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The second-order rate constant of the reaction between the hydrated electron and ferrinitrocytochrome c exhibits a marked pH dependence that could not be fully ascribed to changes in geometrical parameters and in net charge of the protein molecule. The correlation between the pH dependence of the rate constant, the 695-nm absorbance and the ionization state of the nitrated tyrosyl-67 residue indicates that tyrosine-67 is of importance in maintaining the specific structure for the electron transfer mechanism in ferricytochrome c upon reduction.
1. 1. The reaction of hydrated electrons with ferricytochrome c was studied using the pulse-radiolysis technique. 2. 2. In 3.3 mM phosphate buffer (pH 7.2), 100 mM methanol and at a concentration of cytochrome c of less than 20 μM the reduction kinetics of ferricytochrome c by hydrated electrons is a bimolecular process with a rate constant of 4.5 · 1010 M−1 ·s−1 (21 °C). 3. 3. At a concentration of cytochrome c of more than 20 μM the apparent order of the reaction of hydrated electrons with ferricytochrome c measured at 650 nm decreases due to the occurrence of a rate-determining first-order process with an estimated rate constant of 5 · 106 s−1 (pH 7.2, 21 °C). 4. 4. At high concentrations of cytochrome c the reaction-time courses measured at 580 and 695 nm appear to be biphasic. A rapid initial phase (75% and 30% of total absorba...
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