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

James F. Wishart, Ji Sun, Myung Cho, Chang Su, and Stephan S. Isied

J. Phys. Chem. B 101, 687-693 (1997) [Find paper at ACS Publications] or use ACS Articles on Request

Abstract:

Several ruthenium ammine complexes were used to modify horse-heart cytochrome c at histidine-33, creating a series of (NH₃)₄(L)Ru-Cyt c derivatives (L = H₂O/OH^-, ammonia, 4-ethylpyridine, 3,5-lutidine, pyridine, isonicotinamide, N-methylpyrazinium) with a wide range of driving forces for Fe-to-Ru electron transfer (-DG° = -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 × 10⁵ 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 (NH₃)₄(isn)Ru^III-Cyt c^II decreases at high pH, with a midpoint at pH 9.1.