Conformation dependence of the DαDα stretch mode in peptides: Side-chain influence in dipeptide structures.

We have shown by theoretical studies of alanine peptides that the C(α)D(α) stretch frequency could be particularly useful for determining peptide structure because of its sensitivity to the ϕ,ψ torsion angles at the C(α) atom. To demonstrate that this is a robust methodology worthy of experimental exploration, we have also shown that this mode is even more determinative of conformation in aqueous solution, mainly as a result of the development of differential C(α)--D(α)···O(water) interactions. As further assurance, we now determine the influence of the side chain on this band, showing for aliphatic, a polar, and an aromatic side chains that the dependence is minor and explaining why this is also expected for other side chains. These results should stimulate new experimental methodologies in the field of peptide structure determination.