Complete Assignment of1H NMR Spectra and Structural Analysis of Intact BacteriochlorophyllcDimer 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 Qy 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 s...