Arrested Substrate Binding Resolves Catalytic Intermediates in Higher‐Plant Water Oxidation

Among the intermediate catalytic steps of the water‐oxidizing Mn 4 CaO 5 cluster of photosystem II (PSII), the final metastable S 3 state is critically important because it binds one substrate and precedes O 2 evolution. Herein, we combine X‐ and Q‐band EPR experiments on native and methanol‐treated PSII of Spinacia oleracea and show that methanol‐treated PSII preparations of the S 3 state correspond to a previously uncharacterized high‐spin ( S= 6) species. This is confirmed as a major component also in intact photosynthetic membranes, coexisting with the previously known intermediate‐spin conformation ( S= 3). The high‐spin intermediate is assigned to a water‐unbound form, with a Mn IV 3 subunit interacting ferromagnetically via anisotropic exchange with a coordinatively unsaturated Mn IV ion. These results resolve and define the structural heterogeneity of the S 3 state, providing constraints on the S 3 to S 4 transition, on substrate identity and delivery pathways, and on the mechanism of O−O bond formation.