THE FORMATION AND CHARACTERIZATION OF THE IN-VITRO POLYMERIC AGGREGATES OF BACTERIOCHLOROPHYLL-C HOMOLOGS FROM CHLOROBIUM LIMICOLA IN AQUEOUS SUSPENSION IN THE PRESENCE OF MONOGALACTOSYL DIGLYCERIDE

Artificial aggregates of bacteriochlorophyll c (BChl c) were formed in an aqueous medium in the presence of a lipid, monogalactosyl diglyceride (MGDG), and the optical properties of those aggregates were studied by absorption and circular dichroism (CD) mainly. Four BChl c homologs, ([E,E]BChl c(F), [P,E]BChl c(F), [E,M]BChl c(F) and [I,E]BChl c(F)) were, isolated from the green photosynthetic bacterium Chlorobium limicola strain 6230. Above 0.0004%, MGDG induced a red-shift of the absorption maxima of BChl c aggregates. At 0.003% MGDG BChl c aggregates showed absorption maxima in the range of 724 to 745 (+/- 3) nm with a shift of 12 to 24 (+/- 3) nm depending on the homolog species. Four kinds of BChl c-MGDG aggregates showed characteristic CD spectra. [E,M]BChl c(F) gave rise to a CD spectrum similar to that of chlorosomes, while the other three gave spectra of opposite sign. These aggregates are sensitive to 1-hexanol treatment; in a saturating amount (0.85%) of 1-hexanol, all the homologs gave a monomer-like absorption spectrum peaking at 670 nm. At an intermediate concentration (0.5%), [E,M]BChl c(F) showed an enhanced CD intensity, as observed in native chlorosomes. Resonance Raman spectra of the monomer-like BChl c samples indicated that the keto vibrational band at ca. 1640 cm(-1) was considerably weakened by the 0.85% 1-hexanol treatment, however the 1680 cm(-1) band characteristic of a free keto group did not appear. These results indicate that the artificial aggregates formed by purified BChl c homologs and MGDG are good models for studying chlorosomes structure.