Suga, M. Ago, H. Akita, F. Hirata, K. Murakami, H. Nakajima, Y. Shen, J.R. Shimizu, T. Ueno, G. Yamamoto, M. Yamashita, K. FRA01 2015 2015-12 The initial reaction of photosynthesis takes place in Photosystem II (PSII), a 700kD membrane protein complex which 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 XRD at 1.9Å resolution, which revealed the OEC is a Mn4CaO5-cluster*. However, EXAFS studies showed that the manganese atoms in the OEC are easily reduced by X-ray irradiation, and slight differences were found in the Mn-Mn distances between the results of XRD, EXAFS and theoretical studies. We present a radiation-damage-free structure of PSII from Thermosynechococcus vulcanus in the S1 state at 1.95Å resolution using femtosecond X-ray pulses of SACLA. Compared with the structure from XRD, the OEC in the XFEL structure has Mn-Mn distances that are shorter by 0.1-0.2Å. The valences of each manganese atom were assigned as Mn1D(III), Mn2C(IV), Mn3B(IV) and Mn4A(III). One of the oxo-bridged oxygens, O5, has significantly longer Mn-O distances in contrast to the other oxo-oxygen atoms, suggesting that it is a hydroxide ion instead of a normal oxygen dianion and therefore may serve as one of the substrate oxygen atoms**.