ENERGY-TRANSFER AND TRAPPING IN ISOLATED PHOTOSYSTEM-II REACTION CENTERS OF GREEN PLANTS AT LOW-TEMPERATURE - A STUDY BY SPECTRAL HOLE-BURNING

Spectral hole burning has been performed on the Q(y)-region of the iso lated reaction center of photosystem II, the D1-D2-cytochrome b559 com plex (PSII RC), between 665 and 688 nm, at liquid He temperatures. The ''effective'' homogeneous line width Gamma'(hom) at 682 nm, in the re d wing of the Q(y)-band, follows a T-1.3+/-0.1 power law between 1.2 a nd 4.2 K characteristic of glasses and extrapolates to Gamma'(0) = (2 pi T-1)(-1) for T --> 0 with T-1 = (4 +/- 1) ns, the fluorescence life time of the pigments. At these low temperatures, the red-absorbing ''t rap'' pigments are unable to transfer energy to other pigments. The sp ectral distribution of the traps has been determined from hole depth v s lambda(exc) experiments. Their linear electron-phonon coupling stren gth was found to be rather weak, S = 0.73 +/- 0.05. For lambda(exc) < 678 nm, ''downhill'' energy transfer takes place. Spectral distributio ns of pigments characterized by decay times of 200 and 12 ps have furt her been identified in this spectral region. The data have been used t o reconstruct the fluorescence excitation and absorption spectra.