The Structure of the Aggregate Form of Bacteriochlorophyll c Showing the Qy Absorption above 740 nm as Determined by the Ring‐current Effects on 1H and 13C Nuclei and by 1H–1H Intermolecular NOE Correlations

ABSTRACT 13C‐enriched bacteriochlorophyll c (S[I, E] BChl cF) was suspended in a 1:3 mixture of methylene chloride and carbon tetrachloride to form an aggregate showing the Qy absorption above 740 nm; changes in the 13C chemical shifts were traced when methanol was titrated to dissolve the aggregate, and then, the changes were correlated to the ring‐current effects due to the neighboring macrocycles in the aggregate. A pair of aggregate structures has been proposed based on the ring‐current effects on both 1H and 13C nuclei; the monomeric units are stacked together to form an inclined column with different sliding directions, in which the y‐axis of the molecule is parallel to the long axis of the column. In order to confirm this pair of models, the ring‐current effects on the 1H and 13C nuclei were calculated based on both the magnetic‐dipole and the loop‐current approximations. Further, an application of three‐dimensional F1 13C‐edited F3 13C‐filtered heteronuclear single‐quantum correlation‐nuclear Overhauser effect spectroscopy to the above aggregate consisting of a 1:1 mixture of 13C‐labeled and unlabeled BChl c succeeded in detecting selectively the 1H–1H intermolecular nuclear Overhauser effect correlations, which established the coexistence of the above pair of stacked structures in the aggregate.