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 multiconfig urational second-order perturbation (CASPT2) methods. It has been foun d that polypeptides, in addition to the well-known n --> pi (W) and n --> pi (NV1) intrapeptide excitations near 220 nm (5.6 eV) and 190 n m (6.5 eV), respectively, have a characteristic band at 165-170 nm (7. 5-7.3 eV) corresponding to charge transfers between neighboring peptid e units. The intensity of these transitions depends on the conformatio n of the polypeptide. The present results are consistent with experime ntal information.