An analysis of the amide I mode in the vibrational spectra of unordered polypeptides and proteins

The difference in the observed frequency of the amide I mode between the Raman and ir spectra of many unordered polypeptides is interpreted in terms of a model in which sequences of amide groups have similar ϕ, ψ angles. The splittings due to intramolecular interactions for the infinite helices generated by all ϕ, ψ angles are calculated by assuming transition dipole coupling. The effect of finite sequence length and distortions are then considered. This hypothesis accounts for the general features of the amide I mode of unordered polypeptides.