Structure, ligands and substrate coordination of the oxygen-evolving complex of photosystem II in the S-2 state: a combined EPR and DFT study

The S-2 state of the oxygen-evolving complex of photosystem II, which consists of a Mn4O5Ca cofactor, is EPR-active, typically displaying a multiline signal, which arises from a ground spin state of total spin S-T = 1/2. The precise appearance of the signal varies amongst different photosynthetic species, preparation and solvent conditions/compositions. Over the past five years, using the model species Thermosynechococcus elongatus, we have examined modifications that induce changes in the multiline signal, i.e. Ca2+/Sr2+-substitution and the binding of ammonia, to ascertain how structural perturbations of the cluster are reflected in its magnetic/electronic properties. This refined analysis, which now includes high-field (W-band) data, demonstrates that the electronic structure of the S-2 state is essentially invariant to these modifications. This assessment is based on spectroscopies that examine the metal centres themselves (EPR, Mn-55-ENDOR) and their first coordination sphere ligands (N-14/N-15- and O-17-ESEEM, -HYSCORE and -EDNMR). In addition, extended quantum mechanical models from broken-symmetry DFT now reproduce all EPR, Mn-55 and N-14 experimental magnetic observables, with the inclusion of second coordination sphere ligands being crucial for accurately describing the interaction of NH3 with the Mn tetramer. These results support a mechanism of multiline heterogeneity reported for species differences and the effect of methanol [Biochim. Biophys. Acta, Bioenerg., 2011, 1807, 829], involving small changes in the magnetic connectivity of the solvent accessible outer Mn-A4 to the cuboidal unit Mn3O3Ca, resulting in predictable changes of the measured effective 55Mn hyperfine tensors. Sr2+ and NH3 replacement both affect the observed O-17-EDNMR signal envelope supporting the assignment of O5 as the exchangeable mu-oxo bridge and it acting as the first site of substrate inclusion.