Mechanism of the Methane → Methanol Conversion Reaction Catalyzed by Methane Monooxygenase:  A Density Functional Study

Abstract

The hybrid density functional (DFT) method B3LYP was used to study the mechanism of the methane hydroxylation reaction catalyzed by a non-heme diiron enzyme, methane monooxygenase (MMO). The key reactive compound Q of MMO was modeled by (NH 2 )(H 2 O)Fe(μ-O) 2 (η 2 -HCOO) 2 Fe(NH 2 )(H 2 O), I . The reaction is shown to take place via a bound-radical mechanism and an intricate change of the electronic structure of the Fe core is associated with the reaction process. Starting with I, which has a diamond-core structure with two Fe IV atoms, L 4 Fe IV (μ-O) 2 Fe IV L 4, the reaction with methane goes over the rate-determining H-abstraction transition state III to reach a bound-radical intermediate IV, L 4 Fe IV (μ-O)(μ-OH(···CH 3 ))Fe III L 4, which has a bridged hydroxyl ligand interacting weakly with a methyl radical and is in an Fe III −Fe IV mixed valence state. This short-lived intermediate IV easily rearranges intramolecularly through a low barrier at transition state V for addition of the methyl radical to the hydroxyl ligand to give the methanol complex VI, L 4 Fe III (OHCH 3 )(μ-O)Fe III L 4, which has an Fe III −Fe III core. The barrier of the rate-determining step, methane H-abstraction, was calculated to be 19 kcal/mol. The overall CH 4 oxidation reaction to form the methanol complex, I + CH 4 → VI, was found to be exothermic by 39 kcal/mol.