Picosecond kinetics of fluorescence and absorbance changes in photosystem II particles excited at low photon density

Oxygen-evolving photosystem II particles (from Synechococcus) with about 80 chlorophyll molecules per primary electron donor (P(680)) were used for a correlated study of picosecond kinetics of fluorescence and absorbance changes, detected by the single-photon-timing technique and by a pump-probe apparatus, respectively. Chlorophyll fluorescence decays were biexponential with lifetimes tau(1) = 80 +/- 20 ps and tau(2) = 520 +/- 120 ps in open reaction centers and tau(1) = 220 +/- 30 ps and tau(2) = 1.3 +/- 0.15 ns in closed reaction centers. The corresponding fluorescence yield ratio F(max)/F(o) was 3-4. Absorbance changes were monitored in the spectral range of 620-700 nm after excitation at 675 nm with 10-ps pulses sufficiently weak ( P(680) (+) I Q(A) (-) (where I is the primary electron acceptor and Q(A) is the first quinone acceptor) (tau = 510 +/- 50 ps), and (iii) the reduction of P(680) (+) by the intact donor side (tau > 10 ns). With closed reaction centers, the absorbance changes were biexponential with lifetimes tau(1) = 170-260 ps and tau(2) = 1.6-1.75 ns. The results are explained in terms of a kinetic model that assumes P(680) to constitute a shallow trap. The results show that Q(A) reduction in these photosystem II particles decreases both the apparent rate and the yield of the primary charge separation by a factor of 2-3 and increases the mean lifetime of excitons in the antenna by a factor of 3-4. Thus, we conclude that the long-lived, nanosecond chlorophyll fluorescence is not charge-recombination luminescence but rather emission from equilibrated excited states of antenna chlorophylls.