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 H arvesting Complex II (LHC-II) from spinach. The results enable a decom position of the absorption spectrum into components parallel and perpe ndicular to the trimeric plane. For the first time, it is shown quanti tatively that the strong absorption band around 676 nm is polarized es sentially 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 colle ctive excitations of different pigments. Nevertheless, the average ang le between the Q(y) transition dipole moments of all chlorophyll a pig ments in LHC-II and the trimeric plane could be determined and was fou nd 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, mo st of the, fluorescence stems from a weak band above 676 nm and the co rresponding transition dipole moments are oriented further out of plan e than the dipole moments corresponding to the 676-nm band. The result s are shown to be of crucial significance for understanding the relati on between the LHC-II structure and its spectroscopy.