Distance dependence of electron transfer across peptides with different secondary structures: The role of peptide energetics and electronic coupling

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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·In|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 = 0.7 Å-1) than the other secondary structures (β ≈ 1.3 Å-1). Similar calculations on charged dipeptide derivatives, [CH3CONHCH2CONHCH3]+/-, showed that peptide-peptide interaction is more dependent on conformation in the cationic than in the anionic dipeptides. The α-helix and polyproline II-helix both have large peptide-peptide interactions (|HDA| > 800 cm-1) which arise from the angular dependence of their π-orbitals. Such an interaction is much weaker than in the β-strand peptides. These combined results were found to be consistent with electrontransfer rates experimentally observed across short peptide bridges in polyproline II (n = 1-3). These results can also account for directional electron transfer observed in an α-helical structure (different ET rates versus the direction of the molecular dipole).

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Journal of the American Chemical Society

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