A Reappraisal of the S2 State of Nature’s Water Oxidizing Complex in Its Low and High Spin Forms

Density functional theory calculated ¹⁴N hyperfine couplings are obtained for the Mn1 ligated π-N of residue His332 of the photosystem 2 water oxidizing complex. An open cubane, O4H, model closely matches the experimental coupling obtained for the high spin S = 5/2 form of the S ₂ state, supporting an open cubane structure for this state. We also investigate the unusual geometric features for the S ₂ state obtained by X-ray free electron laser structure determinations and rationalize it as an equilibrium occurring at room temperature between W1/O4 deprotonated and protonated forms of the open cubane structure.