Role of azaamino acid residue in β-turn formation and stability in designed peptide

The structural perturbation induced by C(α)H→N(α) exchange in azaamino acid-containing peptides was predicted by ab initio calculation of the 6-31G* and 3-21G* levels. The global energy-minimum conformations for model compounds, For-azaXaa-NH₂ (Xaa=Gly, Ala, Leu) appeared to be the β- turn motif with a dihedral angle of Φ = ±90°, ψ =0°. This suggests that incorporation of the azaXaa residue into the i+2 position of designed peptides could stabilize the β-turn structure. The model azaLeu-containing peptide, Boc-Phe-azaLeu-Ala-OMe, which is predicted to adopt a β-turn conformation was designed and synthesized in order to experimentally elucidate the role of the azaamino acid residue. Its structural preference in organic solvents was investigated using ¹H NMR, molecular modelling and IR spectroscopy. The temperature coefficients of amide protons, the characteristic NOE patterns, the restrained molecular dynamics simulation and, IR spectroscopy defined the dihedral angles [ (Φi+1, ψi+1) (Φi+2, ψi+2)] of the Phe-azaLeu fragment in the model peptide, Boc-Phe-azaLeu-Ala- OMe, as [(-59°, 127°) (107°, -4°)]. This solution conformation supports a βII-turn structural preference in azaLeu-containing peptides as predicted by the quantum chemical calculation. Therefore, intercalation of the azaamino acid residue into the i+2 position in synthetic peptides is expected to provide a stable β-turn formation, and this could be utilized in the design of new peptidomimetics adopting a β-turn scaffold.