The nonheme diiron toluene/o-xylene monooxygenase (ToMO) is the most studied toluene monooxygenase that mediates an aromatic hydroxylation reaction. In this work, QM/MM calculations were performed to understand the reaction mechanism. It is revealed that the μ-η :η peroxodiferric species is the reactive intermediate after the binding of the O molecule to the reduced diferrous center. Subsequently, both a stepwise and a concerted mechanism involving the critical O-O bond cleavage and C-O bond formation were considered. The latter was calculated to be more favorable, suggesting that the formation of a high-valent diferryl Q intermediate is not needed. The isomeric formation of the phenol product was found very facile. The first step was calculated to be rate-limiting, with a barrier of 17.6 kcal/mol for the ortho-hydroxylation.
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