CP-MAS C-13-NMR DIPOLAR CORRELATION SPECTROSCOPY OF C-13-ENRICHED CHLOROSOMES AND ISOLATED BACTERIOCHLOROPHYLL-C AGGREGATES OF CHLOROBIUM-TEPIDUM - THE SELF-ORGANIZATION OF PIGMENTS IS THE MAIN STRUCTURAL FEATURE OF CHLOROSOMES

Magic angle spinning (MAS) NMR dipolar correlation spectroscopy was ap plied for the first time to a biologically intact system, the light-ha rvesting chlorosomes of the green photosynthetic bacterium Chlorobium tepidum. The MAS spectra provide evidence that the self-organization o f many thousands of bacteriochlorophyll c (BChl c) molecules is the pr edominant structural feature of the chlorosome. C-13-Enriched chloroso mes were prepared from nonuniformly labeled cultures grown with (NaHCO 3)-C-13 as the main carbon source and from a uniformly C-13-labeled cu lture grown with (NaHCO3)-C-13 as the sole carbon source. For the nonu niformly labeled samples, the positions of the chlorin macrocycle orig inating from C-4 and C-5 of 5-aminolevulinic acid contained >95% C-13 while the remaining positions, which could have originated also from u nlabeled acetate, were labeled to similar to 60% with C-13. The 1-D an d 2-D MAS data of the labeled chlorosomes, when compared with data on the isolated labeled BChl c aggregated in n-hexane, show that the majo r component of the MAS signals in the chlorosomes is from BChl c, and only minor signal contributions arise from lipids and proteins. The C- 13 MAS signals of the BChl c aggregates were fully assigned by MAS 2-D dipolar correlation spectroscopy, using data on monomeric BChl c in C DCl3/CD3OD as reference. The 2(1)-, 3-, 3(2)-, 5-, 12(1)-, 13-, and 13 (1)-carbons are shifted by 2.5 ppm or more upfield with respect to the solution data. The 2-D response of the BChl c in intact chlorosomes i s virtually indistinguishable from that of the in vitro aggregate with respect to chemical shifts, line widths, and relative intensities of the cross-peaks. This corroborates previous evidence that self-assembl y of BChl c, without the interaction with protein, provides the struct ural basis for the BChl c organization in vivo.