Complete assignment of H-1 NMR spectra and structural analysis of intact bacteriochlorophyll c dimer in solution

Intact farnesyl (3(1)R)-bacteriochlorophyll (BChl) c in carbon tetrachlorid e forms a stable dimer at room temperature characterized by two resonances resolved for each individual proton in the NMR spectrum and by a long wavel ength shift of the a, absorption band to 710 nm. All the proton resonances are precisely assigned on the basis of two-dimensional H-C and H-H correlat ion experiments. Authentic farnesyl acetate is used for assistance in the a ssignment. Extensive nuclear Overhauser effects (NOE) are observed, from wh ich distances between intermolecular proton pairs are evaluated. Geometry o f the macrocycles determined from the distance information and refined by a molecular mechanics program is found to clearly explain the observed compl exation shifts. Strong intermolecular NOE signals observed for 10-H/20(1)-H and 10-H/2(1)-H exclude a face-to-face arrangement but support an antipara llel "piggy-back" conformation for the BChl c dimer. Farnesyl protons do no t show significant complexation shifts, and it is suggested that the farnes yl side chain may adopt a folding-back confirmation with most of the group fluctuating around the periphery of the macrocycle in a restricted motion. The two-dimensional exchange experiment demonstrates chat 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 stereochem ical effect of chirality at the 3(1) position on the aggregation behavior a nd possible relationships among 680, 710, and 740 nm species are discussed.