Magic-angle spinning nuclear magnetic resonance under ultrahigh field reveals two forms of intermolecular interaction within CH₂Cl₂-treated (3¹R)-type bacteriochlorophyll c solid aggregate

(3¹R)-bacteriochlorophyll (BChl) c solid aggregates with an absorbance around 740 nm were formed from BChl c dimers, and 2-D homonuclear ¹³C-¹³C 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 ¹H-¹³C correlation data have been recorded using the frequency- and phase-switched Lee-Goldburg technique to assign the ¹H response. ¹⁵N chemical shifts are assigned from 2-D heteronuclear ¹⁵N-¹³C 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 ¹³C correlation network A or B. The ¹³C 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.