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

We report on the vertical electronic excitation spectra of neutral polypeptides and their radical ions. Model systems including tri-, tetra-. and octamers in an alpha -helix and beta -sheet alike conformation are studied by various computational methods. The results allow us to assign the band centered 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 electron transfer in polypeptide cations.