Conformational Dependence of Electronic Coupling Across Peptide Bonds: A Ramachandran Map

Abstract

Electronic coupling across peptide bonds has been determined throughout the peptide bond conformational space. Using the side chains of tyrosine (Tyr) and tryptophan (Trp) as donors (D) and acceptors (A), respectively, a plot of the electronic coupling matrix element H DA versus (ϕ,ψ) torsional angles has been constructed for Trp-peptide-Tyr molecules. The H DA values were obtained using the generalized Mulliken−Hush approach with electronic transition energies, permanent and transition dipole parameters derived from semiempirical quantum mechanical electronic structure calculations (INDO/S). The computed H DA values and the corresponding electron transfer (ET) rates show that specific helical peptide conformations situated in a narrow ϕ region of the full H DA-(ϕ,ψ) map play a significant role in developing strong electronic coupling for promoting ET. The H DA-(ϕ,ψ) map clearly defines the angular regions in space, where strong Trp-peptide-Tyr coupling occurs and peptide-mediated ET results, as well as regions of weak peptide coupling where the presence of the peptide has only marginal effects on electronic coupling.

Electron transfer in peptides

“…40 More support for the importance of peptide chain length and conformations comes from the work of Isied and coworkers, who demonstrated that distances between donors and acceptors and torsional angles within peptides are critical. 41…”

Electron transfer in proteins: theory, applications and future perspectives

“…The results emphasized the impact of structural fluctuations on the electronic coupling and hence also on protein electron transfer, being well established in the literature. [115][116][117][118][119][120][121][122] Over the past few years, there has been evidence supporting the idea that many inter-protein ET complexes are highly transient as a characteristic consequence of their biological function. These complexes are optimized by evolution to react fast, not to achieve high specificity.…”

The influence of dipole moments on the mechanism of electron transfer through helical peptides

“…5 During superexchange, the amino acids mediate ET and do not undergo a change in their redox states while during hopping, certain amino acids are reversibly oxidized or reduced and serve as relay stations (stepping stones) for the ET process. Important parameters that influence ET mechanisms are peptide conformations, 6,7 peptide flexibility, 8 and redox potentials of the peptide side chains that might become stepping stones for a hopping process if their redox potentials are low enough. 5,9 Of special interest are a-and 3 10 -helices as their high dipole moments could strongly change the redox potential of the amide groups of the peptide backbone.…”

“…Recent calculations of Isied et al have shown that in a-/3 10 -helical peptides the electron coupling between the amide groups is strong. 7 In addition, H-bonds could be used as a pathway for the ET process.…”