ADSORPTION AND REDOX THERMODYNAMICS OF STRONGLY ADSORBED CYTOCHROME-C ON TIN OXIDE ELECTRODES

An examination of the redox and adsorption thermodynamics for adsorbed horse heart cytochrome c on fluorine-doped tin oxide electrodes has been performed using cyclic voltammetry with a nonisothermal electrochemical cell. Cytochrome c was found to adsorb strongly in neutral to slightly alkaline low ionic strength solutions with near-monolayer coverage resulting under optimum conditions. Variation in electroactive surface coverage with solution pH and ionic strength indicated that the adsorption is primarily controlled by electrostatic interactions. The surface formal potential of adsorbed cytochrome c was found to be ca. +240 mV, which is 20-25 mV lower than the solution value. A reaction entropy, ΔS°rc, of ca. -75 to -80 J/(K·mol) was determined from the temperature coefficient of the surface formal potential. This value is about 25 J/(K·mol) more negative than the literature value for solution cytochrome c. These results are accounted for by a model in which ferricytochrome c adsorbs more strongly on tin oxide than ferrocytochrome c due to a greater degree of adsorption-induced structural alteration in the former species. The implications of this study for understanding similar formal potential shifts that result when cytochrome c binds to mitochondrial membranes and other biocomponents are discussed. © 1990 American Chemical Society.