Two-step concerted mechanism for methane hydroxylation on the diiron active site of soluble methane monooxygenase.

A new concerted mechanism is proposed for the conversion of methane to methanol on intermediate Q of soluble methane monooxygenase (sMMO), the active site of which is considered to involve an Fe2(mu-O)2 diamond core. A hybrid density functional theory (DFT) method is used for our mechanistic study on the important reactivity of the bare FeO+ complex and a diiron model of intermediate Q. The reaction pathway for the methane hydroxylation on the diiron complex is essentially identical to that for the gas-phase reaction by the bare FeO+ complex. Methane is highly activated on the dinuclear iron model through the formation of a methane complex, in which a coordinatively unsaturated iron plays a central role in the bonding interaction between the diiron model and substrate methane. A H atom abstraction via a four-centered transition state and a recombination of the OH and CH3 groups via a three-centered transition state successively occur on the dinuclear iron-oxo species, leading to the formation of a methanol complex that corresponds to intermediate T. These electronic processes take place in a concerted manner. Our mechanism for methane hydroxylation by sMMO is different from the radical mechanism that has been widely accepted for enzymatic hydrocarbon hydroxylation, especially by cytochrome P450.