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(μ-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. Copyright (C) 1999 Elsevier Science Inc.