Low-temperature electron transfer suggests two types of QA in intact photosystem II

The correlation between the reduction of QA and the oxidation of TyrZ or Car/ChlZ/Cytb559 in spinach PSII enriched membranes induced by visible light at 10 K is studied by using electron paramagnetic resonance spectroscopy. Similar g = 1.95–1.86 QA-
• EPR signals are observed in both Mn-depleted and intact samples, and both signals are long lived at low temperatures. The presence of PPBQ significantly diminished the light induced EPR signals from QA-
• , Car+
• /Chl+
• and oxidized Cytb559, while enhancing the amplitude of the S1TyrZ
• EPR signal in the intact PSII sample. The quantification and stability of the g = 1.95–1.86 EPR signal and signals arising from the oxidized TyrZ and the side-path electron donors, respectively, indicate that the EPR-detectable g = 1.95–1.86 QA-
• signal is only correlated to reaction centers undergoing oxidation of the side-path electron donors (Car/ChlZ/Cytb559), but not of TyrZ. These results imply that two types of QA-
• probably exist in the intact PSII sample. The structural difference and possible function of the two types of QA are discussed.