Evidence for spatially separate bacteriochlorophyll c and bacteriochlorophyll d pools within the chlorosomal aggregate of the green sulfur bacterium Chlorobium limicola

We have studied the organization of the bacteriochlorophylls (BChl) in isol ated chlorosomes of the green sulfur bacterium Chlorobium limicola UdG6040 containing about 50% BChl d and BChl c each. When the chlorosomes are treat ed in acidic buffer (pH 3.0) two phases in the conversion from BChl to bact eriopheophytin (BPhe) are observed as evidenced by the changes in the absor ption spectrum. In the early phase the pheophytinization of BChl d occurs m uch faster than that of BChl c. In the later phase BChl c and BChl d are co nverted at similar rates. The delayed BChl c conversion observed in intact chlorosomes is interpreted in terms of spatial separation within the same c hlorosome that makes BChl d more accessible to reaction with acid than BChl c. This was supported by acid treatment of in vitro pigment-lipid aggregat es which showed that the pheophytinization of aggregates consisting of only BChl c or BChl d takes place with the same rate. Moreover in mixed in vitr o aggregates where BChl d and BChl c are supposed to be scrambled the two p igments are converted to BPhe simultaneously. Acid treatment of hexanol exp osed chlorosomes indicates that the spatial separation of BChl d and BChl c within the chlorosomes is maintained even if the excitonic interaction bet ween BChls has been disturbed by hexanol. Based on these findings it is sug gested that BChl d and BChl c in the chlorosome are located distal and prox imal, respectively, relative to the chlorosome baseplate.