Snap shots of one of life's central processes

Human Frontier Science Program provides funding of 900,000 US Dollars in support of international collaboration on photosynthesis.

Photosynthetic water-oxidation is one of the central processes of life on Earth but remains incompletely understood. Now, a German-American team of scientists has set out to observe the intermediate stages of this complex catalytic reaction using ultrashort snap shots taken at light sources including BESSY II in Berlin and the Linac Coherent Light Source at Stanford. To support their efforts, the Human Frontier Science Program has now pledged funding of approximately 900,000 US Dollars for a total three-year period. The team includes HZB physicist Dr. Philippe Wernet, chemist Prof. Dr. Athina Zouni of Humboldt University of Berlin, Dr. Uwe Bergmann of the SLAC National Accelerator Laboratory and Dr. Junko Yano of the Lawrence Berkeley National Laboratory, who is in charge of the project.

In spite of the fact that all aerobic organisms consume oxygen, thankfully, we don't ever run out of it. Because as part of photosynthesis, green plants, algae, and cyanobacteria all assemble carbohydrates from CO₂, water, and sunlight, and in the process once again give off oxygen. Here, splitting of oxygen from water, the central reaction in photosystem II, a membrane bound multi-subunit protein, only becomes possible through the presence of a catalyst, a complex molecule with an Mn₄CaO₅ core. A team of HZB researchers, their Berlin and US colleagues is exploring the reactions involving this natural catalyst. New insights would not just be fundamentally exciting but could potentially also contribute to the storage of solar energy in the form of solar fuels to help solve one of the major challenges in the transitioning energy economy.

At the same time, the team has recently come up with a new approach that goes far beyond conventional X-ray crystallography and spectroscopy at low temperatures. Because as long as the investigations are done at temperatures near absolute zero, they don't even come close to resembling real-life conditions. What is more, the X-rays also damage the catalyst molecules. The intense and ultrashort femtosecond X-ray pulses at the Linac Coherent Light Source, the SLAC National Accelerator Laboratory's free electron laser in Stanford, USA, are capable of collecting data at room temperature and, in the process, detecting signals before the sample is destroyed. "What we're doing here is taking sort of a snap shot of the reaction," explains Philippe Wernet.

The researchers want to study protein structure and reaction dynamics of the Mn₄CaO₅  cluster, specifically while additional light is being absorbed and water is oxidized to yield oxygen. "We're planning a series of time-resolved X-ray diffraction and spectroscopy experiments to examine the reaction at room temperature and in the process image all the various intermediate stages," explains Wernet. The scientists' hope is to glean highly specific insights into the reactions as they take place at the Mn₄CaO₅ cluster and identify the intermediate stages that are necessary for the photosynthetic oxidation of water.

Here, the four experts complement each other beautifully. Junko Yano of the Lawrence Berkeley National Laboratory and chemist Athina Zouni of Humboldt-University Berlin are renowned photosystem II protein spectroscopy and X-ray crystallography experts, respectively. Uwe Bergmann and his team at the Linac Coherent Light Source, USA, will be involved with the spectroscopy instrumentation for taking the snap shots using hard X-rays. HZB's own Philippe Wernet will be examining the time-resolved samples using soft X-rays at BESSY II. Lastly, the joint measurements at the Linac Coherent Light Source will be of central importance.

Human Frontier Science Program