Abstract
The initial reaction of photosynthesis takes place in photosystem II（PSII）, a 700 kDa membrane protein complex that catalyzes water-splitting reaction 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 Å resolution, which revealed that the OEC is a Mn4CaO5 cluster coordinated by a well-defined protein environment. However, extended X-ray absorption fine structure（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 determined by XRD and EXAFS studies. Very recently, it was demonstrated that radiation damage free structure can be obtainable using X-ray free electron lasers（XFEL）. In this paper, we report a radiation damage free structure of PSII in the S1 state at a resolution of 1.95 Å using femtosecond X-ray pulses of the SACLA facility. Compared with the structure from XRD, the OEC in the XFEL structure has Mn-Mn distances that are shorter by 0.1～0.2 Å. Based on the XFEL structure, the valences of each manganese atom were tentatively assigned as Mn1（＋3）, Mn2（＋4）, Mn3（＋4）and Mn4（＋3）in the S1 state and that O5 is a hydroxide ion and may serve as one of the substrate oxygen atoms. These findings provide a structural basis for the mechanism of oxygen evolution.
