Vibronic Exciton Approach to NV1 Absorption Bands of Polypeptides. I. α-Helix

From bond-order arguments it is found that the length of the CN bond in the peptide residue increases appreciably upon the NV 1 transition. This effect on the NV 1 absorption band of α-helical polypeptides has been examined theoretically on the basis of the approximation proposed first by Briggs and Herzenberg. The exciton splitting is found to be enhanced markedly. In the second calculation the effect of the hypochromism also has been taken into account by assuming the simplest of possible mechanisms of the hypochromism. Agreement of the obtained absorption spectrum with experiment is excellent. All the necessary parameters have been evaluated semi-empirically.