Peculiarities of the Electronic Structure of Cytochrome P450 Compound I: CASPT2 and DFT Modeling.

Abstract

CASSCF/CASPT2 ab initio formalism has been applied to a thiolate model of cytochrome P450 compound I. A2u ground state of porphyrin radical character was found in agreement with experimental results. A strong mixing of CASSCF reference states in multistate CASPT2 was observed, which is an interesting phenomenon, rare for the ground state near the equilibrium geometry. Details of the CASSCF/CASPT2 procedure (including the construction of the active space) are discussed. Parallel DFT calculations revealed that relative energies and the scheme of spin coupling are qualitatively reproduced by hybrid DFT (B3LYP); however, results from nonhybrid functionals (BLYP, BP86) are significantly different in these aspects.