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... Show moreMagic 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 C-13 and N-15 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 Cd-113 MAS NMR signals indicate a five-coordination of the Cd metal. In particular, the combined NMR data show a (HOCd)-Cd-... coordination, very similar to the (HOMg)-Mg-... coordination in the natural system. Anomalously large H-1 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 A 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 C-13-C-13 correlations obtained from CP3 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. Show less
Umetsu, M.; Hollander, J.G.; Matysik, J.; Wang, Z.Y.; Adschiri, T.; Nozawa, T.; Groot, H.J.M. de 2004
(3(1)R)-bacteriochlorophyll (BChl) c solid aggregates with an absorbance around 740 nm were formed from BChl c dimers, and 2-D homonuclear C-13-C-13 radio frequency-driven dipolar recoupling as... Show more(3(1)R)-bacteriochlorophyll (BChl) c solid aggregates with an absorbance around 740 nm were formed from BChl c dimers, and 2-D homonuclear C-13-C-13 radio frequency-driven dipolar recoupling as well as proton-driven spin diffusion dipolar correlation NMR spectra have been collected in ultrahigh magnetic field. Doubling of signals is observed for most carbons in the BChl c macrocycle, leading to two correlation networks. In this way, two major fractions denoted types A and B are identified. Some of the ring carbons show multiple resonances, revealing additional slight differences in microstructural environment. 2-D heteronuclear H-1-C-13 correlation data have been recorded using the frequency- and phase-switched Lee-Goldburg technique to assign the H-1 response. N-15 chemical shifts are assigned from 2-D heteronuclear N-15-C-13 correlation experiments using spectrally induced filtering in combination with cross polarization. Also the nitrogen atoms in the pyrrole rings I, II, and IV (N-I, N-II, and N-IV, respectively) show two sets of resonances, each of which is connected to a single C-13 correlation network A or B. The C-13 chemical shifts are compared with the signals from antiparallel dimers in solution and with the response from chlorosomes previously reported. The data clearly show that the stacking in CH2Cl2-treated aggregates is different from the stacking in the chlorosomes and hexane-treated aggregates. Some degree of similarity with the antiparallel dimer form in solution transpires, in particular for the type A species. It is proposed that the CH2Cl2 precipitate represents a structural intermediate between the antiparallel dimer and the parallel stack as found in the chlorosome. Show less
Alia, A.; Matysik, J.; Boer, I. de; Gast, P.; Gorkom, H.J. van; Groot, H.J.M. de 2004
