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 o n a diiron model of soluble methane monooxygenase (sMMO) and on dicopper mo dels of particulate methane monooxygenase (pMMO) using the hybrid density-f unctional-theory B3LYP method. Methane is proposed to be reasonably convert ed into methanol in a two-step concerted manner on the dinuclear enzyme mod els. The first step in our proposal is concerted H atom abstraction of meth ane via a four-centered transition state (TS1) and the second step is conce rted methyl migration via a three-centered transition state (TS2). The gene ral features of the electronic process are identical to those of the gas-ph ase process for the methane-methanol conversion by the bare FeO+ complex. T he concerted H atom abstraction and the direct H atom abstraction via a tra nsition state with a linear C-H-O(Fe) array are compared using the dinuclea r models. The transition state for the direct H atom abstraction (TSd) on t he diiron model is found in the spin undecet state; however, that on the di copper models is found in the doubler stare. Kinetic isotope effects (k(H)/ k(D)) are calculated and analyzed for the concerted and the direct H atom a bstraction mechanisms using the transition state theory. Calculated k(H)/k( D) values for the concerted process and the direct process are 9 and 14, re spectively, at 300 K.