Solid-state NMR is an emerging method to obtain structural information in molecular biology and nanotechnology for systems that are inaccessible to solution NMR or diffraction methods. While solution NMR generally converges upon families of structures in a bottom-up approach, solid NMR structure determination will have to take into account the top-down constraints that follow from the additional requirement that the entire 3D space must be packed in an orderly fashion. We used MAS NMR together with molecular modeling calculations in steps to establish a detailed model of the local crystal structure of an aggregate of uniformly 13C- and 15N-labeled Cd-chlorophyllide d, a model for the chlorosomal antennae. In this way we converge upon a space group P21 with a = 14.3 A, b = 27.3 A, c = 6.4 A, beta = 147.2 degrees and Z = 2.
Magic angle spinning NMR spectroscopy has been used to investigate the self-organization of bacteriochlorophylls in chlorosomal light-harvesting antennae. Two model cadmium chlorins were studied that were uniformly 13 C and 15 N enriched in the ring moieties. The chlorin models differ from the natural BChl c in the central metal and the 3-, 12-, 17-, and 20-side chains. One model system has the farnesyl tail replaced by a methyl, whereas the other has a stearyl tail. The 113 Cd MAS NMR signals indicate a five-coordination of the Cd metal. In particular, the combined NMR data show a HO‚‚‚Cd coordination, very similar to the HO‚‚‚Mg coordination in the natural system. Anomalously large 1 H ring-current shifts of up to 10 ppm reveal a dense orderly stacking of the molecules in planar layers, for which a correlation length of at least 24 Å was defined from long-range ring-current shift calculations. In addition, our model structures confirm and validate the essential role of the [3 1 R] and [3 1 S] stereoisomers in the formation of the chlorosomal antennae, as tubular structures are not formed without this chirality. The 3D arrangement of the layers is revealed by intermolecular 13 C-13 C correlations obtained from CP 3 CHHC experiments. With the tail truncated to methyl, a microcrystalline solid is formed with favorable interactions between the planar sheets in a head-to-tail orientation. The stearyl tails lead to a considerably disordered aggregate consisting of both syn and anti layers similar to the chlorosomes, as indicated by a doubling of the N-D signal. These results reveal a balance between relatively strong local interactions and contributions to the free energy of the system associated with a longer length scale. This leads to a robust chlorosome structure, stable against thermodynamic noise, and allows for fine-tuning of the structure.
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