Photosystem II (PSII) is a protein-pigment complex responsible for production of oxygen in higher plants, algae, and cyanobacteria. Since elucidation of the PSII water-splitting mechanism opens a way to create efficient homogeneous and heterogeneous water-oxidation catalysts, the information on various cofactors binding is of extreme importance. Ammonia binds directly to the oxygen-evolving complex (OEC) of PSII upon formation of the S2 intermediate, as evidenced by electron paramagnetic resonance spectroscopy. We explore the binding mode by using quantum mechanics/molecular mechanics methods and simulations of extended X-ray absorption fine structure spectra. We find that NH3 binds as an additional terminal ligand to the dangling Mn4, instead of exchanging with terminal water. Because water and ammonia are electronic and structural analogues, these findings suggest that water binds analogously during the S2 → S3 transition, leading to rearrangement of ligands in a carrousel around Mn4. We therefore propose a model of native S3 state and show that it is consistent with XRD and EXAFS data.

NH3 binding to the S2 state of the O2-evolving complex of Photosystem II points at H2O binding during the S2 → S3 transition: Insights from QM/MM, EXAFS, and femtosecond x-ray diffraction

253rd National Meeting (2017)

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