Complete assignment of¹H NMR spectra and structural analysis of intact bacteriochlorophyll c dimer in solution

Intact farnesyl (³¹R)-bacteriochlorophyll (BChl) c in carbon tetrachloride forms a stable dimer at room temperature characterized by two resonances resolved for each individual proton in the NMR spectrum and by a long wavelength shift of the Q_y absorption band to 710 nm. All the proton resonances are precisely assigned on the basis of two-dimensional H-C and H-H correlation experiments. Authentic farnesyl acetate is used for assistance in the assignment. Extensive nuclear Overhauser effects (NOE) are observed, from which distances between intermolecular proton pairs are evaluated. Geometry of the macrocycles determined from the distance information and refined by a molecular mechanics program is found to clearly explain the observed complexation shifts. Strong intermolecular NOE signals observed for 10-H/20¹-H and 10-H/2¹-H exclude a face-to-face arrangement but support an antiparallel "piggy-back" conformation for the BChl c dimer. Farnesyl protons do not show significant complexation shifts, and it is suggested that the farnesyl side chain may adopt a folding-back conformation with most of the group fluctuating around the periphery of the macrocycle in a restricted motion. The two-dimensional exchange experiment demonstrates that molecules in the dimer experience slow exchange between the two nonequivalent configurations with an exchange rate constant of about 1.8 s^-1. Finally, the stereochemical effect of chirality at the 3¹ position on the aggregation behavior and possible relationships among 680, 710, and 740 nm species are discussed.