New, detailed Snapshots capture Photosynthesis at Room Temperature

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New, detailed snapshots capture photosynthesis at room temperature

A femtosecond X-ray pulse from an X-ray free electron laser intersecting a droplet that contains photosystem II crystals, the protein extracted and crystallized from cyanobacteria. Credit: SLAC National Accelerator Laboratory

The living machinery of photosynthesis is still not fully understood. One of its molecular mysteries involves how a protein complex, photosystem II, harvests energy from sunlight and uses it to split water into hydrogen and oxygen. This process generates the oxygen in the air that we all breathe. New X-ray methods at the DOE SLAC National Accelerator Laboratory have captured the highest resolution room-temperature images of this protein complex, which allows scientists to closely watch how water is split during photosynthesis at ambient temperature. The research team took diffraction images using the bright, fast pulses of Xrays at SLAC’s Xray free-electron laser – the Linac Coherent Light Source (LCLS), a DOE Office of Science User Facility.

Previously, the resting state of photosystem II had been seen in detail using samples that were frozen. In this latest study, the researchers were able to see 2 key steps in photosynthetic water splitting under conditions as it occurs in nature, a big step to decoding how the process works in detail. A damage-free, room temperature study means there are fewer artifacts in the images. Understanding this process might help develop ways to create artificial photosynthesis devices that can serve as potential clean energy sources.

“The eventual goal is to emulate what photosynthesis has been doing for about three billion years. This has been a research challenge for decades,” said Junko Yano, principal investigator and senior scientist at Lawrence Berkeley National Laboratory. “We now have the right tool, the femtosecond X-ray laser pulses created at LCLS, that allows us to observe the water-splitting reaction as it happens, in real time, and as it happens in nature.” “We want to have enough snapshots of this process to see how exactly the oxygen is formed,” added Uwe Bergmann, a distinguished scientist at SLAC.

“The beauty of the LCLS is that the laser pulses are so short – only 40 femtoseconds in duration, but very intense – that you can collect the data before the sample is destroyed,” said Jan Kern, scientist at Berkeley Lab and SLAC. “It’s very new, and there are only two places in the world, LCLS and the SPring-8 Angstrom Compact free electron LAser (SACLA), where this can be done at present.”

Watching how plants make oxygen

The structure of the oxygen evolving complex in photosystem II in a light-activated state. Water molecules are shown as blue spheres, the four manganese ions in purple, the calcium ion in green and the bridging oxygen ions in red. The blue mesh is the experimental electron density, and the blue sticks are the protein side chains holding the catalytic complex. Credit: Johannes Messinger

The scientists placed droplets of the sample in a solution with small, crystallized forms of the photosystem II on a moving conveyor belt and illuminated the samples with pulses of green light from a laser to initiate the water-splitting reaction. After two light pulses, they captured images of the crystals using X-rays, with a resolution finer than 2.5 angstroms – significantly better than previously achieved at room temperature.

The water-splitting reaction takes place at a metal catalyst within the photosystem II protein, known as the oxygen-evolving complex, that is made up of 4 Mn atoms and one Ca atom. The complex uses the energy from light to form pure oxygen from 2 water molecules. The 4 manganese atoms are critical in shuffling electrons through the cycle, but it is unknown where exactly in the complex the involved water is located or where the oxygen formation occurs.

To sort this out, they used ammonia, a water substitute, to narrow down where oxygen atoms from 2 water molecules combine to form an oxygen molecule. If the ammonia was bound to a site, and the reaction still proceeded, that site is unlikely to be part of the oxygen molecule formation. The results from this study offered a surprise – the data do not seem to support 2 leading theories for how the reaction proceeds within the oxygen-evolving complex. In future studies they hope to capture more images at different steps of the process, which will allow them to further refine the details of the water-splitting reaction.

“The chemistry is so unusual,” Vittal Yachandra at Berkeley Lab, said “Learning how exactly this water-splitting process works will be a breakthrough in our understanding, and it can help in the development of solar fuels and renewable energy.” https://www6.slac.stanford.edu/news/2016-11-21-new-detailed-snapshots-capture-photosynthesis-room-temperature.aspx