What affects the quartet-doublet energy splitting in peroxidase enzymes?

Density functional theory calculations have been performed on the active species (Compound I) of cytochrome c peroxidase (CcP) and ascorbate peroxidase (APX) models. We have calculated a large model containing oxo-iron porphyrin plus a hydrogen-bonded network of the axial bound imidazole ligand connected to an acetic acid and an indole group, which mimic the His(175), Asp(235), and Trp(191) amino acids in cytochrome c peroxidase. Our optimized geometries are in good agreement with X-ray and crystallographic structures and give an electronic ground state in agreement with EPR and ENDOR results. We show that the quartet-doublet state ordering and the charge distribution within the model are dependent on small external perturbations. In particular, a single point charge at a distance of 8.7 A is shown to cause delocalization of the charge and radical characters within the model, thereby creating either a pure porphyrin cation radical state or a tryptophan cation radical state. Thus, our calculations show that small external perturbations are sufficient to change the electronic state of the active species and subsequently its catalytic properties. Similar effects are possible with the addition of an electric field strength along a specific coordination axis of the system. The differences between the electronic ground states of CcP and APX Cpd I are analyzed on the basis of external perturbations.