| Research Abstract |
The aggregation behavior of two homologs of bacteriochlorophyll c (BChl c) in a solvent mixture was investigated in terms of the effect of pH and a salt, and the corresponding structure was analyzed by FT-IR spectroscopy. R-[P, E] BChl c_F (R-form of BChl c with a propyl group at the C-8 position and an ethyl group at the C-12 position) and R-[E, E] BChl c_F (R-form of BChl c with two ethyl groups at positions C-8 and C-12) were isolated from the green sulfur bacterium Chlorobium limicola f.thiosulfatophilum. Each homologs showed a pH-dependent shift of the absorption maximum ; at low pH, the peak moved to the red. This tendency was also revealed by the CD spectra. The red-shift of the peaks was also induced by a high concentration of salt (NaCl) or buffer for both homologs, but only R-[E, E] BChl c_F was sensitive to the changes in solvent property. The FT-IR spectrum indicates that under the low pH condition, both homologs formed a common rather amorphous structure. On the other hand
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, a regular structure of R-[P, El BChl c_F was indicated in a solvent -water mixture, which was stabilized by a triangular interaction among three pigment molecules through the Mg ・・・OH (3^1) ・・・O=C (13^1) linkage. This structure was not found for R-[E, E] BChl c_F. These results indicate that the replacement of the side chain at the C-8 position on the macrocycle induced a different aggregation behavior. A possible heterogeneity of the in vivo rod structure of chlorosomes in green sulfur bacteria was discussed based on the above results. We found that the growth condition under limited potassium source ([K^+] = 0.2mM) was effective to enrich the BChl d homologs coexisting with a set of BChl c in chlorosomes of green sulfur bacteria Cb.limicola. As major BChl d homologs, [E, E]- and [P, E] BChl d_F were isolated and identified by ^1H-NMR signal of BChls and by Soret and Qy bands at 425nm and 650nm, respectively, which were blue-shifted than those of BChl c. The reversed phase HPLC elution profiles indicate the same proportionality for the set of four homologs ([E, M]-, [E, E]-, [P, E]- and [I, E]-) between BChl c and BChl d. The concentration of Na^+, Mg^<2+>, Ca^<2+> and Fe^<2+> influenced only to the growth of living cells, but did not onto the conversion of BChl d to BChl c. It was suggested that growth condition under limitation of potassium amount depresses the biosynthesis from BChl d to BChl c in chlorosomes. Less
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