Storage of abnormal oxidants ‘∑ 1’, ‘∑ 2’ and ‘∑ 3’ in photosynthetic water oxidases inhibited by Cl −-removal

Abstract

The Cl − requirement of photosynthetic O 2 evolution was studied by thermoluminescence measurements with purified Photosystem II-containing membrane particles from chloroplast thylakoids. When Cl anions had been removed from the particles either by an alkaline shock in a Cl −-free medium, or by treatment with SO 2− 4, the pattern of the thermoluminescence emission after illumination with increasing numbers of flashes suggested that the oxidant storage in the water oxidase could only proceed up to the final step. The final step itself. i.e., the advance to the water-oxidizing S 4 state, apparently was blocked. An upward shift of the emission temperatures of the thermoluminescence bands was seen both in the absence and in the presence of 3-(3′,4′-dichlorophenyl)-1,1-dimethyl urea, indicating that the stored oxidants had redox properties different from those of normal, Cl −-sufficient preparations. These properties were readily interconvertible by addition or removal of Cl −. We postulate that in Cl −-deficient water oxidases abnormal S 1, S 2 and S 3 states, symbolized as ∑ 1, ∑ 2 and ∑ 3, respectively, are formed which are in a Cl −-dependent equilibrium with the corresponding normal S states. An oxidation of ∑ 3 to a ∑ 4 state is not possible. It is proposed that Cl − controls the oxidation potential of the stored oxidants by regulating events associated with the binding and/or oxidative modification of water molecules at the water oxidase.