CHRONOABSORPTOMETRIC DETERMINATION OF ADSORPTION-ISOTHERMS FOR CYTOCHROME-C ON TIN OXIDE ELECTRODES

The adsorption isotherms for ferri- and ferrocytochrome c on tin oxide electrodes were measured as a function of ionic strength and electrolyte using the spectroelectrochemical technique chronoabsorptometry. In low ionic strength pH 7.0 phosphate buffers, a high-affinity adsorption isotherm was obtained. As the ionic strength of the buffer was increased, the adsorption affinity of cytochrome c for tin oxide diminished, and a Langmuir-type isotherm was obtained. No apparent hysteresis was observed in the adsorption isotherms of ferricytochrome c upon dilution with buffer or higher ionic strength solutions. In contrast, the adsorption isotherms for ferrocytochrome c in low ionic strength phosphate buffer exhibited considerable hysteresis, with some denaturation of ferrocytochrome c being evident. The saturation surface coverage (GAMMA(satn)) and equilibrium adsorption constant (K) for ferricytochrome c on tin oxide were determined by fitting the Langmuir adsorption model to the data. A ca. 2-fold reduction in GAMMA(satn) and a ca. 20-fold reduction in K was observed as the ionic strength of the phosphate buffer was changed from 40 to 150 mM due to decreased electrostatic attractions. Slightly larger GAMMA(satn) and K values were obtained for ferricytochrome c in solutions containing only phosphate compared to solutions in which chloride or nitrate was a major component. The ionic strength dependence of K for the phosphate data was analyzed in terms of simple Debye-Huckel theory, and a value of 5 X 10(8) M-1 at zero ionic strength was obtained. These results indicate that the major driving force for spontaneous adsorption is the strong electrostatic attraction between positively charged cytochrome c and the negatively charged tin oxide electrode.