Photosynthesis is a crucial process for sustaining life on earth, but many aspects of it remain a mystery. Scientists have been working to unlock the secrets of photosynthesis for years, and now researchers from the Department of Energy’s Lawrence Berkeley National Laboratory and SLAC National Accelerator Laboratory have succeeded in cracking a key secret of photosystem II, a protein complex in plants, algae and cyanobacteria.

Using SLAC’s Linac Coherent Light Source (LCLS) and the SPring-8 Angstrom Compact free electron LAser (SACLA) in Japan, the researchers have captured, for the first time in atomic detail, what happens in the final moments leading up to the release of breathable oxygen. The data reveal an intermediate reaction step that had not been observed before.

The findings, published in Nature, will help scientists develop artificial photosynthetic systems that mimic photosynthesis to harvest natural sunlight to convert carbon dioxide into hydrogen and carbon-based fuels. “The more we learn about how nature does it, the closer we get to using those same principles in human-made processes, including ideas for artificial photosynthesis as a clean and sustainable energy source,” said co-author Jan Kern, a scientist at Berkeley Lab.

During photosynthesis, photosystem II’s oxygen-evolving center facilitates a series of challenging chemical reactions that act to split apart a water molecule to release molecular oxygen. The center cycles through four stable oxidation states, known as S0 through S3, when exposed to sunlight. In their experiments, the researchers probed this center by exciting samples from cyanobacteria with optical light and then probing them with ultrafast X-ray pulses from LCLS and SACLA. The data revealed the atomic structure of the cluster and the chemical process around it.

Using this technique, the scientists for the first time imaged the transient state, or S4, where two atoms of oxygen bond together and an oxygen molecule is released. The data showed that there are additional steps in this reaction that had never been seen before. “It’s really going to change the way we think about photosystem II,” said co-author Uwe Bergmann, a scientist and professor at the University of Wisconsin-Madison.

The researchers plan to conduct more experiments to capture many more snapshots of the process. “There are more snapshots we really want to take which would bridge the remaining gaps and tell the whole story,” said co-author Jan Kern. To do so, they need to push the quality of their data even further. When an LCLS upgrade, called LCLS-II, comes online later this year, the repetition rate will skyrocket from 120 pulses per second to up to a million per second. “These new capabilities will continue to drive this research forward and shed new light on photosynthesis,” said Bergmann.

Chemistry

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