EPR of Compound I: An Illustrated Revision of the Theoretical Model

Abstract

AbstractCompound I has been postulated to be the reactive species in many heme catalysts, which performs different chemistry and shows different properties in different enzymes. The aim of this review is to present a comprehensive model which has been successfully used to interpret the EPR spectra of various Compound I species. The theoretical approach established by seminal articles will be revisited and its ability to explain experimental results will be illustrated by simulating selected spectra from the literature. Compound I stores two oxidizing equivalents, one in the paramagnetic iron(IV)-oxo moiety, and another one as a free radical on the porphyrin ligand or an amino acid in the protein. To describe the interactions of the two paramagnetic species with each other and with their local environment, the spin Hamiltonian of the system is built step by step. The Fe(IV) center is described using a two-hole model. The effect of the crystal-field and spin–orbit coupling on the energy levels is calculated with this simple approach, which allows to obtain spin Hamiltonian parameters like zero-field splitting and effectiveg-values for the iron. The magnetic interaction between the Fe(IV) center and the free radical is considered and allowed to vary in sign (ferromagnetic to antiferromagnetic) and magnitude to interpret the EPR of Compound I species in different systems. Since orbital overlap is crucial for exchange interaction, special emphasis is made in obtaining the orientation of Fe semi-occupied orbitals by extending the counter-rotation concept, which relates the directions of magnetic, electronic, and molecular axes.