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Structural model of the OEC of PSII in the S1 and S2 states at the transition state configurations for water exchange from Mn(4). The water-exchange energy barriers in the S1 and S2 states are 7.9 and 8.4 kcal mol−1, respectively. Substrate and exchanging water molecules are labelled Wf (Wfast) and W*, respectively. All amino acid residues correspond to the D1 protein subunit, unless otherwise indicated.
