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 applied for the first time to a biologically intact system, the light-harvesting chlorosomes of the green photosynthetic bacterium Chlorobium tepidum. The MAS spectra provide evidence that the self-organization of many thousands of bacteriochlorophyll c (BChl c) molecules is the predominant structural feature of the chlorosome. C-13-Enriched chlorosomes were prepared from nonuniformly labeled cultures grown with (NaHCO3)-C-13 as the main carbon source and from a uniformly C-13-labeled culture grown with (NaHCO3)-C-13 as the sole carbon source. For the nonuniformly labeled samples, the positions of the chlorin macrocycle originating from C-4 and C-5 of 5-aminolevulinic acid contained >95% C-13 while the remaining positions, which could have originated also from unlabeled acetate, were labeled to similar to 60% with C-13. The 1-D and 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 major 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 CDCl3/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 is 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-assembly of BChl c, without the interaction with protein, provides the structural basis for the BChl c organization in vivo.