The reduction of cytochrome c by iron EDTA-like complexes: implications on charge effect corrections to mediator-protein rate constants

The reduction of cytochrome c by a series of ferrous EDTA-like complexes was analyzed by the relative Marcus and Tunneling theories. All of the rate constants were calculated at infinite ionic strengths by the use of the Wherland-Gray equation to eliminate electrostatic effects. Both of the theories predicted the rate constants quite well for most of the complexes. But, for those complexes that cannot completely fill all the coordination sites of the iron (HEDTA, NTA), the rate constants were anomalously low. This would be consistent with an increased tunneling distance due to an increase in the size of the complex due to hydrogen bonding between the coordinated hydroxide and the solvent, or due to poorer interaction between the protein and the more hydrophobic mediators.