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

Oxygen-evolving photosystem II particles (fromSynechococcus ) with about 80 chlorophyll molecules per primary electron donor (P680 ) 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 τ1 = 80 ± 20 ps and τ2 = 520 ± 120 ps in open reaction centers and τ1 = 220 ± 30 ps and τ2 = 1.3 ± 0.15 ns in closed reaction centers. The corresponding fluorescence yield ratioF 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 (<7 × 1012 photons/cm2 per pulse) to avoid singlet-singlet annihilation. With open reaction centers, the absorbance changes could be fit to the sum of three exponentials. The associated absorbance difference spectra were attributed to (i ) exciton trapping and charge separation (τ = 100 ± 20 ps), (ii ) the electron-transfer step P680 + I- QA → P680 + I QA - (where I is the primary electron acceptor and QA is the first quinone acceptor) (τ = 510 ± 50 ps), and (iii ) the reduction of P680 + by the intact donor side (τ > 10 ns). With closed reaction centers, the absorbance changes were biexponential with lifetimes τ1 = 170-260 ps and τ2 = 1.6-1.75 ns. The results are explained in terms of a kinetic model that assumes P680 to constitute a shallow trap. The results show that QA 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.