Methane-To-Methanol Converter’s Diamond Core Clarified

The catalytic intermediate of an enzyme that microbes use to convert methane to methanol has a diiron site bridged by two oxygen atoms that originate from O₂, report Rahul Banerjee and John D. Lipscomb of the University of Minnesota, Twin Cities, and Yegor Proshlyakov and Denis A. Proshlyakov of Michigan State University (Nature 2015, DOI: 10.1038/nature14160). The structure of the soluble methane monooxygenase intermediate, known as compound Q, has long been debated. A better understanding of Q and how it does its chemistry could point researchers toward more efficient ways of converting waste methane into a liquid fuel and chemical feedstock. The two popular candidates for Q’s structure have been the doubly bridged “diamond core” and an “open core,” with one bridging oxygen and one terminal oxygen (Fe⁴⁺=O). In the new work, Banerjee and colleagues used a continuous-flow system to collect resonance Raman spectra of Q formed through reactions of the enzyme with ¹⁶O₂, ¹⁸O₂, and ¹⁶O¹⁸O. The results indicate that Q has a diamond core formed from homolytic cleavage of O₂ to provide the bridging ligands. Monitoring the reaction further, the researchers determined that a subsequent species, T, retains a single oxygen as a bridge while the other oxygen is transferred to methanol.