The tetranuclear Mn complex in photosystem II functions to accumulate oxidizing equivalents and to catalyze the four-electron oxidation of water to molecular oxygen. In the water-oxidation cycle, the Mn complex is advanced through five intermediate oxidation states called S(i) states (i = 0-4), where i denotes the number of stored oxidizing equivalents. Mechanistic studies of the photosynthetic water oxidation process have been aimed at characterizing the structure and properties of the Mn complex in each of the different S states. Electron paramagnetic resonance and X-ray absorption spectroscopies have been especially powerful methods to probe the Mn complex. However, to use these spectroscopic methods, it is necessary to prepare highly concentrated samples in a specific S state. The photochemical methods used to prepare the different S states are described and the results of studies of the S1 and S2 states are summarized. The structure of the Mn complex is considered in light of recent studies.
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