Structural variability in wild-type and bchQ bchR mutant chlorosomes of the green sulfur bacterium Chlorobaculum tepidum

The self-aggregated state of bacteriochlorophyll (BChl) c molecules in chlorosomes belonging to a bchQ bchR mutant of the green sulfur bacteria Chlorobaculum tepidum, which mostly produces a single 17²-farnesyl- (R)-[8-ethyl,12-methyl]BChl c homologue, was characterized by solid-state nuclear magnetic resonance (NMR) spectroscopy and high-resolution electron microscopy. A nearly complete ¹H and ¹³C chemical shift assignment was obtained from well-resolved homonuclear ¹³C- ¹³C and heteronuclear ¹H-¹³C NMR data sets collected from ¹³C-enriched chlorosome preparations. Pronounced doubling (1:1) of specific ¹³C and ¹H resonances revealed the presence of two distinct and nonequivalent BChl c components, attributed to all syn- and all anti-coordinated parallel stacks, depending on the rotation of the macrocycle with respect to the 3¹-methyl group. Steric hindrance from the 20-methyl functionality induces structural differences between the syn and anti forms. A weak but significant and reproducible reflection at 1/0.69 nm^-1 in the direction perpendicular to the curvature of cylindrical segments observed with electron microscopy also suggests parallel stacking of BChl c molecules, though the observed lamellar spacing of 2.4 nm suggests weaker packing than for wild-type chlorosomes. We propose that relaxation of the pseudosymmetry observed for the wild type and a related BChl d mutant leads to extended domains of alternating syn and anti stacks in the bchQ bchR chlorosomes. Domains can be joined to form cylinders by helical syn-anti transition trajectories. The phase separation in domains on the cylindrical surface represents a basic mechanism for establishing suprastructural heterogeneity in an otherwise uniform supramolecular scaffolding framework that is well-ordered at the molecular level. (Figure Presented).