The effects of temperature and electrolyte at acidic and alkaline pH on the electron transfer reactions of cytochrome c at In2O3 electrodes

Spectroelectrochemical and electrochemical methods were used to investigate the characteristics of heterogeneous electron transfer between cytochrome c and indium oxide electrodes. A linear temperature dependence of the formal potential of cytochrome c was observed from 5 to 75 °C in acidic media. This behavior is attributed to a linear variation in the conformation of ferricytochrome c that results in an increase in solvent exposure of the solvent-exposed heme edge. A break in the linear temperature dependence of the formal potential occurred at 40 °C in alkaline media. This reflects a distinct conformational change that accompanies the onset of thermal denaturation of ferricytochrome c. The small change in reaction center entropy, ΔS°_rc, of ca. −54 J K⁻¹ mol⁻¹ in neutral and acidic media (5 to ⩾ 55 °C) and in alkaline media (below 40 °C) is consistent with a small shift to a more stable conformation of cytochrome c that occurs upon reduction.Adsorption of reactant and product was detected. The strength and type of adsorption were found to be temperature- and pH-dependent. The characteristics of electron transfer between cytochrome c and an electrode depend on bulk solvent properties and electrode surface characteristics.