Photosynthesis, respiration, nitrogen fixation, drug metabolism, DNA synthesis, and immune response are among the scores of biological processes that rely heavily on long-range (10 to 25 Angstrom) protein electron-transfer (ET) reactions. Semiclassical theory predicts that the rates of these reactions depend on the reaction driving force -Delta G degrees, a nuclear reorganization parameter lambda, and the electronic-coupling strength H-AB between reactants and products at the transition state : ET rates (k(ET)degrees) reach their maximum values when the nuclear factor is optimized (-Delta G degrees = lambda); these k(ET)degrees values are limited only by the strength (H-AB(2)) of the electronic interaction between the donor (D) and acceptor (A). Coupling-limited Cu+ to Ru3+ and Fe2+ to Ru3+ ET rates have been extracted from kinetic studies on several Ru-modified proteins. In azurin, a blue copper protein, the distant D/A pairs are relatively well coupled (k(ET)degrees decreases exponentially with R(Cu-Ru); the decay constant is 1.1 Angstrom(-1)). In contrast to the extended peptides found in azurin and other beta-sheet proteins, helical structures have tortuous 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 the alpha helix in myoglobin and the photosynthetic reaction center are between 1.25 and 1.6 Angstrom(-1).