular, the Aib residues tend to stabilize a 3 10 ‐helix more effectively. Therefore, the –(Aib–Δ Z Phe) 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. © 2003 Wiley Periodicals, Inc. Biopolymers 70: 310–322, 2003