CONSPECTUS: Efficient photoelectrochemical water oxidation may open a way to produce energy from renewable solar power. In biology, generation of fuel due to water oxidation happens efficiently on an immense scale during the light reactions of photosynthesis. To oxidize water, photosynthetic organisms have evolved a highly conserved protein complex, Photosystem II. Within that complex, water oxidation happens at the CaMn4O5 inorganic catalytic cluster, the so-called oxygen-evolving complex (OEC), which cycles through storage "S" states as it accumulates oxidizing equivalents and produces molecular oxygen. In recent years, there has been significant progress in understanding the OEC as it evolves through the catalytic cycle. Studies have combined conventional and femtosecond X-ray crystallography with extended X-ray absorption fine structure (EXAFS) and quantum mechanics/molecular mechanics (QM/MM) methods and have addressed changes in protonation states of μ-oxo bridges and the coordination of substrate water through the analysis of ammonia binding as a chemical analog of water. These advances are thought to be critical to understanding the catalytic cycle since protonation states regulate the relative stability of different redox states and the geometry of the OEC. Therefore, establishing the mechanism for substrate water binding and the nature of protonation/redox state transitions in the OEC is essential for understanding the catalytic cycle of O2 evolution. The structure of the dark-stable S1 state has been a target for X-ray crystallography for the past 15 years. However, traditional Xray crystallography has been hampered by radiation-induced reduction of the OEC. Very recently, a revolutionary X-ray free electron laser (XFEL) technique was applied to PSII to reveal atomic positions at 1.95 Å without radiation damage, which brought us closer than ever to establishing the ultimate structure of the OEC in the S1 state. However, the atom positions in this crystal structure are still not consistent with high-resolution EXAFS spectroscopy, partially due to the poorly resolved oxygen positions next to Mn centers and partial reduction due to extended dark adaptation of the sample. These inconsistencies led to the new models of the OEC with an alternative low oxidation state and raised questions on the protonation state of the cluster, especially the O5 μ-oxo bridge. This Account summarizes the most recent models of the OEC that emerged from QM/MM, EXAFS and femtosecond X-ray crystallography methods. When PSII in the S1 state is exposed to light, the S1 state is advanced to the higher oxidation states and eventually binds substrate water molecules. Identifying the substrate waters is of paramount importance for establishing the water-oxidation mechanism but is complicated by a large number of spectroscopically similar waters. Water analogues can, therefore, be helpful because they serve as spectroscopic markers that help to track the motion of the substrate waters. Due to a close structural and electronic similarity to water, ammonia has been of particular interest. We review three competing hypotheses on substrate water/ammonia binding and compile theoretical and experimental evidence to support them. Binding of ammonia as a sixth ligand to Mn4 during the S1 ? S2 transition seems to satisfy most of the criteria, especially the most compelling recent EPR data on D1-D61A mutated PSII. Such a binding mode suggests delivery of water from the "narrow" channel through a "carousel" rearrangement of waters around Mn4 upon the S2 ? S3 transition. An alternative hypothesis suggests water delivery through the "large" channel on the Ca side. However, both water delivery paths lead to a similar S3 structure, seemingly reaching consensus on the nature of the last detectable S-state intermediate in the Kok cycle before O2 evolution.
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