The temperature dependence of the redox potential of horse heart cytochrome c in sodium chloride solutions

The E⁰′ values for the conversion of horse heart cytochrome $\text{c}$ from the oxidized to the reduced form as a function of temperature have been measured in 0.10 M NaCl, 0.10 M sodium phosphate, pH 7.0 solutions in H₂O and D₂O. In H₂O, the decrease in the E⁰′ value is linear with increasing temperature up to 42°C. Above this temperature, the decrease is again linear but with a much greater slope. In D₂O solutions, however, this biphasic behavior was not observed but instead a single line was obtained over the temperature range studied (25°C to 50°C). These results are interpreted in terms of the ability of NaCl to cause a destructuring of the bulk H₂O above 42°C but not in the more stable D₂O (Kreishman, Foss, Inoue and Leifer (1976) $\text{Biochemistry}\text{,}\text{15}$, 5431–5435). This decrease in water structure results in a shift in the equilibrium to the larger oxidized form as indicated by the decrease in E⁰′.