A radical rebound mechanism for the methane oxidation reaction promoted by the dicopper center of a pMMO enzyme: a computational perspective

Abstract

In this article, we investigated the hydroxylation of methane catalyzed by the binuclear copper site of a pMMO enzyme, through a radical rebound mechanism. All intermediates and transition states along the reaction coordinate were located and the energies involved in the mechanism calculated using the B3LYP functional including dispersion effects. Our B3LYP-D2 results show that the singlet state of the (μ-1,2-peroxo)Cu(II)₂ complex plays an important role as the lowest energy species prior to C–H bond activation. A crossing between the singlet and triplet PES is suggested to occur before the cleavage of the C–H bond of methane, where the triplet (bis-μ-oxo)Cu(III)₂ is very reactive towards activation of the strong C–H bond of methane. The C–H bond activation is the rate-determining step of the reaction, with an activation energy of 18.6 kcal mol⁻¹ relative to the singlet (μ-1,2-peroxo)Cu(II)₂ species. Comparison with previous theoretical results for a non-synchronous concerted mechanism suggests the radical rebound mechanism as a possible alternative pathway.