Reversible Structural Isomerization of Nature’s Water Oxidation Catalyst Prior to O–O Bond Formation

Photosynthetic water oxidation is catalyzed by a manganese-calcium oxide cluster, which experiences five "S-states" during a light-driven reaction cycle. The unique "distorted chair"-like geometry of the Mn 4 CaO 5(6) cluster shows structural flexibility that has been frequently proposed to involve "open" and "closed"-cubane forms from the S 1 to S 3 states. The isomers are interconvertible in the S 1 and S 2 states, while in the S 3 state, the open-cubane structure is observed to dominate in Thermosynechococcus elongatus (cyanobacteria) samples. In this work, using density functional theory calculations, we go beyond the S 3 + Y z state to the S 3 n Y z • → S 4 + Y z step, and report for the first time that the reversible isomerism, which is suppressed in the S 3 + Y z state, is fully recovered in the ensuing S 3 n Y z • state due to the proton release from a manganese-bound water ligand. The altered coordination strength of the manganese-ligand facilitates formation of the closed-cubane form, in a dynamic equilibrium with the open-cubane form. This tautomerism immediately preceding dioxygen formation may constitute the rate limiting step for O 2 formation, and exert a significant influence on the water oxidation mechanism in photosystem II.