STUDIES ON THE RECOMBINATION KINETICS OF THE RADICAL PAIR P680+PHEO- IN ISOLATED PS-II CORE COMPLEXES FROM SPINACH

Yield and decay kinetics of the laser flash-induced primary radical pair P680'Pheo- were analyzed by measuring flash-induced absorption changes at 820 nm (DELTAA820) and fluorescence decay kinetics in PS II core complexes under different redox conditions for the primary plastoquinone acceptor, Q(A). lf Q(A) is chemically reduced in the dark by Na2S2O4, the yield of long-lived (tau > 1 ns) P680+Pheo- states is 30% with lifetimes of 4 ns (about 2/3 of the total decay) and 30 ns (about 1/3 of the total decay). The fluorescence data suggest that the 30 ns radical pair and probably the 4 ns radical pair as well decay predominately by recombination to P680*Pheo. Irradiation of PS II core complexes in the presence of Na2S2O4 for only 10 s with intense visible light increases the yield of the 30 ns primary radical pair by a factor of 3. This result can be explained by a double reduction of Q(A) and subsequent protonation leading to Q(A)H-2. At short irradiation times this effect is reversible because subsequent addition of K3[Fe(CN)6] fully restores the capability for a stable charge separation giving rise to P680+Q(A) formation. However, longer irradiation of Na2S2O4-treated PS II core complexes leads to an irreversible impairment of the primary charge separation.