EXCITON EQUILIBRATION IN THE LIGHT-HARVESTING COMPLEX OF PHOTOSYSTEM-II OF HIGHER-PLANTS

The exciton equilibration in the major chlorophyll a/b light-harvesting complex from spinach thylakoid membranes was analysed by non-linear transmission spectroscopy and pump-probe spectroscopy under quasi-stationary conditions at room temperature. Investigations of the non-linear transmission in the Q(y) transitions of Chl a/b absorption indicate that excited state absorption dominates in the wavelength regions between 620 nm and 650 nm and between 685 nm and 690 nm, whereas bleaching and stimulated emission is predominant from 650 nm to 685 nm. The pump-probe transmission spectra suggest that in isolated light-harvesting complexes of Photosystem II (LHC II) the excitons are thermally equilibrated over all spectral chlorophyll forms nearly independent of the excitation wavelength. An analogous phenomenon is not observed in Photosystem II (PS II) membrane fragments. Regardless of differences between isolated LHC II and PS II membrane fragments in both samples, the maximum of the exciton density is located at around 680 nm independent of excitation intensity and wavelength. The maximum of exciton distribution in the Chl b absorption region is located at the absorption maximum of Chl b at 650 nm. The room-temperature exciton equilibration in the wavelength region of Chl a absorption can be described by a Boltzmann distribution. On the other hand, an inclusion of the Chl b excitons in the same distribution can be achieved only under the assumption of a strongly increased oscillator strength of Chl b states.