Characterization of a Resting State Model of Peroxidases by ab Initio Methods: Optimized Geometries, Electronic Structures, and Relative Energies of the Sextet, Quartet, and Doublet Spin States

Abstract

Ferric aquo heme complexes comprise the active site of three major families of heme proteins, the peroxidases, the cytochrome P450s, and metmyoglobin. There is ample evidence from a variety of spectroscopic studies of wild type (wt) and mutant enzymes that in these resting state complexes, the sextet, quartet, and doublet states are very close in energy and the predominant spin state observed is a sensitive function of the number, nature, and geometry of the axial ligands. One correlation that is very difficult to determine experimentally is the relationship between spin state and geometry in the same heme complex. This coupling of geometry and spin state in the same complex can be functionally important since spin state changes are often a key part of function, for example, in the enzymatic cycle of the cytochrome P450s. To further explore the relationship between geometry and spin state, we report here for the first time the use of ab initio methods to calculate optimized geometries and electronic struc...