A theoretical study of electronic factors affecting hydroxylation by model ferryl complexes of cytochrome P-450 and horseradish peroxidase

Density functional theory (DFT) is used to study model ferryl species of cytochrome P-450 and horseradish peroxidase (HRP), as well as of the product complex due to oxidation of H-2 by the P-450 species (1-4 and 7). The ferryl species studied include neutral and cation radical states of the porphyrin, as well as high- and low-spin situations. A few issues are addressed concerning the mechanism of alkane hydroxylation, and theoretical support is provided for: (i) the contention that spin inversion occurs along the reaction path, (ii) that the cation radical state of the porphyrin is an essential feature required to accommodate an excess electron from the ferryl moiety and thereby stabilize the ground state of the hydroxylation product, and (iii) that the donor property of the proximal ligand has a significant influence on the energy of the ferryl-to-ring charge-transfer states which are essential to convert the reactant state to the hydroxylation product state. In this sense, our study sheds some light on the difference between the oxidized and reduced HRP forms, HRP(I) and HRP(II),and suggests that the combination of a cation radical porphyrin state and a good pi-donor proximal ligand like thiolate, could be the underlying reason for the potent hydroxylation ability of the P-450 ferryl-complex.