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

13 C-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 13 C-NMR signals suggesting the presence of nonequivalent BChl c molecules in the form of the piggyback dimer.(2) Changes in the 13 C 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 13 C and 1 H nuclei (the 1 H 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 13 C-edited F3 13 C-filtered heteronuclear single-quantum nuclear-Overhauser-effect spectroscopy was applied to the aggregate consisting of a 1 : 1 mixture of 13 C-labeled and unlabeled BChl c in order to selectively detect the intermolecular 1 H-1 H NOE correlations. The NOE correlations were explained in terms of a straight column, supporting the above model.

Complete Assignment of1H NMR Spectra and Structural Analysis of Intact BacteriochlorophyllcDimer in Solution

“…Although several models have been proposed, the structure of the 740 nm species is still little known because of lack of convincing experimental results, and controversy still remains over the orientation and stacking form of the pigment molecules in the aggregates. 10,22,23 This situation may be attributed to the heterogeneous nature of the 740 nm samples formed in vitro, as can be judged from the broadness of the Q y absorption band and by the fact that there are no diffractions observed from the samples prepared by the usual hexane treatment (data not shown). To obtain more detailed information on the structure of the 740 nm species, structurally homogeneous and highly ordered samples are thought to be a prerequisite, and as an essential step, the formation process of both R-and S-type 740 nm species needs to be well controlled and thoroughly investigated.…”

Atomic structure of the bacteriochlorophyll c assembly in intact chlorosomes from Chlorobium limicola determined by solid-state NMR

“…2c). Similar doublets were ascribed to the piggyback-dimer structure from absorption spectra, ring current shifts and NOEs in solution NMR studies (Wang et al 1999a;Wang et al 1999b;Mizoguchi et al 1999). The signals of the dimer can split because the two molecules are not equivalent owing to the lack of two-fold symmetry.…”

“…The doublets for the chlorosomes are similar to those for the dimers in solution (Mizoguchi et al 1999) in terms of the chemical shift difference and the intensity ratio for the component signals. The correlation coefficient between the chemical shift differences for 33 carbon resonances in the chlorosomes and the solution was 0.63, indicating the significant positive correlation.…”

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

“…11 The aggregation shift of each signal at "0 L" methanol was determined, when necessary, by extrapolation of chemical-shift values at low methanol concentrations by the use of a function of the type, y ϭ a[1 Ϫ exp(Ϯx/b)]ϩc, where x is the amount of methanol, y is the chemical shift, and a, b, and c are constants. A set of assignments of 13 C signals was obtained by the use of two-dimensional HMQC (heteronuclear multiple-quantum correlation), HMBC (heteronuclear multiple-bond correlation), and DEPT (distortionless enhance-ment by polarization transfer) spectra.…”

“…Here, we note that no definitive information has been available concerning the size of these aggregates except for the dimer. 11 (1) Comparison of the observed aggregation shifts (changes in chemical shifts due to aggregate formation in reference to the monomeric state) with the calculated ring-current effects due to the neighboring macrocycles: The 1 H and 13 C aggregation shifts were determined by tracing the changes in chemical shifts when methanol was titrated to dissolve the aggregate which was formed in a 1 : 3 mixture of methylene chloride and carbon tetrachloride, whereas the ring-current effects were calculated by using both the magneticdipole and the loop-current approximations. The composition of these aggregate forms in a BChl c suspension depends on the stereoisomeric configuration of the hydroxyethyl group and the sidechains attached to 8 and 12 carbons of the BChl c molecule, and the BChl c concentration and the solvent in the suspension.…”