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Publications (6)17.15 Total impact

  • Biochemistry - BIOCHEMISTRY-USA. 04/2002; 26(5).
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    ABSTRACT: The energy transfer and charge separation kinetics of a photosystem I (PS I) core particle of an antenna size of 100 chlorophyll/P700 has been studied by combined fluorescence and transient absorption kinetics with picosecond resolution. This is the first combined picosecond study of transient absorption and fluorescence carried out on a PS I particle and the results are consistent with each other. The data were analyzed by both global lifetime and global target analysis procedures. In fluorescence major lifetime components were found to be 12 and 36 ps. The shorter-lived one shows a negative amplitude at long wavelengths and is attributed to an energy transfer process between pigments in the main antenna Chl pool and a small long-wavelength Chl pool emitting around 720 nm whereas the longer-lived component is assigned to the overall charge separation lifetime. The lifetimes resolved in transient absorption are 7-8 ps, 33 ps, and [unk]1 ns. The shortest-lived one is assigned to energy transfer between the same pigment pools as observed also in fluorescence kinetics, the middle component of 33 ps to the overall charge separation, and the long-lived component to the lifetime of the oxidized primary donor P700(+). The transient absorption data indicate an even faster, but kinetically unresolved energy transfer component in the main Chl pool with a lifetime <3 ps. Several kinetic models were tested on both the fluorescence and the picosecond absorption data by global target analysis procedures. A model where the long-wave pigments are spatially and kinetically connected with the reaction center P700 is favored over a model where P700 is connected more closely with the main Chl pool. Our data show that the charge separation kinetics in these PS I particles is essentially trap limited. The relevance of our data with respect to other time-resolved studies on PS I core particles is discussed, in particular with respect to the nature and function of the long-wave pigments. From the transient absorption data we do not see any evidence for the occurrence of a reduced Chl primary electron acceptor, but we also can not exclude that possibility, provided that reoxidation of that acceptor should occur within a time <40 ps.
    Biophysical Journal 06/1993; 64(6):1813-26. · 3.67 Impact Factor
  • Biophys. J. 01/1993; 64:1813-1826.
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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. · 3.67 Impact Factor
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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. · 9.81 Impact Factor
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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.