SPECTROSCOPIC CHARACTERIZATION OF 3 DIFFERENT MONOMERIC FORMS OF THE MAIN CHLOROPHYLL A B BINDING-PROTEIN FROM CHLOROPLAST MEMBRANES/

A detailed comparison has been made between dichroic steady-state spec troscopic properties at 77 K of several trimeric and monomeric forms o f the major chlorophyll a/b binding protein (LHC-II) from pea. Monomer ic forms were obtained by applying high concentrations of nonionic det ergents, by a lipase treatment, or by a chymotrypsin/trypsin treatment . The latter treatments removed phosphatidyl glycerol essential for tr imer formation. The absorption and dichroism spectra indicate that for trimeric LHC-II the chlorophyll b absorption region is centered aroun d 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 arou nd 670 nm and is composed of at least five bands near 661, 668, 671, 6 73, and 676 nm. The chrorophyll b band near 647 and 652 nm and the chl orophyll a bands near 668 and 673 nm are absent in the circular dichro ism spectrum after monomerization. A configuration in which pigments o f the same nature located on different monomers become excitonically c oupled in the trimer could explain these results. In monomers obtained in high concentrations of nonionic detergents, no additional bands ha ve disappeared, but the absorption spectra of the other two types of m onomers lack the bands at 640 and 661 nm. These monomers have lost som e chlorophyll a and b according to the fluorescence emission spectra, which show contributions from free chlorophyll a and b. The results su ggest that phosphatidyl glycerol not only is involved in trimer format ion but also has a structural role within the monomers.