MODELS FOR THE PIGMENT ORGANIZATION IN THE CHLOROSOMES OF PHOTOSYNTHETIC BACTERIA - DIASTEREOSELECTIVE CONTROL OF IN-VITRO BACTERIOCHLOROPHYLL C(S) AGGREGATION

The (3(1)R)- and (3(1)S)-epimeric stearyl bacteriochlorophyllides c (BChl-c(s)) are contained, together with other bacteriochlorophyllide-c esters, in the form of large aggregates in the chlorosomes, the light-harvesting antennae, of the photosynthetic green bacterium Chloroflexus aurantiacus. Their dynamic in-vitro aggregation was studied in dichloromethane and chloroform solutions by FT-IR, NMR, and UV-vis spectroscopy. Aggregate formation was spontaneous with both the separate epimers and the (R:S) mixture of ca. 2:1 which occurs naturally. A large chlorosome-type aggregate, of type II, possesses a ''piggy-back'' stack geometry and involves coordination of the C-3(1) hydroxy group of one BChl-c molecule to the central magnesium atom of a second BChl-c, with simultaneous hydrogen bonding to the C-13(1) keto group of a third molecule, i.e., Mg...O-H...O=C <, as the two main interactions. The type II aggregate is in equilibrium with the monomers which, in turn, reversibly form dimers and relatively small oligomers (type I) thereof. The extent of aggregation and the ratio of type I to type II aggregates depend on the nature of the solvent, the concentration of BChl-c(s), and the epimeric composition. In particular, the (3(1)S) epimer preferentially gives type II aggregates whereas (3(1)R) and the (R + S) mixture tend to form preferentially type I oligomers.