文摘
A mixture of bacteriochlorophyll (BChl) c isomers was extracted from the cells of Chlorobiumlimicola that were grown in the media of 13C-enriched and natural-abundance isotopic compositions. Themagic-angle spinning 13C NMR proton-driven spin-diffusion spectra were recorded with mixing times of50, 100, and 250 ms for two different kinds of in vitro aggregates, one consisting of pure [13C]BChl c andthe other consisting of a 1:1 mixture of [13C]BChl c and [12C]BChl c; those peaks whose intensities werereduced to ~1/4 by this dilution were assigned to intermolecular 13C-13C dipolar correlation peaks. Onthe other hand, the nearest-neighbor intermolecular carbon-carbon close contacts with distances of 4-6Å were simulated, to predict observed correlation peaks, for six different models of BChl c assembly.They include weakly overlapped monomers forming structure 1 and structure 2, strongly overlapped dimersforming straight and inclined columns, and weakly overlapped dimers forming aligned and displacedlayers. Comparison between the observed correlation peaks and the predicted carbon-carbon close contacts,for both the macrocycles and the side chains, led us to a conclusion that the weakly overlapped dimersforming displaced layers are most likely the assembly of the BChl c molecules in the aggregate.
