Structures and Organization of Bacteriochlorophyll c’s in Chlorosomes from a New Thermophilic Bacterium Chlorobium tepidum

A new thermophilic bacterium Chlorobium tepidum contains two major bacteriochlorophyll c components whose periphery substituents at 4-position are either an ethyl or a propyl group. Both components possess ethyl groups at 5-position, and the ester alkyl groups at 7-position are farnesyl groups. As an aggregate structure in CDCl3 a piggy-back type dimer was suggested on the basis of variations of 1H NMR chemical shifts accompanied with the aggregate formation and of values of calculated ring current shifts. Bacteriochlorophyll c (BChl c) aggregates with absorption maxima around 677, 705, and 740 nm showed stepwise interchanges in hexane-dichloromethane with variations in BChl c concentrations and in solvent compositions. Exciton theoretical analysis of the stepwise spectral shifts predicted high ordered organization of BChl c, implying a model for rod-elements in chlorosomes.