ULTRAFAST EXCITATION-ENERGY TRANSFER AND EXCITON-EXCITON ANNIHILATION PROCESSES IN ISOLATED LIGHT-HARVESTING COMPLEXES OF PHOTOSYSTEM-II (LHC-II) FROM SPINACH

Excitation energy transfer and exciton-exciton annihilation in the isolated light-harvesting chlorophyll a/b protein complex of spinach photosystem II (LHC II) has been studied by two-color absorption difference spectroscopy with femtosecond time resolution. After selectively exciting Chl b at 645 nm, the transient absorption changes were monitored at wavelengths where either Chl b (655 nm) or Chl a (680 nm) dominates the absorption of LHC II. From the good correspondence of the lifetimes obtained from a numerical analysis of the very fast relaxation in the CN b absorption band (160 +/- 20 fs) and the rise kinetics in the Chl a absorption band (145 +/- 20 fs), it is suggested that the Chl b --> Chl a excitation energy transfer occurs on a time scale of about 150 fs. In addition, at both probe wavelengths (655 and 680 nm) lifetimes of 3-7 ps were observed which likely arise from excitation energy transfer processes connected with spectral shifting. The kinetic curves of the transient absorption changes at 680 nm show a remarkable intensity dependence which is ascribed to exciton-exciton annihilation. Since at a probe wavelength of 655 nm no intensity effect on the kinetics was observed, it is concluded that annihilation processes preferably occur among excited singlet states of Chl a molecules. From the time course of the transient absorption changes at 680 nm, the bimolecular annihilation constant (gamma 2') was estimated to be 2.5 x 10(-10) cm(3) s(-1). The experimental results on the exciton dynamics in LHC II are discussed in view of the new structural data on this antenna protein.