An analysis of the structural changes of the oxygen evolving complex of Photosystem II in the S₁ and S₃ states revealed by serial femtosecond crystallography

Photosystem II (PSII) is a unique natural catalyst that converts solar energy into chemical energy using earth abundant elements in water at physiological pH. Understanding the reaction mechanism will aid the design of biomimetic artificial catalysts for efficient solar energy conversion. The Mn₄O₅Ca cluster cycles through five increasingly oxidized intermediates before oxidizing two water molecules into O₂ and releasing protons to the lumen and electrons to drive PSII reactions. The Mn coordination and OEC electronic structure changes through these intermediates. Thus, obtaining a high-resolution structure of each catalytic intermediate would help reveal the reaction mechanism. While valuable structural information was obtained from conventional X-ray crystallography, time-resolution of conventional X-ray crystallography limits the analysis of shorted-lived reaction intermediates. Serial Femtosecond X-ray crystallography (SFX), which overcomes the radiation damage by using ultra short laser pulse for imaging, has been used extensively to study the water splitting intermediates in PSII. Here, we review the state of the art and our understanding of the water splitting reaction before and after the advent of SFX. Furthermore, we analyze the likely Mn coordination in multiple XFEL structures prepared in the dark-adapted S₁ state and those following two-flashes which are poised in the penultimate S₃ oxidation state based on Mn coordination chemistry. Finally, we summarize the major contributions of the SFX to our understanding of the structures of the S₁ and S₃ states.