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.