MECHANISTIC INFORMATION FROM THE FIRST VOLUME PROFILE ANALYSIS FOR A REVERSIBLE INTERMOLECULAR ELECTRON-TRANSFER REACTION INVOLVING PENTAAMMINE(ISONICOTINAMIDE)RUTHENIUM AND CYTOCHROME-C

The reversible intermolecular electron-transfer reaction between penta ammine(isonicotinamide)ruthenium(II/III) and horse-heart cytochrome c iron(III/II) was subjected to a detailed kinetic and thermodynamic stu dy as a function of temperature and pressure. Theoretical calculations based on the Marcus-Hush theory were employed to predict all rate and equilibrium constants as well as activation parameters. There is an e xcellent agreement between the kinetically and thermodynamically deter mined equilibrium constants and associated pressure parameters. These data are used to construct a volume profile for the overall process, f rom which it follows that the transition state lies halfway between th e reactant and product states on a volume basis. The reorganization in the transition state has reached a similar degree in both directions of the electron-transfer process and corresponds to a lambda(double da gger) value of 0.44 for this reversible reaction. This is the first co mplete volume profile analysis for a reversible intermolecular electro n-transfer reaction.