¹³C- and ¹⁵N-NMR studies on the intact bacteriochlorophyll c dimers in solutions

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-famesyl 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 thai the paramagnetic shielding effect is predominant and N(IV) nitrogen is most sensitive to the dimer formation, followed by N(II), N(I), and N(III), 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.