Theory of chemical bonds in metalloenzymes. VII. Hybrid-density functional theory studies on the electronic structures of P450

A first principle investigation has been carried out for intermediate states of the catalytic cycle of a cytochrome P450. To elucidate the whole catalytic cycle of P450, the electronic and geometrical structures are investigated not only at each ground state but also at low-lying energy levels. Using the natural orbital analysis, the nature of chemical bonds and magnetic interactions are investigated. The ground state of the Compound 1 (cpd1) is calculated to be a doublet state, which is generated by the antiferromagnetic coupling between a triplet Fe(IV)=O moiety and a doublet ligand radical. We found that an excited doublet state of the cpd1 is composed of a singlet Fe(IV)=O and a doublet ligand radical. This excited state lies 20.8 kcal mol^-1 above the ground spin state, which is a non-negligible energy level as compared with the activation energy barrier of ΔE^# = 26.6 kcal mol^-1. The reaction path of the ground state of cpd1 is investigated on the basis of the model reaction: ³O(³p) + CH₄. The computational results suggest that the reactions of P450 at the ground and excited states proceed through abstraction (³O-model) and insertion (¹O-model) mechanisms, respectively.