SPECTROSCOPIC PROPERTIES OF LHC-II, THE MAIN LIGHT-HARVESTING CHLOROPHYLL-A/B PROTEIN COMPLEX FROM CHLOROPLAST MEMBRANES

The major chlorophyll a/b light-harvesting complex LHC-II was isolated from spinach thylakoid membranes after solubilization with the non-ionic detergent n-dodecyl-beta-D-maltoside. The complex is characterized by a chlorophyll a to b ratio of 1.5 +/- 0.1. The spectroscopic properties of this trimeric complex were analyzed and found to be very similar to those observed in the intact thylakoid membrane. By low-temperature absorption, linear dichroism and circular dichroism spectroscopy it is shown that the chlorophyll Q(y)(0-0) absorption region of the complex is characterized by at least six and perhaps nine chlorophyll transitions with significant differences in energy, orientation and rotational strength. Excitation of the pigments resulted in a sharp fluorescence band peaking near 680 nm at 77K. Presumably, this fluorescence originates from the main and red-most chlorophyll a Q(y) transition, which peaks at 676 nm and is oriented at a small angle with the plane of the particle. Fitting of polarized excitation spectra indicated that the polarization of Chl676 is about 0.1; this argues against a single emitting species, but is in line with a model in which efficient energy transfer occurs among several (> 3)Chl676 molecules which form a circularly degenerate oscillator in a plane. Since the shapes of the linear dichroism and polarized excitation spectra were found to be virtually identical between 640 and 676 nm, this plane is probably the same as the plane of the particle. Above 676 nm, the polarization rises gradually to a value of about 0.2, which is interpreted as a result of inhomogeneous line broadening of the 676 nm transition. A model that describes the chlorophyll organization of LHC-II predominantly in terms of exciton interactions between several chlorophyll a and several chlorophyll b molecules is discussed.