Thermodynamic and kinetic aspects of the electron transfer reaction of bovine cytochrome c immobilized on 4-mercaptopyridine and 11-mercapto-1-undecanoic acid films

Abstract

Bovine cytochrome c (cyt c) was adsorbed on a polycrystalline gold electrode coated with 4-mercaptopyridine and 11-mercapto-1-undecanoic acid self-assembled monolayers (SAMs) and the thermodynamics and kinetics of the heterogeneous protein-electrode electron transfer (ET) reaction were determined by cyclic voltammetry. The E°′ values for the immobilized protein were found to be lower than those for the corresponding diffusing species. The thermodynamic parameters for protein reduction ( $$ \Updelta {H}_{{\rm rc}} ^{{\circ \prime }} $$ and $$\Updelta{S}_{{\rm rc}}^{{\circ \prime }} $$ ) indicate that the stabilization of the ferric state due to protein–SAM interaction is enthalpic in origin. The kinetic data suggest that a tunneling mechanism is involved in the ET reaction: the distance between the redox center of the protein and the electrode surface can be efficiently evaluated using the Marcus equation.