Hydroxylation mechanism of methane and its derivatives over designed methane monooxygenase model with peroxo dizinc core

Abstract

The peroxo dizinc Zn₂O₂ complex Q coordinated by imidazole and carboxylate groups for each Zn center has been designed to model the hydroxylase component of methane monooxygenase (MMO) enzyme, on the basis of the experimentally available structure information of enzyme with divalent zinc ion and the MMO with Fe₂O₂ core. The reaction mechanism for the hydroxylation of methane and its derivatives catalyzed by Q has been investigated at the B3LYP*/cc-pVTZ, Lanl2tz level in protein solution environment. These hydroxylation reactions proceed via a radical-rebound mechanism, with the rate-determining step of the C–H bond cleavage. This radical-rebound reaction mechanism is analogous to the experimentally available MMOs with diamond Fe₂O₂ core accompanied by a coordinate number of six for the hydroxylation of methane. The rate constants for the hydroxylation of substrates catalyzed by Q increase along CH₄ < CH₃F < CH₃CN ≈ CH₃NO₂ < CH₃CH₃. Both the activation strain ΔE^≠_strain and the stabilizing interaction ΔE^≠_int jointly affect the activation energy ΔE^≠. For the C–H cleavage of substrate CH₃X, with the decrease of steric shielding for the substituted CH₃X (X = F > H > CH₃ > NO₂ > CN) attacking the O center in Q, the activation strain ΔE^≠_strain decreases, whereas the stabilizing interaction ΔE^≠_int increases. It is predicted that the MMO with peroxo dizinc Zn₂O₂ core should be a promising catalyst for the hydroxylation of methane and its derivatives.