Vibronic Exciton Approach to NV1 Absorption Bands of Polypeptides. II. β-Form

The NV 1 absorption band of polypeptides in the β-form is discussed on the basis of vibronic exciton treatment. The adopted model is different from the one of free-exciton theory merely in that the freedoms of the CN stretching vibration in individual residues are taken into account explicitly. The calculated absorption bands for the antiparallel and the parallel sheets are compared with the experimental data of poly-L-lysine in the β-form observed by Rosenheck et al . It is concluded, in agreement with Pysh, that the experimental observations correspond to the antiparallel sheet. Numerical values of transition monopoles, bond lengths and force constants of the CN bond are all assumed to be the same as those of the isolated peptide group.