Primary Processes and Structure of the Photosystem II Reaction Center: A Photon Echo Study,

Abstract

An experimental and theoretical photon echo (PE) study of the primary charge separation process in the photosystem II reaction center (PS II RC) at low temperature (T = 1.33 ± 0.01 K) is reported. Experiments were carried out at low excitation intensities of 5 × 1012 photons/cm2 with time and spectral resolution of about 0.5 ps and 1 nm, respectively, using the two-pulse photon echo technique (2PE). The data were interpreted in the framework of the exciton model. For that purpose the theory of the PE formation and energy transfer in an excitonically coupled system, including explicitly the electron-bath interaction, is developed. By comparing the measured and the simulated PE kinetics, we draw the conclusion that the accessory chlorophyll in the active branch of the RC core is the primary electron donor. The charge separation occurs with an intrinsic time constant of ≈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. 1989, 86, 524; Jankowiak, R.; Tang, D.; Small, G. J.; Seibert, M. J. Phys. Chem. 1989, 93, 1649; Tang, D.; Jankowiak, R.; Seibert, M.; Small, G. J. Photosynth. Res. 1991, 27, 19). However, the dipole−dipole interaction between pigments leads to a very wide distribution of the effective charge separation kinetics ranging from 1.5 ps up to a few nanoseconds. Thus, the experimentally observable effective distributive charge separation rate differs strongly from the intrinsic one. In this work the effect of the charge separation process in an excitonically coupled system is described for the first time. Energy transfer rates, calculated on the basis of developed theory, show that the energy transfer occurs in the 100−200 fs time domain in agreement with our own experimental observations, and previously published data. This 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 primary charge separation process.