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Publications (5)16.84 Total impact

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    ABSTRACT: A detailed model for the kinetics and energetics of the exciton trapping, charge separation, charge recombination, and charge stabilization processes in photosystem (PS) II is presented. The rate constants describing these processes in open and closed reaction centers (RC) are calculated on the basis of picosecond data (Schatz, G. H., H. Brock, and A. R. Holzwarth. 1987. Proc. Natl. Acad. Sci. USA. 84:8414-8418) obtained for oxygen-evolving PS II particles from Synechococcus sp. with approximately 80 chlorophylls/P(680). The analysis gives the following results. (a) The PS II reaction center donor chlorophyll P(680) constitutes a shallow trap, and charge separation is overall trap limited. (b) The rate constant of charge separation drops by a factor of approximately 6 when going from open (Q-oxidized) to closed (Q-reduced) reaction centers. Thus the redox state of Q controls the yield of radical pair formation and the exciton lifetime in the Chl antenna. (c) The intrinsic rate constant of charge separation in open PS II reaction centers is calculated to be approximately 2.7 ps(-1). (d) In particles with open RC the charge separation step is exergonic with a decrease in standard free energy of approximately 38 meV. (e) In particles with closed RC the radical pair formation is endergonic by approximately 12 meV. We conclude on the basis of these results that the long-lived (nanoseconds) fluorescence generally observed with closed PS II reaction centers is prompt fluorescence and that the amount of primary radical pair formation is decreased significantly upon closing of the RC.
    Biophysical Journal 10/1988; 54(3):397-405. DOI:10.1016/S0006-3495(88)82973-4
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    ABSTRACT: 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 (<7 x 10(12) photons/cm(2) 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 (tau = 100 +/- 20 ps), (ii) the electron-transfer step P(680) (+) I(-) Q(A) --> 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.
    Proceedings of the National Academy of Sciences 12/1987; 84(23):8414-8. DOI:10.1073/pnas.84.23.8414
  • Biochemistry 03/1987; 26(5). DOI:10.1021/bi00379a030
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    ABSTRACT: The lifetimes and relative quantum yields of the three fluorescence components of the P/sub r/ (red-absorbing) form of 124-kDa oat phytochrome in HâO and DâO solutions have been determined by single-photon-timing techniques. Lifetime and quantum yield of the main component, which is the shortest lived and reflects the photochromic properties of P/sub r/, are not affected by DâO. The medium-lifetime component, which is attributable to a photochromic source behaving comparable to that responsible for the main component, is not affected either. Only the least occurring and longest lived component, which is nonphotochromic, i.e., unaffected by red irradiation, markedly increases in lifetime. Thus, its relative contribution to the total fluorescence yield increases in DâO. The spectra of the individual fluorescence components, obtained by a global analysis of the decay traces from 124-kDa P/sub r/ in HâO solution at different observation wavelengths, differ only slightly in wavelength. The authors conclude that the primary photoreaction in the P/sub r/ transformation to P/sub fr/ (far-red-absorbing form of phytochrome) of the main phytochrome component exhibiting the shortest lived fluorescence does not involve a proton transfer as has been suggested in the literature.