INVESTIGATION OF THE AMMONIUM-CHLORIDE AND AMMONIUM ACETATE INHIBITION OF OXYGEN EVOLUTION BY PHOTOSYSTEM-II

Using EPR and EXAFS spectroscopies we show that high concentrations of ammonium cations at alkaline pH are required for (1) inhibition of ox ygen evolution: (2) an alteration of the EPR properties of the oxygen evolving complex: (3) the ability to detect Y-Z(.); and (4) the slow r eduction of the Mn complex leading to the appearance of EPR detectable Mn2+. The inhibition of S state cycling, slowing of Y-Z(.) reduction, appearance of Mn2+ and the yield of a Hpp < 10 mT S-3 type EPR signal are decreased by calcium addition. This indicates that these effects were probably associated with calcium depletion arising from the high concentration of ammonium cation. The ammonia-induced changes to the S -2 multiline EPR signal are not affected by calcium addition. The appe arance of Mn2+ is shown to be reversible on illumination, suggesting t hat the Mn reduced from the native state is located at or near the nat ive site. Simulations of the interaction which gives rise to the S-3 E PR signal are also presented and discussed. These indicate that linesh ape differences occur through small changes in the exchange component of the interaction between the manganese complex and organic radical, probably through minor structural changes between the variously treate d samples.