Work by Stanford chemical engineers could lead to greener fuels, fertilizers and petrochemicals
Making hydrogen easily and cheaply is a dream goal for clean, sustainable energy. Bacteria have been doing exactly that for billions of years, and now chemists at the University of California, Davis, and Stanford University are revealing how they do it, and perhaps opening ways to imitate them.
A study published Oct. 25 in the journal Science describes a key step in assembling the hydrogen-generating catalyst.
"It's pretty interesting that bacteria can do this,” said David Britt, professor of chemistry at UC-Davis and co-author of the paper. “We want to know how nature builds these catalysts. From a chemist's perspective, these are really strange things.”
The results reported in Science build on years of chemical engineering and bioengineering groundwork.
“Our lab here at Stanford has spent a decade working with these enzymes, and this allowed us to produce the samples required for these studies,” said James Swartz, the James H. Clark Professor in the School of Engineering at Stanford, and a member of the research team.
The bacterial catalysts are based on precisely organized clusters of iron and sulfur atoms, with side groups of cyanide and carbon monoxide. Those molecules are highly toxic unless properly controlled, Britt noted.
The cyanide and carbon monoxide groups were known to come from the amino acid tyrosine, Britt said. Jon Kuchenreuther, a postdoctoral researcher in Britt's laboratory, used a technique called electron paramagnetic resonance to study the structure of the intermediate steps.
The researchers found a series of chemical reactions involving a type of highly reactive enzyme called a radical SAM enzyme. The tyrosine is attached to a cluster of four iron atoms and four sulfur atoms, then cut loose leaving the cyanide and carbon monoxide groups behind.
“People think of radicals as dangerous, but this enzyme directs the radical chemistry, along with the production of normally poisonous CO and CN, along safe and productive pathways,” Britt said.
Kuchenreuther, Britt and colleagues also used another technique, Fourier Transform Infrared to study how the iron-cyanide-carbon monoxide complex is formed. That work will be published separately.
“Together, these results show how to make this interesting two-cluster enzyme,” Britt said. “This is unique, new chemistry.”
Swartz said his Stanford lab plans to use the insights gained in these experiments to develop new ways to produce hydrogen for use in fuels, fertilizers and petrochemicals. Industrial hydrogen is produced today through processes that involve the use of methane. Swartz said roughly 1 to 2 percent of the carbon dioxide vented into the atmosphere worldwide results from these methane-based processes.
He and his students are working on two alternatives that would employ these enzymes to make hydrogen with far less carbon dioxide residue. One alternative would produce hydrogen through a series of processes starting with the waste material from plants. The other is an ongoing effort to bioengineer a photosynthetic bacterium to be capable of using these enzymes to convert sunlight and water into hydrogen and oxygen.
The work was supported by grants from the U.S. Department of Energy.
Other authors on the paper are UC-Davis postdoctoral researchers William Myers and Troy Stich, project scientist Simon George and graduate student Yaser NejatyJahromy.
Andy Fell is a science writer for the UC Davis News Service
Tom Abate is the Associate Director of Communications for Stanford Engineering
