Closing Kok's cycle of nature's water oxidation catalysis

The Mn4CaO5(6) cluster in photosystem II catalyzes water splitting. Recent experimental breakthroughs have illuminated the S3(S4) → S0 transition, but the S0-state reconstitution remained unclear. This study uses molecular dynamics simulations and density functional calculations to propose a mechanism for closing the cycle by restoring the first catalytic state. The simulations reveal the formation of closed-cubane intermediates with hexa-coordinate Mn, followed by proton release, water dissociation, and ligand transfer to produce the open-cubane structure of the S0 state. This identifies previously unknown structural isomerism in the S0 state, suggesting structural flexibility is functionally important for water oxidation catalysis.

Yu Guo, Lanlan He, Yunxuan Ding, Lars Kloo, Dimitrios A. Pantazis, Johannes Messinger, Licheng Sun