Theory of chemical bonds in metalloenzymes. XIV. Correspondence between magnetic coupling mode and radical coupling mechanism in hydroxylations with methane monooxygenase and related species

Broken-symmetry (BS) and approximate spin-projected (AP) BS hybrid density functional theory (DFT) calculations were performed to elucidate possible mechanisms of hydroxylation reactions of methane and alkanes with soluble methane monooxygenase (sMMO) and related metalloenzymes. The BS HDFT (UB3LYP) method as employed to elucidate electronic and spin structures of the key intermediate "Q" and to locate transition structures for hydroxylation reactions in the lowest-spin (LS) singlet and the highest-spin (HS) states of sMMO. The spin density populations and chemical indices obtained by the BS B3LYP calculations were found to be consistent with orbital interaction models for hydroxylation with MMO. However these indices in turn indicated significant spin contamination errors in the BS LS solution. The elimination of the errors with the AP procedure indeed reduced the barrier height for the recombination step of alkyl and hydroxyl radicals in the pure LS singlet state, leading to a rebound process. Then present computational results indicated that hydroxylation reactions proceed through the continuous diradical (diradicaloid) mechanism without discreet free radical fragments in the pure LS singlet state. The computational results are, respectively, compatible with local singlet (SD) and local triplet (TD) diradical mechanisms for hydroxylation in the LS and HS states; those were already applied to P450 successfully. Thus magnetic (exchange) coupling modes (LS and HS) in MMO, P450 and related metalloenzymes are directly related to local SD and TD mechanisms for hydroxylation, indicating the correspondence between the magnetic coupling mode and the radical reaction mechanism. These theoretical results enable us to examine recent BS hybrid DFT computational results for hydroxylation reactions with sMMO by several groups. Implications of the present theoretical and computational results are also discussed in relation to several experimental aspects of hydroxylation reactions.