Intermolecular Interaction Effects in the Amide I Vibrations of β Polypeptides

Previous perturbation treatments of the Amide I vibrations of beta 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 beta polypeptides provides a satisfactory interpretation of splittings in the Amide I modes.