Fate of the excited triplet state of zinc cytochrome c in the presence of iron(III), iron(II), iron-free, and heme-free forms of cytochrome c

We study, by laser flash photolysis, the mechanism of quenching the triplet state of zinc cytochrome c, 3Zncyt, by the iron(III), iron(II), iron-free, and heme-free forms of cytochrome c. The method of inserting zinc(II) ion into the heme group in a protein overcomes the difficulties in studying redox reactions between two heme proteins having similar absorption spectra. Quenching of positively charged 3Zncyt by these positively charged reactants is promoted at high ionic strength, μ=1500 mM, which shields electrostatic charge and weakens protein–protein repulsion. Upon addition of potential quenchers, the rate constant for 3Zncyt decay becomes higher in the following order: ferrocytochrome c