Possible intermediates for the conversion of methane to methanol on dinuclear iron centers of methane monooxygenase models

Possible binding modes of dioxygen to the dinuclear non-heme iron centers of methane monooxygenase (MMO) and their reactivity with methane are examined theoretically, using the approximate extended Hückel method. The μ-η¹:η¹ mode is the most favorable binding mode of dioxygen to the supposed active site, one where an exogenous bridging ligand is removed from a model complex for the crystallographically observed structure. This binding mode corresponds to the core structure of the first intermediate P in the catalytic cycle of MMO. The role of diiron(III) peroxides in the catalytic cycle for the enzymatic function of MMO is discussed. Symmetrical, locally six-coordinated diiron(III) peroxides with a μ-η¹:η¹-O₂^2- or μ-η²:η^2-O₂^2- binding mode (and their related O⋯O nonbonded dioxides) appear in our calculations to have no direct reactivity with methane. We show that methane in theory can be activated or coordinated if a less symmetric five-coordinate iron is generated among the dinuclear iron centers of an MMO model. A C_3v distortion of methane, as suggested by Shestakov and Shilov, also appears to play a role. Intermediate Q, which can interact directly with methane, is proposed by us to contain a five-coordinate iron as well as an active ferryl species in its structure.