NH₃ Binding to the S₂ State of the O₂‑Evolving Complex of Photosystem II: Analogue to H₂O Binding during the S₂ → S₃ Transition

Ammonia binds directly to the oxygen-evolving complex of photosystem II (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.