Description
Application of Marcus theory to outer-sphere electron transfer reactions of classical inorganic complexes is accepted because of its general success. However, certain facets of molecular dynamics remain problematic because they are either not considered by the theory or have not been adequately investigated. Included in this group of problems are electrostatic reactant-reactant and reactant-solvent interactions, specific ligand-ligand interactions and non-adiabaticity. In this work investigations of the above-mentioned complications to the use of the theory have been made. These studies include the dynamics of the reaction of Fe(EDTA)⁻ with Ru(NH₃)₆²⁺ and Ru(NH₃)₄phen³⁺ and the analysis of the kinetic results, including the activation parameters, using Marcus theory. The rate constants for the following reactions were determined via stopped-flow kinetic measurements. Reaction 1 was investigated because of anomalous results for the calculated protein self-exchange rate constants calculated from the reactions of Ru(NH₃)₆²⁺ and Fe(EDTA)⁻ with h.h. cyt c. Also, the importance of electrostatic interaction is addressed by this reaction. Comparison of the observed rate constants and activation parameters with those calculated indicates excellent agreement. The calculated values for ΔG≠, ΔH≠ and ΔS≠ are 7.8 kcal/mole, 3.1kcal/mole and -16 e.u. versus 8.2, 3.1, and -17, respectively. These values were calculated at zero ionic strength and included electrostatic work terms appropriate to those conditions. The observed values were extrapolated from the observed ionic strength dependence of the activation free energy. The activation entropy was assumed to be invariant with ionic strength. The importance of electrostatics in the reactions of metalloproteins with small inorganic molecules was investigated by measuring the kinetics of Reactions (2), (4), (5) and (6). The cytochrome c₅₅₀ from Para. denitrificans and the HIPIP from Rps. gelatinosa are of the opposite charge type than the previously studied h.h. cyt c and Chr. vinosum HIPIP. The rate constants and activation parameters for Reactions (2) and (4) are very similar to those measured previously for the reaction of Ru(NH₃)₆²⁺ with h.h. cyt c and Chr. vinosum HIPIP. This evidence indicates that electrostatic effects are of minimal importance at the conditions at which these experiments were run (I>0.10M). This evidence does not necessarily support the "site-model" of electrostatic interaction. The virtual elimination of "normal" electrostatic effects simplifies interpretation of the calculated protein self-exchange activation parameters. The results from this work and previous work allow comparison of the protein self-exchange activation parameters calculated from the protein's cross-reaction with five inorganic species, Co(phen)₃³⁺, Ru(NH₃)₄phen³⁺, Ru(NH₃)₅py³⁺/²⁺, Ru(NH₃)₆²⁺ and Fe(EDTA)²⁻. Comparison of the calculated values foreach protein and between the two proteins indicate that specific interaction of the protein with the small molecule's ligands is important to the reaction dynamics. Hydrophobic ligands have a distinct advantage with both proteins. Size and orientations are also important as is indicated by comparison of the Ru(NH₃)₅py³⁺/²⁺ results with those from Ru(NH₃)₄phen³⁺ and Co(phen)₃³⁺. Distortion of the proteins' tertiary structure and desolvation of both protein and small molecule contribute to the activation barriers calculated. Kinetic results for the reaction of Rps. palustris cyt c' with Ru(NH₃)₆²⁺ indicate that a preequilibrium step is kinetically important for this reaction.
