Dependence of Intramolecular Electron-Transfer Rates on Driving Force, pH, and Temperature in Ammineruthenium-Modified Ferrocytochromes c

Several ruthenium ammine complexes were used to modify horse-heart cytochrome c at histidine-33, creating a series of (NH 3 ) 4 (L)Ru−Cyt c derivatives (L = H 2 O/OH -, ammonia, 4-ethylpyridine, 3,5-lutidine, pyridine, isonicotinamide, N -methylpyrazinium) with a wide range of driving forces for Fe-to-Ru electron transfer (−Δ G ° = −0.125 to +0.46 eV). Electron-transfer rates and activation parameters were measured by pulse radiolysis using azide or carbonate radicals. The driving-force dependence of electron-transfer rates between redox centers of the same charge types obeys Marcus−Hush theory. The activationless rate limit for all of the ruthenium derivatives except the N -methylpyrazinium complex is 3.9 × 10 5 s -1 . Thermodynamic parameters obtained from nonisothermal differential pulse voltammetry show that the electron-transfer reactions are entropy-driven. The thermodynamic and kinetic effects of phosphate ion binding to the ruthenium center are examined. The rate of intramolecular electron transfer in (NH 3 ) 4 (isn)Ru III −Cyt c II decreases at high pH, with a midpoint at pH 9.1.