Reaction profile of the last step in cytochrome P-450 catalysis revealed by studies of model complexes

A series of oxoiron(IV) porphyrin cation radical complexes was investigated as compound I analogs of cytochrome P-450. Both the spectroscopic features and the reactivities of the complexes in oxygen atom transfer to olefins were examined as a function of only one variable, the axial ligand trans to the oxoiron(IV) bond. The results disclosed two important kinetic steps - electron transfer from olefin to oxoiron(IV) and intramolecular electron transfer from metal to porphyrin radical - which are affected differently by the axial ligands. The large kinetic barrier of the latter step in the reaction of olefins with the perchlorato-bound oxoiron(IV) porphyrin cation radical complex enabled the trapping of a reaction intermediate in which the metal, but not the porphyrin radical, is reduced. The first electron transfer step is probably followed by a-bond formation, which readily accounts for formation of isomerized organic products at low temperatures. It is finally postulated that part of the enhanced oxygenation activities of cytochrome P-450 monooxygenases and chloro-peroxidases is due to a lowering of the energy barrier for the second electron transfer step via participation of their redox-active cysteinate ligand.