Effect of Differential Geminal Substitution of γAmino Acid Residues at the (i + 2) Position of αγTurn Segments on the Conformation of Template β-Hairpin Peptides

Abstract

The effect of insertion of three geminally dimethyl substituted γamino acid residues [γ2,2 (4-amino-2,2-dimethylbutanoic acid), γ3,3 (4-amino-3,3-dimethylbutanoic acid), and γ4,4 (4-amino-4,4-dimethylbutanoic acid)] at the (i + 2) position of a two-residue αγC12 turn segment in a model octapeptide sequence Leu-Phe-Val-Aib-Xxx-Leu-Phe-Val (where Xxx = γamino acid residues) has been investigated in this study. Solution conformational studies (NMR, CD, and IR) and ab initio calculations indicated that γ3,3 and γ4,4 residues were well accommodated in the β-hairpin nucleating αγC12 turns, which gave rise to well-registered hairpins, in contrast to γ2,2, which was unable to form a tight C12 β-hairpin nucleating turn and promote a well-registered β-hairpin. Geminal disubstitution at the Cα carbon in γ2,2 led to unfavorable steric contacts, disabling its accommodation in the αγC12 hairpin nucleating turn unlike the γ3,3 and γ4,4 residues. Geminal substitutions at different carbons along the backbone constrained backbone torsion angles for the three γamino acid residues differently, generating diverse conformational preferences in them. Folded hairpins were energetically more stable (∼8 to 9 kcal/mol) than the unfolded peptides. Conformational preference of the peptides was independent of the N-terminal protecting group. Such fundamental understanding will instrumentalize the future directed design of foldamers.