Polarized fluorescence and absorption of macroscopically aligned Light Harvesting Complex II

Polarized absorption and fluorescence measurements have been performed at 77 K on isotropic and anisotropic preparations of trimeric Light Harvesting Complex II (LHC-II) from spinach. The results enable a decomposition of the absorption spectrum into components parallel and perpendicular to the trimeric plane. For the first time, it is shown quantitatively that the strong absorption band around 676 nm is polarized essentially parallel to the plane of the trimer, i.e., the average angle between the corresponding transition dipole moments and this plane is at most 12 degrees. The different absorption bands for LHC-II should not be considered as corresponding to individual pigments but to collective excitations of different pigments. Nevertheless, the average angle between the Qy transition dipole moments of all chlorophyll a pigments in LHC-II and the trimeric plane could be determined and was found to be 17.5 degrees +/- 2.5 degrees. For the chlorophyll b pigments, this angle is significantly larger (close to 35 degrees). At 77 K, most of the fluorescence stems from a weak band above 676 nm and the corresponding transition dipole moments are oriented further out of plane than the dipole moments corresponding to the 676-nm band. The results are shown to be of crucial significance for understanding the relation between the LHC-II structure and its spectroscopy.