Oxidised metallophenolicporphyrins as models for compound I of the peroxidases. Reduction of oxidised iron(III), nickel(II) and zinc(II) complexes of meso-tetrakis(3,5-di-t-butyl-4-hydroxyphenyl)prophyrin by the peroxidase substrate p-cresol

Oxidised metal complexes of meso-tetrakis(3,5-di-t-butyl-4-hydroxyphenyl)porphyrin (3), M-TBHPP(ox), react with peroxidase substrates such as p-cresol regenerating the parent metalloporphyrins cleanly and quantitatively. The kinetics of the reduction of the oxidised iron(III), nickel(II) and zinc(II) porphyrin complexes in chloroform and methanol have been investigated. Generally rate expressions of the form, rate={k1+k2[p-cresol]}[M-TBHPP(ox)] were observed where the k1 term represents an auto- or solvent-assisted reduction pathway while the k2 term represents a reduction pathway involving p-cresol. For the reduction of Ni-TBHPP(ox) in chloroform however a rate expression of the form rate={k1+k3[p-cresol]2}[Ni-TBHPP(ox)] was observed, the second order dependence of rate on p-cresol concentration being attributed to pre-equilibrium formation of a p-cresol dimer which in the rate determining step is the active reductant. Rate constants for the above reactions are reported as well as activation parameters for the reduction of the oxidised nickel(II) complex in both solvents.