Mar. 8, 1999 A test-tube photosynthesis system--described in the March 5 issue of Science--mimics a metal cluster that helps green plants harness sunlight to turn water into oxygen, says University of Delaware chemist Arnold L. Rheingold, a coauthor of the journal article, who analyzed the Yale University invention.
"We owe our lives to oxygen, and virtually all of the oxygen we breathe is produced by plants and some bacteria, through photosynthesis," says Rheingold, one of the world's 10 most frequently cited chemists. "Yet, our understanding of photosynthesis has been limited by its complexity. This relatively simple, artificial system should shed light on how life-giving oxygen is produced on Earth, which points to our origins and how the atmosphere evolved on our planet."
A UD professor of chemistry and biochemistry, Rheingold doesn't foresee any immediate practical uses for fake photosynthesis. In industrial settings, oxygen can already be mass-produced by fractionating liquid air, he notes. But, he adds, "Next-generation solar power will require more efficient water-splitting techniques," so the artificial photosynthesis system may suggest new strategies for converting sunlight into electricity. And, he says, such fundamental knowledge enhances our appreciation of the natural world.
Rheingold teamed up with UD graduate student Louise M. Liable-Sands to precisely map the molecular structure of the test-tube photosynthesis system, developed by Yale graduate student Julian Limburg, lead author of the Science article, with graduate student John S. Vrettos and Profs. Robert H. Crabtree and Gary W. Brudvig.
"I doubt there's a commercial need for the Yale oxygen-making factory," Rheingold says. "But, it may allow us to understand how our planet came to have oxygen. The early environment on Earth was quite inhospitable. The evolution of photosynthetic plants created oxygen and completely changed the atmosphere on our planet. That's an amazing process. Now, we can more fully explore it!"
Mimicking Nature's magic tricks
To change water into one of its constituent elements, dioxygen (O2), the Yale research team needed a catalyst to trigger the reaction. Their solution was a metal-based, "dioxygen-evolving complex (OEC)"--essentially, a metal cluster of two manganese atoms activated by bleach-which serves as the basis of the Photosystem II (PSII).
The synthetic, metal cluster is patterned after a naturally occurring, four-manganese cluster involved in plant functions, says Rheingold, who describes his X-Ray Crystallography Laboratory as "the Supreme Court of chemistry," where researchers worldwide send samples to be deciphered.
The natural protein is highly complex, Rheingold says, and "it would be hard to use a leaf to make oxygen in the lab." So, the Yale scientists painstakingly developed a simpler, synthetic version, which Rheingold describes as "an individual work of art."
When they identified a synthetic complex that seemed to turn water into oxygen, the Yale scientists sent their crystals to Rheingold, who determined the extent to which they resembled their natural cousins. In Rheingold's lab, a single crystal, when bombarded by a beam of X-rays, produced a pattern of scattered beams related to the arrangement of atoms in the molecules in the crystal. Using a computer, the UD researchers could then position the atoms to create a color-coded "map" of the molecular architecture of the Yale sample, Rheingold explains.
To understand X-ray crystallography, he says, "Think of a mirrored disco ball, hanging over a dance floor, reflecting spots of light onto the surrounding walls." Mapping the spots reveals the shape of the disco ball. Similarly, diffracted X-rays can be analyzed to determine a material's structure.
In the case of the metal cluster in the Yale photosynthesis system, he says, "It was a bit of a miracle that we were able to determine the structure because of the inherent weakness of the reflections of this crystal. The structure was solved largely because of the persistence of my graduate student."
UD web site - http://www.udel.edu/arcade/arnrhein.htm
Yale web site - http://www.chem.yale.edu/~brudvig/
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