Crystallographic characterization of the α,γ C12 helix in hybrid peptide sequences

The solid-state conformations of two αγ hybrid peptides Boc-[Aib-γ(4) (R)Ile]4 -OMe 1 and Boc-[Aib-γ(4) (R)Ile]5 -OMe 2 are described. Peptides 1 and 2 adopt C12 -helical conformations in crystals. The structure of octapeptide 1 is stabilized by six intramolecular 4 → 1 hydrogen bonds, forming 12 atom C12 motifs. The structure of peptide 2 reveals the formation of eight successive C12 hydrogen-bonded turns. Average backbone dihedral angles for αγ C12 helices are peptide 1, Aib; φ (°) = -57.2 ± 0.8, ψ (°) = -44.5 ± 4.7; γ(4) (R)Ile; φ (°) = -127.3 ± 7.3, θ1 (°) = 58.5 ± 12.1, θ2 (°) = 67.6 ± 10.1, ψ (°) = -126.2 ± 16.1; peptide 2, Aib; φ (°) = -58.8 ± 5.1, ψ (°) = -40.3 ± 5.5; ψ(4) (R)Ile; φ (°) = -123.9 ± 2.7, θ1 (°) = 53.3 θ 4.9, θ 2 (°) = 61.2 ± 1.6, ψ (°) = -121.8 ± 5.1. The tendency of γ(4) -substituted residues to adopt gauche-gauche conformations about the C(α) -C(β) and C(β) -C(γ) bonds facilitates helical folding. The αγ C12 helix is a backbone expanded analog of α peptide 310 helix. The hydrogen bond parameters for α peptide 310 and α-helices are compared with those for αγ hybrid C12 helix. Copyright © 2016 European Peptide Society and John Wiley & Sons.