Ultrafast spectroscopy of trimeric light-harvesting complex II from higher plants

Time-resolved femtosecond transient absorption measurements have been carried out at room temperature on Light-harvesting chlorophyll alb protein complex of photosystem II (LHC II) trimers prepared from spinach. Exciting in the chlorophyll (Chi) b region at 650 nm with very low intensity, virtually annihilation-free two-color transient absorption measurement of the kinetics over 100 ps, between 645 and 690 nm, yield global lifetimes of 175 fs, 625 fs, and 5 ps and a long component (greater than or equal to 790 ps) where the three fastest lifetimes reflect Chi b to Chi a energy transfer. Using a camera detection system, kinetics over 400 ps at still low annihilation levels and with much higher spectral resolution have been obtained. Short lifetime components of 180 fs, 480 fs, and 6 ps are comparable with the two-color data, but in addition, 34 and 85 ps components with small amplitudes are resolved and a long component (3.6 ns) is fixed at the longest lifetime value determined by fluorescence. Annihilation statistics have been calculated to compare these and earlier results. On the basis of these results and recent electron diffraction structural data, a preliminary three-pool Chi a, three-pool Chi b kinetic model is proposed. The possible influence of variable xanthophyll composition on quenching in LHC II preparations isolated from light- and dark-adapted leaves has been investigated using time-resolved picosecond fluorescence at room temperature. Global Lifetimes of 5 ps, 170 ps and 3.6 ns, the lifetimes of the terminal LHC II excited state, were obtained. No discernable quenching effect due to the presence of zeaxanthin was observed.