Methane monooxygenase (MMO) catalyses the O2-dependent conversion of methane to methanol in methanotrophic bacteria, thereby preventing the atmospheric egress of approximately one billion tons of this potent greenhouse gas annually. The key reaction cycle intermediate of the soluble form of MMO (sMMO) is termed compound Q (Q). Q contains a unique dinuclear Fe IV cluster that reacts with methane to break an exceptionally strong 105 kcal mol -1 C-H bond and insert one oxygen atom. No other biological oxidant, except that found in the particulate form of MMO, is capable of such catalysis. The structure of Q remains controversial despite numerous spectroscopic, computational and synthetic model studies. A definitive structural assignment can be made from resonance Raman vibrational spectroscopy but, despite efforts over the past two decades, no vibrational spectrum of Q has yet been obtained. Here we report the core structures of Q and the following product complex, compound T, using time-resolved resonance Raman spectroscopy (TR 3). TR 3 permits fingerprinting of intermediates by their unique vibrational signatures through extended signal averaging for short-lived species. We report unambiguous evidence that Q possesses a bis-μ-oxo diamond core structure and show that both bridging oxygens originate from O 2. This observation strongly supports a homolytic mechanism for O-O bond cleavage. We also show that T retains a single oxygen atom from O2as a bridging ligand, while the other oxygen atom is incorporated into the product. Capture of the extreme oxidizing potential of Q is of great contemporary interest for bioremediation and the development of synthetic approaches to methane-based alternative fuels and chemical industry feedstocks. Insight into the formation and reactivity of Q from the structure reported here is an important step towards harnessing this potential.
|Original language||English (US)|
|Number of pages||4|
|State||Published - Feb 19 2015|
Bibliographical noteFunding Information:
Acknowledgements We thank G. T. Babcock (deceased), S.-K. Lee and J. C. Nesheim (deceased) for initial studies that led to this project and E. Bergeron for technical assistance. This work was supported by the NIH grants GM40466 and GM100943 (to J.D.L.) and grant GM096132 (to D.A.P.).
© 2015 Macmillan Publishers Limited.