Conversion of methane to methanol on diiron and dicopper enzyme models of methane monooxygenase: A theoretical study on a concerted reaction pathway

We present theoretical analyses for the conversion of methane to methanol on a diiron model of soluble methane monooxygenase (sMMO) and on dicopper models of particulate methane monooxygenase (pMMO) using the hybrid density-functional-theory B3LYP method. Methane is proposed to be reasonably converted into methanol in a two-step concerted manner on the dinuclear enzyme models. The first step in our proposal is concerted H atom abstraction of methane via a four-centered transition state (TS1) and the second step is concerted methyl migration via a three-centered transition state (TS2). The general features of the electronic process are identical to those of the gas- phase process for the methane-methanol conversion by the bare FeO⁺ complex. The concerted H atom abstraction and the direct H atom abstraction via a transition state with a linear C-H-O(Fe) array are compared using the dinuclear models. The transition state for the direct H atom abstraction (TSd) on the diiron model is found in the spin undecet state; however, that on the dicopper models is found in the doublet state. Kinetic isotope effects (k(H)/k(D)) are calculated and analyzed for the concerted and the direct H atom abstraction mechanisms using the transition state theory. Calculated k(H)/k(D) values for the concerted process and the direct process are 9 and 14, respectively, at 300 K.