Driving-Force Effects on the Rate of Long-Range Electron Transfer in Ruthenium-Modified Cytochrome c

Two new Ru-modified, Zn-substituted derivatives of horse heart cytochrome c have been prepared, Rua₄L(histi dine-33)-Zn-cyt c (a = NH₃; L = pyridine, isonicotinamide). Molecular mechanics modeling indicates that the 11.7 Å edge-to-edge separation between the redox centers is virtually identical with that reported for the Ru-pentaammine derivative. Rates of photoinduced charge separation and recombination in Rua₄L(His-33)-Zn-cyt c lie in the range of 2.0 X 10⁵-3.3 X 10⁶ s^-1(22 °C). These kinetics, along with those already reported, provide a total of eight intramolecular electron-transfer reactions that have been measured in Ru-M-cyt c (M = Fe, Zn) at driving forces (-ΔG^o) ranging from 0.18 to 1.05 eV. The variation of the rate with driving force is in general agreement with the semiclassical theory of electron-transfer reactions. Fitting the Ru-Zn-cyt c charge-separation data yields a reorganization energy (X) of 1.15 (5) eV and an electronic coupling matrix element (H_AB) of 0.13 (1) cm^-1. The charge-recombination data are fit with the parameters λ= 1.24 (5) eV and H_AB = 0.10 (1) cm^-1, and the Ru-Fe-cyt c electron-transfer rate can be described with λ = 1.2 eV and H_AB= 0.03 cm^-1.