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

摘要

Oxygen-evolving photosystem II particles (from Synechococcus) 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 ratio Fmax/Fo 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 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.