UV-VISIBLE SPECTROSCOPY OF ADSORBED CYTOCHROME-C ON TIN OXIDE ELECTRODES

The conformational state and surface coverage of electrostatically adsorbed cytochrome c on tin oxide electrodes were examined by using visible absorption spectroscopy. Spectral analysis revealed that no major structural changes occur in ferri- or ferrocytochrome c as a result of adsorption from pH 7, low ionic strength, phosphate buffers. Adsorbed ferricytochrome c was found to be conformationally stable at applied potentials below 0.70 V ve Ag/AgCl (1 M Kcl). At applied potentials above 0.70 V, adsorbed cytochrome c was irreversibly oxidized beyond the ferric state with the resulting oxidation product remaining tenaciously adsorbed on the electrode surface. From the close similarity between the total surface coverage, determined from the Soret absorbance, and the electroactive coverage, determined from vottammetry, it was concluded that adsorbed cytochrome c resides in a predominantly electroactive form. Due to the small signals involved and the presence of adsorption-induced optical perturbations in the tin oxide/buff or background absorbance, the development of background subtraction procedures proved necessary. These procedures are discussed in detail.