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

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.