High-Pressure Pulse Radiolysis Study of Intramolecular and Intermolecular Reduction of Cytochrome c by Ruthenium(II) Ammine Complexes

Volumes of activation for intra- and intermolecular electron transfer from ruthenium(II) ammine complexes to two types of cytochrome c were measured by pulse radiolysis using a portable, high-pressure optical cell. Electron transfer rates were measured as a function of pressure from 1 to 1500 atmospheres. The DV^ý values for intramolecular electron transfer in (NH₃)₅Ru^II-His33 horse heart ferricytochrome c and (NH₃)₅Ru^II-His39 Candida krusei ferricytochrome c are -17.7 ± 0.9 and -18.3 ± 0.7 cm³ mol^-1, respectively. For the intermolecular reaction between [Ru(NH₃)₆]²⁺ and horse heart ferricytochrome c, DV^ý is -15.6 ± 0.6 cm³ mol^-1. The results indicate that the electron transfer transition state is significantly more compact than the reactant state in both the intramolecular and intermolecular cases. Most of this volume change can be accounted for by an increase in solvent electrostriction at the ruthenium center.