The β-turn scaffold of tripeptide containing an azaphenylalanine residue Original Research Article

The conformational preferences of azaphenylalanine-containing peptide were investigated using a model compound, Ac-azaPhe-NHMe with ab initio method at the HF/3-21G and HF/6-31G⁎ levels, and the seven minimum energy conformations with trans orientation of acetyl group and the 4 minimum energy conformations with cis orientation of acetyl group were found at the HF/6-31G⁎ level if their mirror images were not considered. An average backbone dihedral angle of the 11 minimum energy conformations is ϕ = ± 91° ± 24°, ψ = ± 18° ± 10° (or ± 169° ± 8°), corresponding to the i + 2 position of β-turn (δR) or polyproline II (βP) structure, respectively. The χ1 angle in the aromatic side chain of azaPhe residue adopts preferentially between ± 60° and ± 130°, which reflect a steric hindrance between the N-terminal carbonyl group or the C-terminal amide group and the aromatic side chain with respect to the configuration of the acetyl group. These conformational preferences of Ac-azaPhe-NHMe predicted theoretically were compared with those of For-Phe-NHMe to characterize the structural role of azaPhe residue.