Low-temperature electron transfer suggests two types of Q(A) 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 S(1)Tyr(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.