Calcium Valence-to-Core X-ray Emission Spectroscopy of the Oxygen-Evolving Complex of Photosystem II

The oxygen-evolving complex (OEC) of Photosystem II (PSII) is the active site of biological water-splitting and one of the most important structures in nature. Advances in theory, spectroscopy and biomimetic synthesis have revealed much of its chemistry, but key details remain unresolved. Last year, we introduced calcium valence-to-core X-ray emission spectroscopy (Ca VtC XES; 4030 eV) and detected subtle changes in Ca coordination environments in a series of synthetic models of the OEC. Here, we propose to use the new tender X-ray spectrometer at ID26 to perform Ca VtC XES on PSII. By correlating spectra for two catalytic states to extensively-validated computational models, we hope to further our understanding of OEC catalysis, while developing the practical knowledge required for future studies of the complete catalytic cycle in this little-studied energy regime. To our knowledge, ID26 offers the only apparatus presently capable of performing Ca VtC XES on dilute biological systems.