Intermolecular Interaction Effects in the Amide I Vibrations of β Polypeptides

Abstract

Previous perturbation treatments of the Amide I vibrations of β polypeptides are inconsistent with a detailed normal coordinate analysis of crystalline polyglycine I. This analysis indicates that the D ₁₀ interaction constant is essentially zero, rather than the large value (about 20 cm^-1) required by the earlier application of the perturbation theory. It is suggested that the previously neglected D ₁₁ term should be included in the perturbation expression, and it is shown that the physical origin of such a term can be accounted for by transition dipole coupling. This mechanism is shown to give a reasonable explanation of splittings of the C=O stretching vibrations in hydrogenbonded carboxylic acid dimers. Its application to β polypeptides provides a satisfactory interpretation of splittings in the Amide I modes.

• vibrational spectra
• perturbation treatment
• transition dipole coupling
• conformational analysis