13C- and 15N-NMR Studies on the Intact Bacteriochlorophyll c Dimers in Solutions
journal contributionposted on 25.09.1999, 00:00 by Zheng-Yu Wang, Mitsuo Umetsu, Masayuki Kobayashi, Tsunenori Nozawa
13C and 15N chemical shifts of the intact farnesyl (31R)-bacteriochlorophyll (BChl) c have been measured in methanol and carbon tetrachloride solutions. Two sets of resonances have been observed in carbon tetrachloride for all carbon and nitrogen atoms, indicating a formation of highly stable dimeric species with asymmetric configurations. Complete assignments have been made based on a combination of homonuclear and heteronuclear correlation experiments using the 13C- and 15N-labeled BChl c samples. Changes of the 13C chemical shift in the two solvents can be interpreted in terms of mixed effects arising from (a) ring current due to the overlap of the macrocycles, (b) coordination state of the central magnesium, (c) excitation state of the π-electron system, (d) polarity of the solvents used, and (e) hydrogen bonding. Substantial ring current effect is observed on the 13C chemical shifts for the carbon atoms around pyrrolic ring I upon the dimer formation. Remarkable differences in the line widths observed for all propionic carbons and some carbons of the farnesyl group suggest that the propionic-farnesyl side chains may adopt a “return” structure over the region from 171 to f2 carbons with much different conformation and mobility in the dimer. No clear evidence is obtained for a hydrogen bond formed with the C131 carbonyl group in CCl4 solution, nor for ring overlap over the ring V. Comparison between the 15N chemical shifts in both solvents indicates that the paramagnetic shielding effect is predominant and NIV nitrogen is most sensitive to the dimer formation, followed by NII, NI, and NIII, respectively. The result reveals a high sensitivity of 15N chemical shift to the electronic state and N−Mg bond length for each nitrogen atom in the dimer.