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In photosystem II (PSII), water oxidation occurs in the Mn 4 CaO 5 cluster with the release of electrons via the redox-active tyrosine (TyrZ) to the reaction-center chlorophylls (P D1 /P D2 ). Using a quantum mechanical/molecular mechanical approach, we report the redox potentials ( E  m ) of these cofactors in the PSII protein environment. The E  m values suggest that the Mn 4 CaO 5 cluster, TyrZ, and P D1 /P D2 form a downhill electron transfer pathway. E  m for the first oxidation step, E  m (S 0 /S 1 ), is uniquely low (730 mV) and is ∼100 mV lower than that for the second oxidation step, E  m (S 1 /S 2 ) (830 mV) only when the O4 site of the Mn 4 CaO 5 cluster is protonated in S 0 . The O4-water chain, which directly forms a low-barrier H-bond with the Mn 4 CaO 5 cluster and mediates proton-coupled electron transfer in the S 0 to S 1 transition, explains why the second lowest oxidation state, S 1 , is the most stable and S 0 is converted to S 1 even in the dark.

Redox Potential of the Oxygen-Evolving Complex in the Electron Transfer Cascade of Photosystem II