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, Rua{sub 4}L(histidine-33)-Zn-cyt c (a = NH{sub 3}; L = pyridine, isonicotinamide). Molecular mechanics modeling indicates that the 11.7 {angstrom} 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 Rua{sub 4}L (His-33)-Zn-cyt c lie in the range of 2.0 {times} 10{sup 5}-3.3 {times} 10{sup 6} s{sup {minus}1} (22{degree}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 (-{Delta}G{degree}) 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 ({lambda}) of 1.15 (5) eV and an electronic coupling matrix element (H{sub AB}) of 0.13 (1) cm{sup {minus}1}. The charge-recombination data are fit with the parameters {lambda} = 1.24 (5) eV and H{sub AB} = 0.10 (1) cm{sup {minus}1}, and the Ru-Fe cyt c electron-transfer rate can be described with {lambda} = 1.2 eV and H{sub AB} = 0.03 cm{supmore » {minus}1}.« less