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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 Ca2+. The water-exchange energy barriers in the S1 and S2 states are 19.3 and 16.6 kcal mol−1, respectively. Substrate and exchanging water molecules are labelled Ws (Wslow) and W1*, respectively. All amino acid residues correspond to the D1 protein subunit, unless otherwise indicated.
