Photoinduced Oxidation of Microperoxidase-8:  Generation of Ferryl and Cation-Radical Porphyrins

The electron-transfer reactivity of microperoxidase-8 (MP8), the heme octapeptide derived from enzymatic cleavage of cytochrome c, has been studied by nanosecond flash photolysis methods. Ferric MP8 is rapidly oxidized by photogenerated Ru(bpy)3 3+ in acidic solutions to a ferric cation-radical porphyrin (k ∼ 5.6 × 109 M-1 s-1); the oxidation product in alkaline solutions is ferryl MP8 (k ∼ 2.2 × 109 M-1 s-1). Numerical simulations of the kinetics for the direct oxidation of ferric to ferryl MP8 predict a marked pH effect on the rate of reaction in alkaline solutions; however, only a very weak pH dependence is observed in the range 7−8.5, indicating that the ferryl species is generated by intramolecular electron transfer within a ferric cation-radical porphyrin. Transient spectra taken between pH 6 and 8.5 show increasing ferryl absorption as the pH is increased, demonstrating a pH-dependent equilibrium between the two oxidized forms of MP8.