Self-aggregates of bacteriochlorophylls-c, d and e in a light-harvesting antenna system of green photosynthetic bacteria: Effect of stereochemistry at the chiral 3-(1-hydroxyethyl) group on the supramolecular arrangement of chlorophyllous pigments

Green photosynthetic bacteria possess light-harvesting antenna systems, called chlorosomes, in which a number of bacteriochlorophyll(BChl)-c/d/e molecules self-aggregate to form rod-shaped oligomers. All BChls-c/d/e have a 1-hydroxyethyl group at the 31-position and are 31-epimeric mixtures at various ratios. These stereoisomers were isolated and structurally determined by several experimental techniques. In vitro studies on their self-aggregation indicated that the 31-stereochemistry affected the supramolecular structures of the chlorosomal assemblies. Some recent reports suggested that both stereoisomers are required for making rod-shaped chlorosome-type aggregates, i.e., the 31R/S-stereoisomers co-operate in the construction of the antenna system. The synthetic models of BChls possessing a chiral secondary alcoholic group also exhibited stereochemical control of the self-aggregation. In this review, we will discuss the unique stereochemical control on the chlorosomal BChl self-aggregates.