Up-Hill Electron Transfer in Pentaammineruthenium(III)-Modified Ferrocytochrome c: Rates, Thermodynamics, and the Mediating Role of the Ruthenium Moiety

At moderate to high ionic strengths (>0.1 M), Co(oxalate)₃^3- oxidizes native cytochrome c very slowly, however it undergoes a rapid reaction with pendant ruthenium complexes covalently attached to the surface of the protein. Under these conditions, the rate of the thermodynamically unfavorable (up-hill) Fe^II-to-Ru^III electron transfer process in pentaammineruthenium-modified horse-heart cytochrome c can be revealed using sufficiently high Co(oxalate) ₃^3- concentrations. Rate measurements performed over a wide range of Co^III concentrations confirm the proposed kinetic scheme. The rate of up-hill intramolecular electron transfer at 25 °C is 0.40 sec^-1, (DH^ý = 12.5 ± 0.2 kcal mol^-1 and DS^ý = -18.3 ± 0.7 cal deg^-1 mol^-1) in 50 mM pH 7.0 phosphate buffer. The net reaction thermodynamics, as obtained from activation parameters, are: DG° = -0.125 eV, DH° = -9.0 ± 0.2 kcal mol^-1 and DS° = -20.7 ± 0.8 cal deg^-1 mol^-1. The thermodynamic results are confirmed by nonisothermal differential pulse voltammetry.