Photosynthesis converts light energy into biologically useful chemical energy vital to life onEarth.The initial reactionof photosynthesis takes place in photosystem II (PSII), a 700-kilodalton homodimeric membrane protein complex that catalyses photo-oxidation of water into dioxygen through an S-state cycle of the oxygen evolving complex (OEC). The structure of PSII has been solved by X-ray diffraction (XRD) at 1.9 a°ngströmresolution, which revealed that theOEC is aMn4CaO5-cluster coordinated by a well defined protein environment1. However, extended X-ray absorption fine structure (EXAFS) studies showed that the manganese cations in the OEC are easily reduced by X-ray irradiation2, and slight differences were found in the Mn-Mn distances determined by XRD1, EXAFS3-7 and theoretical studies8-14.Herewe report a 'radiation-damage-free' structure ofPSII from Thermosynechococcus vulcanus in the S1 state at a resolution of 1.95 a°ngströms using femtosecond X-ray pulses of the SPring-8 ångströmcompact free-electron laser (SACLA) and hundreds of large, highly isomorphousPSII crystals.Compared with the structure from XRD, the OEC in the X-ray free electron laser structure has Mn-Mn distances that are shorter by 0.1-0.2 a°ngströms. The valences of each manganese atom were tentatively assigned as Mn1D(III), Mn2C(IV), Mn3B(IV) andMn4A(III), based on the averageMn-ligand distances and analysis of the Jahn-Teller axis on Mn(III). One of the oxobridged oxygens, O5, has significantly longer distances to Mn than do the other oxo-oxygen atoms, suggesting that O5 is a hydroxide ion instead of a normal oxygen dianion and therefore may serve as oneof the substrate oxygen atoms.These findings provide a structural basis for the mechanism of oxygen evolution, and we expect that this structure will provide a blueprint for the design of artificial catalysts for water oxidation.
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