Distance dependence of electron transfer across peptides with different secondary structures: the role of Peptide energetics and electronic coupling.

The charge-transfer transition energies and the electronic-coupling matrix element, |HDA|, for electron transfer from aminopyridine (ap) to the 4-carbonyl-2,2‘-bipyridine (cbpy) in cbpy-(gly)n-ap (gly = glycine, n = 0−6) molecules were calculated using the Zerner's INDO/S, together with the Cave and Newton methods. The oligopeptide linkages used were those of the idealized protein secondary structures, the α-helix, 310-helix, β-strand, and polyproline I- and II-helices. The charge-transfer transition energies are influenced by the magnitude and direction of the dipole generated by the peptide secondary structure. The electronic coupling |HDA| between (cbpy) and (ap) is also dependent on the nature of the secondary structure of the peptide. A plot of 2·ln|HDA| versus the charge-transfer distance (assumed to be the dipole moment change between the ground state and the charge-transfer states) showed that the polyproline II structure is a more efficient bridge for long-distance electron-transfer reactions (β ...