Charge transfer transitions in neutral and ionic polypeptides: A theoretical study

We report on the vertical electronic excitation spectra of neutral polypept ides and their radical ions. Model systems including tri-, tetra-. and octa mers in an alpha -helix and beta -sheet alike conformation are studied by v arious computational methods. The results allow us to assign the band cente red at about 7.5 eV in the absoption spectra of proteins to charge transfer states. The calculations also support conclusions from recent experiments which propose charge transfer as a possible mechanism for photoinduced elec tron transfer in polypeptide cations.