The effect of axial ligands on the reactivity and stability of the oxoferryl moiety in model complexes of Compound I of heme-dependent enzymes

The contribution of simple inorganic model complexes to the understanding of related processes in biomolecules is demonstrated by a series of Compound I analogs of heme-dependent enzymes. The oxoiron- (IV) porphyrin cation radical state in these synthetic complexes is stable enough to be studied by spectroscopic methods as a function of only one variable, the axial ligand trans to the oxoiron bond. Complementary information from kinetic investigations of the reactivity in epoxidation of olefins enables the separation of the thermodynamic and kinetic effects of the axial ligands. The results clearly indicate that epoxidation by these complexes proceeds by two distinguishable steps, which are affected differently by the axial ligands. The first step is electron transfer from the olefin to the ferryl moiety, probably followed by intramolecular charge rearrangement and product release. It is proposed that part of the enhanced oxygenation activities of cytochrome P-450 monooxygenases and chloroperoxidases is due to a lowering of the energy barrier for the second step via participation of their redox-active cysteinate ligand in charge rearrangement.