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Stacking of Bacteriochlorophyll c Macrocycles in Chlorosome from Chlorobium limicola As Revealed by Intermolecular 13C Magnetic-Dipole Correlation, X-Ray Diffraction, and Quadrupole Coupling in 25Mg NMR

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posted on 13.01.2009, 00:00 by Yoshinori Kakitani, Yasushi Koyama, Yuichi Shimoikeda, Toshihito Nakai, Hiroaki Utsumi, Tadashi Shimizu, Hiroyoshi Nagae
The stacking of the bacteriochlorophyll (BChl) c macrocycles and the role of water in forming an aggregate sheet, in chlorosome, were examined by means of 13C NMR spectroscopy, the measurement of the X-ray diffraction pattern, and 25Mg NMR spectroscopy. (1) The stacking of the macrocycles, i.e., weakly overlapped dimers forming displaced layers, was selected out of six different kinds of stacking so far identified in the aggregates of isomeric BChl c in solution and in the solid aggregate of an isomeric mixture of BChl c extracted from Chlorobium limicola. The selection was based on the comparison of the intermolecular 13C···13C magnetic-dipole correlations with the nearest-neighbor carbon-to-carbon close contacts simulated for the above six different stackings. It has turned out that the stacking of the macrocycles in chlorosome is basically the same as that in the in vitro solid aggregate. (2) The crucial role of water in stabilizing the aggregate structure in chlorosome was shown by tracing the dehydration processes and by comparison with the solid aggregate using the X-ray diffraction pattern. Possible binding sites of water molecules were located, by structural simulation, based on the particular stacking structure. (3) The dimer-based stacking of the macrocycles was evidenced by 25Mg NMR spectroscopy, which exhibited a pair of signals showing different quadrupole coupling, due to the presence or absence of a water molecule in the axial position.

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