The structure of an aggregate form of bacteriochlorophyll c showing the Qy absorption at 705 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 (R[E, E] BChl c_F) was suspended in chloroform to form an aggregate showing the Q_y absorption at 705 nm. (1) The aggregate exhibited several largely split ¹³C-NMR signals suggesting the presence of nonequivalent BChl c molecules in the form of the piggyback dimer. (2) Changes in the ¹³C chemical shifts were traced when methanol was titrated to dissolve the aggregate, and the aggregation shifts (in reference to the monomeric state) were determined as a function of the amount of methanol titrated, and they were analyzed empirically. (3) The ring-current effects were calculated based on the loop-current approximation, and the results were compared with the observed aggregation shifts for ¹³C and ¹H nuclei (the ¹H aggregation shifts were determined by extrapolation of the data taken from Mizoguchi, T.; Limantara, L.; Matsuura, K.; Shimada, K.; Koyama, Y. J Mol Structure 1996, 379, 249–265). The results showed that the assembly of two straight columns consisting of the piggyback dimer stacked in the antiparallel orientation is the best choice as a model for the B705 aggregate. (4) Three-dimensional F1 ¹³C-edited F3 ¹³C-filtered heteronuclear single-quantum nuclear-Overhauser-effect spectroscopy was applied to the aggregate consisting of a 1 : 1 mixture of ¹³C-labeled and unlabeled BChl c in order to selectively detect the intermolecular ¹H–¹H NOE correlations. The NOE correlations were explained in terms of a straight column, supporting the above model. © 1999 John Wiley & Sons, Inc. Biospectroscopy 5: 63–77, 1999