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Published April 17, 1998 | public
Book Section - Chapter

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).

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© 1998 American Chemical Society. Published in print 17 April 1998. Our work on electron transfer in proteins is supported by the National Science Foundation, the National Institutes of Health, and the Arnold and Mabel Beckman Foundation.

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