Theoretical study of long range electron transfer in Phthalimide-Peptide-Methyl Aminoacetate Model molecules

Long-range electron transfer (ET) matrix elements (VPS), rate constants (kET) and reorganization energies for ET from phthalimide radical (pha) moiety to methyl aminoacetate radical (aa) moiety in pa-(gly)n = 0-6-aa (pa = C6H4(CO)2N-(CH2CO), gly = glycine, aa = HNCH2COOCH3) ionic molecules have been investigated using two-state variational method (TSVM) and classical rate model. Calculations on VPS reveal that the overlap between the frontier orbitals of two diabatic states is quite small, which leads to a small value of VPS. kET has a minimum at the range n = 1-3 for β-strand conformation, but linearly increases as the peptide chain length (n) increases for pro II-helix conformation. These results are in good agreement with the experimental predictions. Relevant ET mechanisms are elucidated. The transition energies for charge transfer in such systems are also calculated to test the influences of local dipoles on the potentials of the donor and acceptor. For comparison electron couplings in [pa-(gly)n = 1,3-aa]+ cations are calculated and the effects of electron correlation on inner reorganization energies in pha + pha−/+ self-exchange reactions are examined at different levels of theory respectively. Calculated results are discussed also.