Structure of the oxygen evolving complex of Photosystem II at room temperature.

The functional structure of the Oxygen Evolving Complex, the Mn4Ca cluster of Photosystem II, a critical component of natural photosynthesis, was analyzed at room temperature by EXAFS. An experimental improvement in the form of a spinning sample holder allowed us to efficiently counteract the severe X-ray damage sensitivity of Photosystem II to obtain high quality data subsequently used for a systematic evaluation of atomistic S1 state models. We investigated the accuracy of models collected by both conventional and femtosecond XRD at 1.9 and 1.95 Å resolution, respectively, as well as DFT-based models, to determine the structure most representative of experimental data. Additionally, room temperature EXAFS results do not show a visible reduction in the intensity of the k-space oscillations, supporting a rigid structure of the Mn4Ca cluster at room temperature.