Primary processes and structure of the photosystem II reaction center: A photon echo study

An experimental and theoretical photon echo (PE) study of the primary charg e separation process in the photosystem II reaction center (PS II RC) at lo w temperature (T = 1.33 +/- 0.01 K) is reported. Experiments were carried o ut at low excitation intensities of 5 x 10(12) photons/cm(2) with time and spectral resolution of about 0.5 ps and 1 nm, respectively, using the two-p ulse photon echo technique (2PE). The data were interpreted in the framewor k of the exciton model. For that purpose the theory of the PE formation and energy transfer in an excitonically coupled system, including explicitly t he electron-bath interaction, is developed. By comparing the measured and t he simulated PE kinetics, we draw the conclusion that the accessory chlorop hyll in the active branch of the RC core is the primary electron donor: The charge separation occurs with an intrinsic time constant of approximate to 1.5 ps, in good agreement with previously published data (Wasielewski, M. R .; Johnson, D. G.; Seibert, M.; Govindjee Proc. Natl. Acad. Sci. U.S.A. 198 9, 86, 524; Jankowiak, R.; Tang, D.; Small, G. J.; Seibert, M. J. Phys. Che m. 1989, 93, 1649; Tang, D.; Jankowiak, R.; Seibert, M.; Small, G. J. Photo synth. Res. 1991, 27, 19). However, the dipole-dipole interaction between p igments leads to a very wide distribution of the effective charge separatio n kinetics ranging from 1.5 ps up to a few nanoseconds. Thus, the experimen tally observable effective distributive charge separation rate differs stro ngly from the intrinsic one. In this work the effect of the charge separati on process in an excitonically coupled system is described for the first ti me. Energy transfer rates, calculated an the basis of developed theory, sho w that the energy transfer occurs in the 100-200 fs time domain in agreemen t with our own experimental observations, and previously published data. Th is fast energy transfer contributes to the intense and narrow peak at early delay times in the 2PE kinetics. In contrast, the slow dephasing observed in the 2PE kinetics at time delay above ca. 1 ps reflects mainly the primar y charge separation process.