Research Settles Debate on How Methane Forms
Understanding how methane forms could improve catalysts for fuel production.
Methanogenic archaea produce more than 90 percent of Earth’s atmospheric methane, totaling more than 1 billion tons of methane per year globally. A new study settles a longstanding debate on how this important fuel and powerful greenhouse gas is generated.
By identifying the key intermediate involved in methane formation, this study could lead to the development of better catalysts for fuel production as well as strategies to inhibit microbial production of a potent greenhouse gas.
The mechanism of methane formation puzzled scientists for years, mainly because intermediates in the catalytic cycle had not been identified. The enzyme that catalyzes the chemical step of methane synthesis or oxidation is methyl-coenzyme M reductase (MCR). Two proposed mechanisms for how methane is generated differ in whether the first step in the MCR catalytic reaction involves an organometallic methyl-nickel(III) or a methyl radical intermediate. A third mechanism involving methyl anion and Ni(III)-SCoM species also is possible. All three mechanisms propose formation of distinct intermediates. To uncover the true MCR mechanism, researchers from the University of Michigan, Ann Arbor, and Pacific Northwest National Laboratory combined rapid kinetic studies and spectroscopic approaches with high-performance computing resources at the Environmental Molecular Sciences Laboratory (EMSL), a U.S. Department of Energy scientific user facility, and the National Energy Research Scientific Computing Center located at Lawrence Berkeley National Laboratory. Their rapid kinetic studies revealed no evidence for a methyl-Ni(III) species proposed by the first mechanism. Rather, spectroscopic results provided direct evidence that Ni(II)-thiolate and methyl radical intermediates proposed in the second potential mechanism are key intermediates in methane formation. Moreover, computational analyses revealed the formation of the methyl radical intermediate is thermodynamically favored. Temperature-dependent transient kinetics also closely matched density functional theory predictions of the methyl radical mechanism. Additional calculations ruled out formation of a methyl anion intermediate proposed by the third mechanism. Taken together, the findings provide clear support for a methyl radical–based mechanism of methane formation. Furthermore, the findings have broad applicability for developing technologies to make and activate methane for alternative fuel as well as reducing greenhouse gas warming.
University of Michigan
- Research Settles Debate on How Methane Forms
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Computation at EMSL was supported by the U.S. Department of Energy’s (DOE) Office of Science, Office of Biological and Environmental Research. All other work was supported by DOE’s Office of Science, Office of Basic Energy Sciences and Advanced Research Project Agency–Energy.
Wongnate, T., D. Sliwa, B. Ginovska, D. Smith, M. W. Wolf, N. Lehnert, S. Raugei, and S. W. Ragsdale. 2016. “The Radical Mechanism of Biological Methane Synthesis by Methyl-Coenzyme M Reductase,” Science 352(6288), 953-58. DOI: 10.1126/science.aaf0616.