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

The correlation between the reduction of Q_A and the oxidation of Tyr_Z or Car/Chl_Z/Cyt_b559 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 Q_A^-•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 Q_A^-•, Car^+•/Chl^+• and oxidized Cyt_b559, while enhancing the amplitude of the S₁Tyr_Z• 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 Tyr_Z and the side-path electron donors, respectively, indicate that the EPR-detectable g = 1.95-1.86 Q_A^-• signal is only correlated to reaction centers undergoing oxidation of the side-path electron donors (Car/Chl_Z/Cyt_b559), but not of Tyr_Z. These results imply that two types of Q_A^-• probably exist in the intact PSII sample. The structural difference and possible function of the two types of Q_A are discussed.