THE ORGANIZATION OF BACTERIOCHLOROPHYLL-C IN CHLOROSOMES FROM CHLOROFLEXUS-AURANTIACUS AND THE STRUCTURAL ROLE OF CAROTENOIDS AND PROTEIN -AN ABSORPTION, LINEAR DICHROISM, CIRCULAR-DICHROISM AND STARK SPECTROSCOPY STUDY

The organization of bacteriochlorophyll c (BChl c) molecules was studi ed in normal and carotenoid-deficient chlorosomes isolated from the gr een phototrophic bacterium Chloroflexus aurantiacus. Carotenoid-defici ent chlorosomes were obtained from cells grown in the presence of 60 m u g of 2-hydroxybiphenyl per mi. At this concentration, BChl c synthes is was not affected while the formation of the 5.7 kDa chlorosome poly peptide was inhibited by about 50% (M. Foidl et al., submitted). Absor ption, linear dichroism and circular dichroism spectroscopy showed tha t the organization of BChl c molecules with respect to each other as w ell as to the long axis of the chlorosomes was similar for both types of chlorosomes. Therefore, it is concluded that the organization of BC hl c molecules is largely independent on the presence of the bulk of c arotenoids as well as of at least half of the normal amount of the 5.7 kDa polypeptide. The Stark spectra of the chlorosomes, as characteriz ed by a large difference polarizability for the ground-and excited sta tes of the interacting BChl c molecules, were much more intense than t hose of individual pigments. It is proposed that this is caused by the strong overlap of BChl c molecules in the chlorosomes. In contrast to individual chlorophylls, BChl c in chlorosomes did not give rise to a significant difference permanent dipole moment for the ground-and exc ited states. This observation favors models for the BChl c organizatio n which invoke the anti-parallel stacking of linear BChl c aggregates above those models in which linear BChl c aggregates are stacked in a parallel fashion. The difference between the Stark spectrum of caroten oid-deficient and WT chlorosomes indicates that the carotenoids are in the vicinity of the BChls.