The structure of the aggregate form of bacteriochlorophyll c showing the Q(y) absorption above 740 nm as determined by the ring-current effects on H-1 and C-13 nuclei and by H-1-H-1 intermolecular NOE correlations

摘要

C-13-enriched bacteriochlorophyll c (S[I, E] BChl c(F)) was suspended in a 1:3 mixture of methylene chloride and carbon tetrachloride to form an aggregate showing the Q(y) absorption above 740 nm; changes in the C-13 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 H-1 and C-13 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 H-1 and C-13 nuclei were calculated based on both the magnetic-dipole and the loop-current approximations, Further, an application of three-dimensional F1 C-13-edited F3 C-13-filtered heteronuclear single-quantum correlation-nuclear Overhauser effect spectroscopy to the above aggregate consisting of a 1:1 mixture of C-13-labeled and unlabeled BChl c succeeded in detecting selectively the H-1-H-1 intermolecular nuclear Overhauser effect correlations, which established the coexistence of the above pair of stacked structures in the aggregate. [References: 21]