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 (NH2)(H2O)Fe(m-O)2(η2-HCOO)2Fe(NH2)(H2O), 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, L4F(IV)(μ-O)2Fe(IV)L4, the reaction with methane goes over the rate-determining H-abstraction transition state III to reach a bound-radical intermediate IV, L4Fe(IV)(μ-O)(μ-OH(· · · CH3))Fe(III)L4, 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, L4Fe(III)(OHCH3)(μ- O)Fe(III)L4, 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 CH4 oxidation reaction to form the methanol complex, I + CH4 → VI, was found to be exothermic by 39 kcal/mol.
All Science Journal Classification (ASJC) codes
- Colloid and Surface Chemistry