Self-reduction of chlorohemin in pyridine: Comparison with the photoreduction process

The self-reduction of chlorohemin in pure pyridine has been investigated by voltammetric techniques. The results obtained indicate that the self-reduction follows a first-order kinetic rate law. The reduction is strongly inhibited by the presence of chloride ions suggesting that the monopyridine protoporphyrin iron(III) Fe(III)(PP)(py)]⁺ axial complex is the species undergoing self-reduction. In oxygenated solution the process is strongly retarded since the oxidation of the Fe(II)PP intermediate strongly competes with the formation of a stable Fe(II)(PP)(py)₂ complex. The results are discussed in the light of those obtained in a previous investigation on the photochemistry of chlorohemin under similar conditions.