Thermoluminescence properties of the S2-state in chloride-depleted water oxidizing complexes after reconstituting treatments with various monovalent anions.

Under conditions that assured rebinding of the extrinsic 17 and 23 kDa polypeptides, Cl(-)-depleted Photosystem II membranes isolated from spinach chloroplasts were subjected to reconstituting treatments in media containing NaF, NaCl, NaBr, NaI or NaNO3, or they were kept in a medium without any added salt other than the buffer. After removing most of the unbound reconstituting anions by washing, the O2-evolution activities and thermoluminescence properties of the membranes were compared. While the temperature of maximal thermoluminescence emission was lowest for membranes treated with Cl(-), no uniform correlation was evident between the temperature profile of the thermoluminescence emission and the apparent activating effectiveness of the anions in the membranes' water oxidizing machinery. However, the differences between the thermoluminescence features did conform to a trend according to which the emission temperatures were upshifted as the size of the activating anion increased, and its hydration energy decreased, i.e. Cl(-)<Br(-)<NO3 (-)<I(-). The inactive F(-) anions were not well retained by the membranes. To explain the experimental data it is suggested that the structural environment of the charge accumulating Mn-center is influenced by the ionic conditions encountered by the Photosystem II membranes after Cl(-) removal, further enforced by the binding of compatible anions, and then stabilized by the 17 and 23 kDa extrinsic polypeptides. If, as some concepts imply, the anion binding sites are located at or near the functional Mn, only very exceptional characteristics of the water-oxidizing mechanism may account for the observation that the potentially electron-donating I(-) anion can serve as activator and that it stabilizes rather than destabilizes the S2-state.