Structural Changes in the Oxygen-Evolving Complex of Photosystem II Induced by the S(1) to S(2) Transition: A Combined XRD and QM/MM Study

[Image: see text] The S(1) → S(2) transition of the oxygen-evolving complex (OEC) of photosystem II does not involve the transfer of a proton to the lumen and occurs at cryogenic temperatures. Therefore, it is commonly thought to involve only Mn oxidation without any significant change in the structure of the OEC. Here, we analyze structural changes upon the S(1) → S(2) transition, as revealed by quantum mechanics/molecular mechanics methods and the isomorphous difference Fourier method applied to serial femtosecond X-ray diffraction data. We find that the main structural change in the OEC is in the position of the dangling Mn and its coordination environment.