Kinetics of the reduction of cytochrome c by [FeII(edta)(H2O)]2-:: outer-sphere vs. inner-sphere electron transfer mechanisms

The reduction of horse heart cytochrome c(III) by [Fe-II(edta)(H2O)](2-) [Fe-II(edta)(H2O)](2-) + [cytochrome eI(II)](7+) --> [Fe-III(edta)(H2O)](-) + [cytochrome c(II)](6+) was reinvestigated as a function of all chemical,and physical variables. Possible electrostatic interaction between the oppositely charged redox partners can assist the electron transfer process. Rate and activation parameters determined in the study are discussed in reference to those reported in the literature and compared with theoretical calculations based on the Marcus-Hush theory for outer-sphere electron transfer reactions. The reaction can also be interpreted in terms of an inner-sphere mechanism due to the presence of a very labile water ligand on [Fe-II(edta)(H2O)](2-). Experiments performed at elevated pressure reveal additional mechanistic information in terms of volume changes associated with the electron transfer process.