Snapshot of photosynthesis reveals process

Femtosecond snapshots of photosynthetic water oxidation have enabled researchers to understand how the process works. Using the world’s most powerful x-ray laser at the SLAC National Accelerator Laboratory researchers were able to take detailed images of the four photo-step cycle for water oxidation in photosystem II, the only known biological system to harness sunlight for photosynthesis. The work, published in Nature Communications, could help advance the development of artificial photosynthesis for clean, green renewable energy. “An effective method of solar-based water-splitting is essential for artificial photosynthesis to succeed but developing such a method has proven elusive,” said Vittal Yachandra from the Lawrence Berkeley National Laboratory. Using femtosecond x-ray diffraction (XRD) and x-ray emission spectroscopy (XES) data at room temperature researchers have gone around the four-step catalytic cycle of photosynthetic water oxidation in photosystem II, Yachandra said. “This represents a major advance towards the real-time characterisation of the formation of the oxygen molecule in photosystem II, and has yielded information that should prove useful for designing artificial solar-energy based devices to split water.” Photo-oxidation of water by photosystem II is responsible for most of the oxygen in Earth’s atmosphere – at its core is a manganese-calcium (Mn4Ca) metalloenzyme complex that, when excited by solar photons, catalyses the four photon-step cycle of oxidation states from S0 to S3 to give molecular oxygen. “We report data from the S3 (2-flash) and S0 (3-flash) states, which are the intermediate states directly before and after the evolution of the oxygen molecule,” said Junko Yano, a chemist at Berkeley Lab’s Physical Biosciences Division. “In addition, we report data for the first time from a light-induced transient state between the S3 and S0 states, which opens the window for elucidating the mechanism of oxygen-oxygen bond formation that occurs between these two states.” XRD data revealed an anomalous diffraction signal from MN that is uncomplicated by signals from the overall protein matrix, or the Ca atom it forms a complex with. Researchers believe this detection opens up the possibility for detecting changes relating only to the Mn cluster as it advances through the S-state cycles and oxygen-oxygen bond formation, which is where the catalytic action is taking place. “Knowing how this happens is important for understanding the design principles used in natural photosynthesis,” Yachandra said. Taking snapshots of photosynthetic water oxidation using femtosecond x-ray diffraction and spectroscopy