The structure of an aggregate form of bacteriochlorophyll c showing the Q(y) absorption at 705 nm as determined by the ring-current effects on H-1 and C-13 nuclei and by H-1-H-1 intermolecular NOE correlations

C-13-enriched bacteriochlorophyll c (R[E, E] BChl c(F)) was suspended in ch loroform to form an aggregate showing the Q(y) absorption at 705 nm. (1) Th e aggregate exhibited several largely split C-13-NMR signals suggesting the presence of nonequivalent BChl c molecules in the form of the piggyback di mer. (2) Changes in the C-13 chemical shifts were traced when methanol was titrated to dissolve the aggregate, and the aggregation shifts (in referenc e to the monomeric state) were determined as a function of the amount of me thanol titrated, and they were analyzed empirically. (3) The ring-current e ffects were calculated based on the loop-current approximation, and the res ults were compared with the observed aggregation shifts for C-13 and H-1 nu clei (the H-1 aggregation shifts were determined by extrapolation of the da ta taken from Mizoguchi, T.; Limantara, L.; Matsuura, K.; Shimada, K.; Koya ma, Y. J Mol Structure 1996, 379, 249-265). The results showed that the ass embly of two straight columns consisting of the piggyback dimer stacked in the antiparallel orientation is the best choice as a model for the B705 agg regate. (4) Three-dimensional F1 C-13-edited F3 C-13-filtered heteronuclear single-quantum nuclear-Overhauser-effect spectroscopy was applied to the a ggregate consisting of a 1 : 1 mixture of C-13-labeled and unlabeled BChl c in order to selectively detect the intermolecular H-1-H-1 NOE correlations . The NOE correlations were explained in terms of a straight column, suppor ting the above model. (C) 1999 John Wiley & Sons, Inc.