SUMMARYThe 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.Previously we have shown I , that for the reaction of metmyoglobin and methaemoglobin with hydrated electrons (eaq) the second-order rate constant (kobsd) is pH dependent. This behaviour was ascribed to the pH dependence of the net charge on the protein and the resulting pH dependence of the electrostatic interaction energy of the reactants. For ferricytochrome c, however, between pH 8 and 10 a larger change in kobsd was found than could be expected from the change in net charge on the protein and it was suggested that this is due to structural changes. The conformational change of ferricytochrome c between pH 8 and 10 (refs 2-13), characterized by a decrease in 695-nm absorbance (A69s nm) 4-7 , affects mainly the structure in the vicinity of the haem a-13 and seems to have little effect on the geometrical parameters of the molecule a ,6,7,13,14. That the change in reactivity is caused by a change in radius (see also Braams and Ebert Is ,16) is unlikely, since for the observed decrease in reactivity to about 10% of the value at neutral pH the radius of ferricytochrome c at alkaline pH must at least be halved. We therefore suggest that changes in the structure around the haem affect the reactivity of ferricyto-*Postal address:
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