COMPARATIVE KINETIC-ANALYSIS OF REVERSIBLE INTERMOLECULAR ELECTRON-TRANSFER REACTIONS BETWEEN A SERIES OF PENTAAMMINERUTHENIUM COMPLEXES AND CYTOCHROME-C

In this kinetic and thermodynamic study, the reversible outer-sphere e lectron-transfer reactions between a series of Ru(NH3)(5)L(3+/2+) comp lexes (L = etpy, py, lut) (etpy = 4-ethylpyridine; py = pyridine; lut = 3,5-lutidine) and cytochrome c were investigated as a function of io nic strength, buffer, pH, temperature, and pressure. Due to the low dr iving forces of these systems, it was possible to study all the reacti ons in both redox directions. The observed rate constants for various L are correlated on the basis of the ability of ligands on the rutheni um complex to penetrate the heme groove on cytochrome c. The measureme nts as a function of pressure enabled the construction of volume profi les for all investigated systems. The activation volumes for all of th ese processes are very similar: between -14.9 and -17.8 cm(3) mol(-1) for the reduction and between +14.7 and +17.8 cm(3) mol(-1) for the ox idation of the protein by Ru(MH(3))(5)L(2+/3+), respectively. The over all reaction volume varies between 27 and 35 cm(3) mol(-1), from which it follows that the transition state lies exactly halfway between rea ctant and product states on a volume basis in all cases. There is good agreement throughout between kinetic and thermodynamic data.