“…The marked decrease in oxidation potential of the S 2 state manganese cluster in K ϩ -, Rb ϩ -, and Cs ϩ -substituted OEC as indicated by the upshifted TL band is very similar to that observed in the Ca 2ϩ -depleted sample membranes prepared by low-pH treatment (Ono and Inoue, 1989a;Ono et al, 1992), in which binding of the 24 kDa protein to Ca 2ϩdepleted OEC causes abnormalities of the manganese cluster, including the alteration of the magnetic properties (Ono and Inoue, 1989a;Ono et al, 1992). Interestingly, the Kedge of the Mn XANES spectrum of low-pH treated membranes downshifts, indicating the modified ligation structural of the manganese cluster such as a broken ligation bond to the cluster (Ono et al, 1991(Ono et al, , 1993Latimer et al, 1995;Cinco et al, 1998), whereas no such spectral change has been identified in Ca 2ϩ -depleted samples devoid of the 24-kDa protein (Hatch et al, 1995;Riggs-Gelasco et al, 1996). EXAFS (Yachandra et al, 1993) and FTIR (Noguchi et al, 1995) studies indicated that Ca 2ϩ is connected with the manganese cluster via a carboxylate bridge, and that this structure is responsible for the conformational change that appears upon S 2 state formation by breaking the coordination to Ca 2ϩ , whereas Ca 2ϩ depletion at low-pH liberates the carboxylate ligand from the manganese cluster (Noguchi et al, 1995).…”