An EPR study of the pH dependence of formate effects on Photosystem II

Effects of formate on rates of O2 evolution and electron paramagnetic resonance (EPR) signals were observed in the oxygen evolving PS II membranes as a function of pH. In formate treated PS II membranes, decrease in pH value resulted in the inhibition of the O2 evolving activity, a decrease in the intensity of S2 state multiline signal but an increase in the intensity of the QA-Fe2+ EPR signal. Time-resolved EPR study of the YZ● decay kinetics showed that the light-induced intensity of YZ● EPR signal was proportional to the formate concentration. The change in the pH affected both the light-induced intensities and the decay rates of YZ●, which was found to be faster at lower pH. At 253 K, t1/e value of YZ● decay kinetics was found to be 8-10 s at pH 6.0 and 18-21 s at pH 5.0. The results presented here indicate that the extent of inhibition at the donor and the acceptor side of PS II due to formate is pH dependent, being more effective at lower pH.