Theoretical Study of the Electronic Spectroscopy of Peptides. III. Charge-Transfer Transitions in Polypeptides

The valence excited singlet states of several model polypeptides have been studied using the complete active space (CAS) SCF and multiconfigurational second-order perturbation (CASPT2) methods. It has been found that polypeptides, in addition to the well-known n → π* (W) and π → π* (NV1) intrapeptide excitations near 220 nm (5.6 eV) and 190 nm (6.5 eV), respectively, have a characteristic band at 165−170 nm (7.5−7.3 eV) corresponding to charge transfers between neighboring peptide units. The intensity of these transitions depends on the conformation of the polypeptide. The present results are consistent with experimental information.