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Photosystem II (PSII) catalyzes the light driven oxidation of water into dioxygen, protons and electrons. This reaction takes place at the oxygen evolving complex (OEC) a Mn4CaO5 cluster, through f ...

The photosynthetic water oxidation reaction is energetically demanding and mechanistically complex because of the difficulties in managing the four electron, four proton redox chemistry required for the evolution of molecular oxygen starting from two water molecules. The reaction takes place in Photosystem II (PS II), a multi-subunit membrane protein present in plants, algae, and cyanobacteria. This sunlight-driven reaction is catalyzed by an oxygen-evolving complex (OEC), that consists of an oxo-bridged four Mn and one Ca cluster. O2 is formed and released only after four oxidation equivalents are accumulated at the OEC. The structure of the Mn4CaO5 cluster has been studied by various spectroscopic and diffraction methods. The recent XRD study by Umena et al.[1] has shown the oxo-bridged Mn4Ca cluster structure at 1.9 Å resolution. Based on this high-resolution XRD structure, there have been efforts to obtain chemically optimized structures and structural changes of the Mn4CaO5 cluster during the catalytic cycle using spectroscopic parameters and theoretical approaches. EXAFS spectra of the PS II S states show that the structure of the Mn4CaO5 cluster changes during the catalytic cycle.[2] In particular, the short Mn-Mn distances change in the range of 2.7 to 2.9 Å. Such changes in oxygen-bridged Mn-Mn distances can reflect several chemical parameters; Mn oxidation state changes, protonation state changes of bridging oxygens, ligation modes (e.g. bidentate/monodentate), as well as fundamental changes in geometry. We have also used femtosecond X-ray spectroscopy and crystallography to study the catalytic process of the OEC.[3] The femtosecond X-ray pulses of the free-electron laser allows us to out-run X-ray damage at room temperature, and the time-evolution of the photo-induced reaction can be probed using a visible laser-pump followed by the X-ray-probe pulse. We will discuss a possible water oxidation mechanism based on these results.

Femtosecond-scale XFEL pulses can produce diffraction free from radiation damage, under functional physiological conditions where reaction dynamics can be studied for systems such as photosystem II. However, it has been extremely difficult to derive accurate structure factors from the data since every shot is a still exposure from a distinct specimen. Accuracy can be improved by software methods implemented in the program cctbx.xfel, including optimal indexing and retention of data from multiple lattices, and separate determination of the resolution cutoff for individual lattices. Various techniques can produce well-conforming descriptions of the Bragg spot shape and crystal mosaicity, enabled in part by sub-pixel characterization of the detector geometry. By carefully discriminating between image pixels known to contain diffraction signal and the surrounding pixels containing only background noise, and by extending postrefinement techniques that lead to a better crystal orientation, we derive accurate structure factors with substantially fewer crystal specimen exposures. It is hoped that these developments will make it easier to measure small structure factor differences, such as those from anomalous scattering that will enable the de novo determination of macromolecular structure.