(3 1 R )-bacteriochlorophyll (BChl) c solid aggregates with an absorbance around 740 nm were formed from BChl c dimers, and 2-D homonuclear 13 C− 13 C 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 1 H− 13 C correlation data have been recorded using the frequency- and phase-switched Lee−Goldburg technique to assign the 1 H response. 15 N chemical shifts are assigned from 2-D heteronuclear 15 N− 13 C 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 13 C correlation network A or B. The 13 C 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 CH 2 Cl 2 -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 CH 2 Cl 2 precipitate represents a structural intermediate between the antiparallel dimer and the parallel stack as found in the chlorosome.
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Umetsu et al. (2004) studied this question.