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  • X-Ray Laser Observes Photosynthesis in Action
Feb 2013
MENLO PARK, Calif., Feb. 20, 2013 — The structure and chemical behaviors of a natural catalyst involved in photosynthesis were observed simultaneously for the first time, using an x-ray laser. The discovery opens a new window on the way plants generate the oxygen we breathe.

The work, made possible by the ultrafast, ultrabright x-ray pulses at SLAC National Accelerator Laboratory’s Linac Coherent Light Source (LCLS), is a breakthrough in studying atomic-scale transformation in photosynthesis and other biological and industrial processes that depend on catalysts.

“All life that depends on oxygen is dependent on photosynthesis,” said Junko Yano, a Lawrence Berkeley National Laboratory chemist and co-leader in the experiment. “If you can learn to do this as nature does it, you can apply the design principles to artificial systems such as the creation of renewable energy sources. This is opening up the way to really learn a lot about changes going on in the catalytic cycle.”

Artist’s view of a photosystem II crystal hit by a femtosecond x-ray pulse. The resulting x-ray diffraction pattern (left) is used to reconstruct the overall protein structure. The x-ray emission spectrum (right) is simultaneously collected and used to probe the intactness and oxidation state of the crystal's key manganese complex (Mn4Ca). The results are a pioneering experimental technique for studying photosynthesis and other catalytic reactions that use the unique capabilities of the Linac Coherent Light Source, an x-ray free-electron laser. Courtesy of Greg Stewart, SLAC National Accelerator Laboratory.

Catalysts are vital to a variety of industrial processes, including the production of food, fuels, fertilizers and pharmaceuticals. Natural catalysts are also key to the chemistry of life; a major goal of x-ray science is to determine their purpose in photosynthesis, which produces energy and oxygen from sunlight and water.

The LCLS experiment focused on photosystem II, a protein complex in plants, algae and some microbes that carries out the oxygen-producing stage of photosynthesis. This four-step process takes place in a simple catalyst — a cluster of calcium and manganese atoms. In each step, photosystem II absorbs a photon of sunlight and releases a proton and an electron, which provide the energy to link two water molecules, break them apart and release an oxygen molecule.

The experimental chamber with the large access door open. The x-rays travel from the right to the left, and the x-ray spectrometer is located in the front. The sample is injected from the top and illuminated by an optical laser. The detector for the x-ray diffraction is located on the left (outside the view area). Courtesy of R. Alonso-Mori, LCLS.

Previous studies froze crystals of the catalyst at various stages of the process to study how it looked. But investigators wanted to see the chemistry in action. This was not possible at other x-ray facilities because the fragile crystals had to be frozen to protect them from radiation damage.

The LCLS x-ray laser, however, comes in such brief pulses that the researchers could probe the crystals at room temperature in a chemically active state, before any damage set in, and generate data on two of the four steps in oxygen generation.

“We decided to use two x-ray techniques at once at the LCLS: crystallography to look at the overall atomic structure of photosystem II, and spectroscopy to document the position and flow of electrons in the catalyst,” said Vittal Yachandra, a Berkeley Lab chemist and co-leader of the project. “The electrons are important because they are involved in making and breaking bonds and other processes at the heart of chemical reactions.”

“This result is a critical step in the ultimate goal to watch the full cycle of the splitting of water into oxygen during photosynthesis,” said SLAC physicist and co-leader Uwe Bergmann.

The inside of the experimental chamber showing the spectrometer (front) and the detector (black rectangle in the back) used for the detection of the x-ray emission spectra, and the glass fiber used for illumination of the sample (green). The sample is injected from the top, and the x-rays enter the chamber from the right. The detector for the x-ray diffraction is located on the left (outside the view area). Courtesy of D. Schafer, LCLS.

The use of both techniques also verified that the molecular structure of the samples is not damaged during measurement with the LCLS, Bergmann said. “It’s the first time that we have resolved the structure of photosystem II under conditions in which we know for sure that the machinery that does the water splitting is fully intact.”

In future LCLS experiments, the investigators hope to study all the steps carried out by photosystem II in higher resolution, revealing the full transformation of water molecules into oxygen molecules — considered a key to unlocking the system’s potential use in making alternative fuels.

“Getting a few of the critical snapshots of this transition would be the final goal,” said Jan Kern, a chemist who holds a joint position at Berkeley Lab and SLAC. “It would really answer all of the questions we have at the moment about how this mechanism works.”

The findings appear in Science (doi: 10.1126/science.1234273). 

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