Electron Tunneling in Engineered Proteins

Semiclassical theory predicts that the rates of electron transfer (ET) reactions depend on the reaction driving force (-ΔG°), a nuclear reorganization parameter (λ), and the electronic-coupling strength (H_(AB)) between reactants and products at the transition state. ET rates reach their maximum values (k°_(ET)) when the nuclear factor is optimized (-ΔG° = λ); these k°_(ET) values are limited only by the strength (H^2_(AB)) of the electronic interaction between the donor (D) and acceptor (A). The dependence of the rates of Ru(His33)cytochrome c ET reactions on -ΔG° (0.59-1.4 eV) accords closely with semiclassical predictions. The anomalously high rates of highly exergonic (-ΔG° ≥ 1.4 eV) ET reactions suggest initial formation of an electronically excited ferroheme in these cases. Coupling-limited Cu^+ to Ru^(3+) and Fe^(2+) to Ru^(3+) ET rates for several Ru-modified proteins are in good agreement with the predictions of a tunneling-pathway model. In azurin, a blue copper protein, the distant D-A pairs are relatively well coupled (k°_(ET) decreases exponentially with Cu-Ru distance; the decay constant is 1.1 Å^(-1)). In contrast to the extended peptides found in azurin and other β-sheet proteins, helical structures have torturous covalent pathways owing to the curvature of the peptide backbone. The decay constants estimated from ET rates for D-A pairs separated by long sections of α helix in myoglobin and the photosynthetic reaction center are between 1.25 and 1.6 Å^(-1)