Driving-Force Effects on the Rate of Long-Range Electron Transfer in Ruthenium-Modified Cytochrome c
نویسندگان
چکیده
Two new Ru-modified, Zn-substituted derivatives of horse heart cytochrome c have been prepared, Rua,L(histi* dine-33)-Zn-cyt c (a = NH3; L = pyridine, isonicotinamide). Molecular mechanics modeling indicates that the 11.7 A 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 lo6 s-' (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 (-AGO) 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 (A) of 1.15 (5) eV and an electronic coupling matrix element ( H A B ) of 0.13 ( I ) cm-I. The charge-recombination data are fit with the parameters X = 1.24 (5) eV and H A B = 0.10 ( 1 ) cm-I, and the Ru-Fe-cyt c electron-transfer rate can be described with X = 1.2 eV and H A B = 0.03 cm-'. The aim of our research into the electron-transfer reactions of metalloproteins is to identify and understand the factors that control the rates of these pr0cesses.l Semiclassical* and quantum mechanical3 theories have provided the following simple expression for the rate of nonadiabatic electron transfer between two centers held a t fixed distance and orientation:
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