Photosynthesis Caught in Action

Green crystals, millionths of a millimeter in size, preserve the molecular structure and activity of photosystem II, the molecule that photoxidizes water into molecular oxygen (Jan Kern / Lawrence Berkeley National Laboratory)

“All life that depends on oxygen is dependent on photosynthesis. 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,” said Dr Junko Yano of the Lawrence Berkeley National Laboratory (LBNL), co-leader of the experiment described in the journal Science.

Catalysts are vital to many industrial processes, such as the production of fuels, food, pharmaceuticals and fertilizers. Natural catalysts are also key to the chemistry of life; a major goal of X-ray science is to learn how they function in photosynthesis, which produces energy and oxygen from sunlight and water.
The 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.
Past studies were able to freeze crystals of the catalyst at various stages of the process and see how it looked. But the team wanted to see the chemistry take place.
The scientists probed 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: crystallography to look at the overall atomic structure of Photosystem II, and spectroscopy to document the position and flow of electrons in the catalyst. The electrons are important because they are involved in making and breaking bonds and other processes at the heart of chemical reactions,” said project co-leader Dr Vittal Yachandra, also of the LBNL.
“This result is a critical step in the ultimate goal to watch the full cycle of the splitting of water into oxygen during photosynthesis. The use of both techniques also verified that the molecular structure of the samples is not damaged during measurement. 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,” said project co-leader Dr Uwe Bergmann of the SLAC National Accelerator Laboratory.
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