Single molecule spectroscopy on the light-harvesting complex II of higher plants

Spectroscopic and polarization properties of single light-harvesting comple xes of higher plants (LHC-II) were studied at both room temperature and T < 5 K. Monomeric complexes emit roughly linearly polarized fluorescence ligh t thus indicating the existence of only one emitting state. Most probably t his observation is explained by efficient triplet quenching restricted to o ne chlorophyll a (Chl a) molecule or by rather irreversible energy transfer within the pool of Chi a molecules. LHC-II complexes in the trimeric (nati ve) arrangement bleach in a number of steps, suggesting localization of exc itations within the monomeric subunits. Interpretation of the fluorescence polarization properties of trimers requires the assumption of transition di pole moments tilted out of the symmetry plane of the complex. Low-temperatu re fluorescence emission of trimers is characterized by several narrow spec tral lines. Even at lowest excitation intensities, we observed considerable spectral diffusion most probably due to low temperature protein dynamics. These results also indicate weak interaction between Chls belonging to diff erent monomeric subunits within the trimer thus leading to a localization o f excitations within the monomer. The experimental results demonstrate the feasibility of polarization sensitive studies on single LHC-II complexes an d suggest an application for determination of the Chi transition-dipole mom ent orientations, a key issue in understanding the structure-function relat ionships.