Kinetics of the decomposition of coprohaem compound I in aqueous solution

Abstract

Coprohaem Compound I, formed by oxidation of coproferrihaem {[3,8,13,17-tetramethylporphyrin-2,7,12,18-tetrapropionato(6–)-N²¹,N²²,N²³,N²⁴]ferrate(3–)} with peroxyacids in reactions of 1 : 1 stoicheiometry, decomposes in aqueous solution in the absence of added reductants. The reaction is biphasic and follows series first-order kinetics. The formation of a coprohaem Compound II intermediate has been implicated. When the initial ratio of [peroxyacid] : [coproferrihaem] was ⩽ 1, an approximately constant proportion (75–80%) of the Compound I initially formed was reduced back to coproferrihaem and the remainder was converted into products which were non-absorbing in the Soret and visible spectral regions. It is proposed that coproferrihaem Compound I undergoes a rate-determining reaction with water, probably involving nucleophilic attack at the porphyrin meso positions, to form species which are susceptible to rapid oxidative attack by other Compound I molecules and resulting in the reduction of 3–4 molecules of Compound I for each molecule of Compound I oxidatively destroyed.