Impact of Backbone Pattern and Residue Substitution on Helicity in alpha/beta/gamma-Peptides

We have evaluated the impact of changes in the chemical structure of peptidic oligomers containing alpha-, beta-, and gamma-amino acid residues (alpha/beta/gamma-peptides) on the propensities of these oligomers to adopt helical conformations in aqueous and alcoholic solutions. These studies were inspired by our previous discovery that alpha/beta/gamma-peptides containing a regular alpha gamma alpha alpha beta alpha hexad repeat adopt an alpha-helix-like conformation in which the beta and gamma residues are aligned in a stripe along one side, and the remainder of the helix surface is defined by the alpha residues. This helix was found to be most stable when the beta and gamma residues were rigidified with specific cyclic constraints. Relaxation of the beta residue constraints caused profound conformational destabilization, but relaxation of the gamma residue constraints led to only a moderate drop in helicity. The new work more broadly characterizes the effect of gamma residue substitution on helix stability, based on circular dichroism and two-dimensional NMR measurements. We find that even a fully unsubstituted gamma residue (derived from gamma-aminobutyric acid) supports a moderate helical propensity, which is surprising in light of the strong destabilizing effect of glycine residues on alpha-helix stability. Additional studies examine the effects of altering sequence in terms of amino acid type, by comparing a prototype with the alpha gamma alpha alpha beta alpha hexad pattern to isomers with irregular arrangements of the alpha, beta, and gamma residues along the backbone. The data indicate that the strong helix-forming propensity previously discovered for alpha/beta/gamma-peptide 12-mers is retained when sequence is varied, with small variations detected across diverse alpha-beta-gamma placements. These structural findings suggest that alpha/beta/gamma-peptide scaffolds represent versatile scaffolds for the design of peptidic foldamers that display specific functions.