Spectroscopic characterization of three different monomeric forms of the main chlorophyll a/b binding protein from chloroplast membranes.

A detailed comparison has been made between dichroic steady-state spectroscopic properties at 77 K of several trimeric and monomeric forms of the major chlorophyll a/b binding protein (LHC-II) from pea. Monomeric forms were obtained by applying high concentrations of nonionic detergents, by a lipase treatment, or by a chymotrypsin/trypsin treatment. The latter treatments removed phosphatidyl glycerol essential for trimer formation. The absorption and dichroism spectra indicate that for trimeric LHC-II the chlorophyll b absorption region is centered around 649 nm and is composed of at least five subbands near 640, 647, 649, 652, and 656 nm. The chlorophyll a absorption region is centered around 670 nm and is composed of at least five bands near 661, 668, 671, 673, and 676 nm. The chlorophyll b band near 647 and 652 nm and the chlorophyll a bands near 668 and 673 nm are absent in the circular dichroism spectrum after monomerization. A configuration in which pigments of the same nature located on different monomers become excitonically coupled in the trimer could explain these results. In monomers obtained in high concentrations of nonionic detergents, no additional bands have disappeared, but the absorption spectra of the other two types of monomers lack the bands at 640 and 661 nm. These monomers have lost some chlorophyll a and b according to the fluorescence emission spectra, which show contributions from free chlorophyll a and b. The results suggest that phosphatidyl glycerol not only is involved in trimer formation but also has a structural role within the monomers.