| Abstract
| - Little is known about transient intermediates in photoinduced electron-transfer reactions of metalloproteins.Oxidative quenching of the triplet state of zinc cytochrome c, 3Zncyt, is done at 20 °C, pH 7.00, and ionic strengthof 1.00 M, conditions that suppress the thermal back-reaction and prolong the lifetime of the cation radical,Zncyt+. This species is reduced by [Fe(CN)6]4-, [W(CN)8]4-, [Os(CN)6]4-, [Mo(CN)8]4-, and [Ru(CN)6]4-complexes of similar structures and the same charge. The rate constants and thermodynamic driving forces forthese five similar electron-transfer reactions were fitted to Marcus theory. The reorganization energy of Zncyt+is λ = 0.38(5) eV, lower than that of native cytochrome c, because the redox orbital of the porphyrin cationradical is delocalized and possibly because Met80 is not an axial ligand to the zinc(II) ion in the reconstitutedcytochrome c. The rate constant for electron self-exchange between Zncyt+ and Zncyt, k11 = 1.0(5) × 107 M-1s-1, is large owing to the extended electron delocalization and relatively low reorganization energy. These resultsmay be relevant to zinc(II) derivatives of other heme proteins, which are often used in studies of photoinducedelectron-transfer reactions.
- The cation radical of zinc cytochrome c, Zncyt+, contains an electron “hole” in the porphyrin ring. This radical is reduced by cyano complexes; thus, Zncyt+ + [M(CN)n]4-Zncyt + [M(CN)n]3-. The rate constants kr and thermodynamic driving forces Δfor five cyano complexes were fitted to Marcus theory. The reorganization energy of Zncyt+ is 0.38(5) eV, and the rate constant for electron self-exchange between Zncyt+ and Zncyt is 1.0(5) × 107 M-1 s-1. These two properties of iron-containing and zinc-containing forms of cytochrome c are compared.
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