Heteronuclear 2-D and 3-D magic-angle spinning NMR dipolar correlation spectroscopy was applied to determine solid-state (1)H shifts for aggregated bacteriochlorophyll c (BChl c) in uniformly (13)C-enriched light harvesting chlorosomes of the green photosynthetic bacterium Chlorobium tepidum. A complete assignment of 29 different observable resonances of the 61 protons of the aggregated BChl c in the intact chlorosomes is obtained. Aggregation shifts relative to monomeric BChl c in solution are detected for protons attached to rings I, II, and III/V and to their side chains. The 2(1)-H(3), 3(2)-H(3), and 3(1)-H resonances are shifted upfield by -2.2, -1, and -3.3 ppm, respectively, relative to monomeric BChl c in solution. Although the resonances are inhomogeneously broadened and reveal considerable global structural heterogeneity, the 5-CH and the 7-Me responses are doubled, which provides evidence for the existence of at least two relatively well-defined structurally different arrangements. Ab initio quantum chemical modeling studies were performed to refine a model for the self-assembled BChl c with two different types of BChl stacks. The BChl in the stacks can adopt either anti- or syn-configuration of the coordinative bond, where anti and syn designate the relative orientation of the Mg-OH bond relative to the direction of the 17-17(1) bond. The analogy between aggregation shifts for BChl c in the chlorosome and for self-assembled chlorophyll a/H(2)O is explored, and a bilayer model for the tubular supra-structure of sheets of BChl c is proposed, from a homology modeling approach.

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. 13C-Enriched chlorosomes were prepared from nonuniformly labeled cultures grown with NaH13CO3 as the main carbon source and from a uniformly 13C-labeled culture grown with NaH13CO3 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% 13C while the remaining positions, which could have originated also from unlabeled acetate, were labeled to approximately 60% with 13C. 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 13C 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.(ABSTRACT TRUNCATED AT 250 WORDS)

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