Rationalizing the Geometries of the Water Oxidising Complex in the Atomic Resolution, Nominal S₃ State Crystal Structures of Photosystem II

Three atomic resolution crystal structures of Photosystem II, in the double flashed, nominal S₃ intermediate state of its Mn₄Ca Water Oxidising Complex (WOC), have now been presented, at 2.25, 2.35 and 2.08 Å resolution. Although very similar overall, the S₃ structures differ within the WOC catalytic site. The 2.25 Å structure contains only one oxy species (O5) in the WOC cavity, weakly associated with Mn centres, similar to that in the earlier 1.95 Å S₁ structure. The 2.35 Å structure shows two such species (O5, O6), with the Mn centres and O5 positioned as in the 2.25 Å structure and O5−O6 separation of ∼1.5 Å. In the latest S₃ variant, two oxy species are also seen (O5, Ox), with the Ox group appearing only in S₃, closely ligating one Mn, with O5−Ox separation <2.1 Å. The O5 and O6/Ox groups were proposed to be substrate water derived species. Recently, Petrie et al. (Chem. Phys. Chem., 2017) presented large scale Quantum Chemical modelling of the 2.25 Å structure, quantitatively explaining all significant features within the WOC region. This, as in our earlier studies, assumed a ‘low’ Mn oxidation paradigm (mean S₁ Mn oxidation level of +3.0, Petrie et al., Angew. Chem. Int. Ed., 2015), rather than a ‘high’ oxidation model (mean S₁ oxidation level of +3.5). In 2018 we showed (Chem. Phys. Chem., 2018) this oxidation state assumption predicted two energetically close S₃ structural forms, one with the metal centres and O5 (as OH⁻) positioned as in the 2.25 Å structure, and the other with the metals similarly placed, but with O5 (as H₂O) located in the O6 position of the 2.35 Å structure. The 2.35 Å two flashed structure was likely a crystal superposition of two such forms. Here we show, by similar computational analysis, that the latest 2.08 Å S₃ structure is also a likely superposition of forms, but with O5 (as OH⁻) occupying either the O5 or Ox positions in the WOC cavity. This highlights a remarkable structural ‘lability’ of the WOC centre in the S₃ state, which is likely catalytically relevant to its water splitting function.