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

Abstract

We have studied the organization of the bacteriochlorophylls (BChl) in isolated chlorosomes of the green sulfur bacterium Chlorobium limicola UdG6040 containing about 50% BChl d and BChl c each. When the chlorosomes are treated in acidic buffer (pH 3.0) two phases in the conversion from BChl to bacteriopheophytin (BPhe) are observed as evidenced by the changes in the absorption spectrum. In the early phase the pheophytinization of BChl d occurs much faster than that of BChl c. In the later phase BChl c and BChl d are converted at similar rates. The delayed BChl c conversion observed in intact chlorosomes is interpreted in terms of spatial separation within the same chlorosome that makes BChl d more accessible to reaction with acid than BChl c. This was supported by acid treatment of in vitro pigment-lipid aggregates 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 vitro aggrega tes where BChl d and BChl c are supposed to be scrambled the two pigments are converted to BPhe simultaneously. Acid treatment of hexanol exposed chlorosomes indicates that the spatial separation of BChl d and BChl c within the chlorosomes is maintained even if the excitonic interaction between BChls has been disturbed by hexanol. Based on these findings it is suggested that BChl d and BChl c in the chlorosome are located distal and proximal, respectively, relative to the chlorosome baseplate.