Probing the turnover efficiency of photosystem II membrane fragments with different electron acceptors.

In this study we employ isotope ratio membrane-inlet mass spectrometry to probe the turnover efficiency of photosystem II (PSII) membrane fragments isolated from spinach at flash frequencies between 1Hz and 50Hz in the presence of the commonly used exogenous electron acceptors potassium ferricyanide(III) (FeCy), 2,5-dichloro-p-benzoquinone (DCBQ), and 2-phenyl-p-benzoquinone (PPBQ). The data obtained clearly indicate that among the tested acceptors PPBQ is the best at high flash frequencies. If present at high enough concentration, the PSII turnover efficiency is unaffected by flash frequency of up to 30Hz, and at 40Hz and 50Hz only a slight decrease by about 5-7% is observed. In contrast, drastic reductions of the O(2) yields by about 40% and 65% were found at 50Hz for DCBQ and FeCy, respectively. Comparison with literature data reveals that PPBQ accepts electrons from Q(A)(-) in PSII membrane fragments with similar efficiency as plastoquinone in intact cells. Our data also confirm that at high flashing rates O(2) evolution is limited by the reactions on the electron-acceptor side of PSII. The relevance of these data to the evolutionary development of the water-splitting complex in PSII and with regard to the potential of artificial water-splitting catalysts is discussed. This article is part of a Special Issue entitled: Photosynthesis Research for Sustainability: from Natural to Artificial.