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

C-13 and N-15 chemical shifts of the intact farnesyl (3(1)R)-bacteriochloro phyll (BChl) c have been measured in methanol and carbon tetrachloride solu tions. Two sets of resonances have been observed in carbon tetrachloride fo r all carbon and nitrogen atoms, indicating a formation of highly stable di meric species with asymmetric configurations. Complete assignments have bee n made based on a combination of homonuclear and heteronuclear correlation experiments using the C-13- and N-15-labeled BChl c samples. Changes of the C-13 chemical shift in the two solvents can be interpreted in terms of mix ed effects arising from (a) ring current due to the overlap of the macrocyc les, (b) coordination state of the central magnesium, (c) excitation state of the pi-electron system, (d) polarity of the solvents used, and (e) hydro gen bonding. Substantial ring current effect is observed on the C-13 chemic al shifts for the carbon atoms around pyrrolic ring I upon the dimer format ion. Remarkable differences in the line widths observed for all propionic c arbons and some carbons of the farnesyl group suggest that the propionic-fa rnesyl side chains may adopt a "return" structure over the region from 17(1 ) to f2 carbons with much different conformation and mobility in the dimer. No clear evidence is obtained for a hydrogen bond formed with the C13(1) c arbonyl group in CCl4 solution, nor for ring overlap over the ring V. Compa rison between the N-15 chemical shifts in both solvents indicates that the paramagnetic shielding effect is predominant and N-IV nitrogen is most sens itive to the dimer formation, followed by N-II, N-I, and N-III, respectivel y. The result reveals a high sensitivity of N-15 chemical shift to the elec tronic state and N-Mg bond length for each nitrogen atom in the dimer.