One-electron reduction of a ferrihaem

Abstract

The hydrated electron reacts with hydroxy protoferrihaem dimer with k= 2.0 × 10¹⁰ dm³ mol^–1 s^–1 to form in the first instance a product represented as a half-reduced dimer. The superoxide anion radical does not react with hydroxy protoferrihaem dimer. The half-reduced dimer can undergo a first order reaction in which a hydroxyl group in the inner sphere of a ferrous ion is substituted by a water molecule, k= 9.8 × 10² s^–1 at 25°C, activation energy 27.4 kJ mol^–1. The product of this reaction (compound I) can trasfer an electron to a molecule like itself, 2k= 4 × 10⁹ dm³ mol^–1 s^–1, following which a hydroxyl group is rapidly substituted for one of the water molecules in the oxidized dimer formed. An alternative reaction for the half-reduced dimer is to transfer an electron to a molecule like itself, 2k= 1.2 × 10⁹ dm³ mol^–1 s^–1, following which the hydroxyl group in the fully reduced dimer (compound II) is substituted by a water molecule with the same rate constant and activation energy as the similar reaction occurring in the half-reduced dimer. The half-reduced dimer can also transfer an electron to compound I, k= 2 × 10⁹ dm³ mol^–1 s^–1. The final product of the whole process is a mixture of hydroxy protoferrihaem dimer and the same reduced dimer as is produced by dithionite or ascorbate.