|Time-resolved Serial Femtosecond Protein Crystallography - a step toward elucidating water splitting reaction|
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Arizona State University
Photosystem II (PSII) is the most important enzyme for oxygen production in the atmosphere from planet Earth. The light-driven water splitting in PSII involves a multi-step reaction, where the Oxygen-Evolving Complex (OEC) of PSII cycles through 5 different states, S0 to S4. We carried out time-resolved serial femtosecond crystallography (SFX) experiments  at the free-electron laser LCLS on nano- to micron-sized PSII crystals, injected in a liquid stream at room temperature. The higher S-states were reached by exciting the PSII crystals with two flashes of a visible laser. The undamaged x-ray structure of the dark state (S1) of PSII at 5.0 Å and the first structure of the double excited state (S3), at 5.5 Å, were unraveled based on the analysis of diffraction snapshots from millions of PSII crystals. We discuss the processing of time-resolved SFX data and determination of two different S- states’ x-ray structures. A conformational change of the OEC is observed during the transition from S1 to S3, consistent with the computational model for the higher S- state conformation.