Fake Photosynthesis? Yale Chemists Develop Test-tube System That Sheds Light on How Plants Turn Water into Oxygen

– With a little manganese and some household bleach, Yale University chemists have developed a test-tube system that mimics for the first time the way green plants harness sunlight to turn water into oxygen, according to a report in the March 5 issue of the journal Science.

The discovery could help scientists better understand the complex process of photosynthesis and might even lead to ways to generate electrical power more efficiently from solar energy, although the scientists caution that practical applications may be years away.

“All the oxygen we breathe was made by plants and certain bacteria. Exactly how nature makes 02 molecules from water is an amazingly complex problem, and we think our test-tube model will make it much easier to study,” said graduate student Julian Limburg, who designed the system with fellow graduate student John S. Vrettos under the direction of professors Robert H. Crabtree and Gary W. Brudvig. University of Delaware scientists confirmed the three-dimensional structure of the manganese molecules.

“Artificial photosynthesis has long been a goal of researchers,” said Brudvig, who has been working on the problem for 15 years. “We can’t hope to perform photosynthesis as efficiently as nature does it, but we might learn to aid the process, to supply what is missing. For example, we might be able to make plants more shade tolerant.”

Photosynthesis is a two-stage process that converts water and carbon dioxide in the presence of sunlight into excess oxygen, as well as the sugar and carbohydrates used as food by the plant.

Scientists have a good understanding of one stage of the process, in which the carbon dioxide is broken down to produce carbon and oxygen. But the splitting of oxygen from water has been “one of the most poorly understood of biological processes, considering its major impact,” said Brudvig, who earlier discovered five changes that manganese molecules undergo as they act as a catalyst to split oxygen from water. His photosynthesis research was carried out in spinach plants.

To split 02 molecules from water with fake photosynthesis, the Yale research team needed a simpler, synthetic chemical structure to trigger the reaction. Their solution was a cluster of two metal atoms (manganese) activated by bleach. This oxygen-evolving complex is patterned after a naturally occurring four-manganese cluster known to play a role in photosynthesis, said Limburg in describing the design of his Photosystem II (PSII) device.

After the Yale scientists identified a synthetic complex that seemed to work, they sent their crystals to University of Delaware chemist Arnold L. Rheingold, who determined the extent to which the synthetic manganese complex resembled its natural cousin.

In Rheingold’s lab, a single crystal, when bombarded by a beam of X-rays, produced a pattern of scattered beams that reflects the arrangement of atoms in the crystallized molecules. Using a computer, the researchers could then position the atoms to create a color-coded “map” of the molecular architecture of the Yale sample, Rheingold said.

“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,” said Rheingold, who collaborated on the X-ray crystallography analysis of the ions with his graduate student Louise M. Liable-Sands. The structure closely resembled that predicted by the Yale chemists.

In industrial settings, oxygen can already be mass-produced by fractionating liquid air, but next-generation solar power will require more efficient water-splitting techniques, Rheingold said, so the artificial photosynthesis system may suggest new strategies for converting sunlight into electricity.