In vitro self-assemblies of bacteriochlorophylls-c from Chlorobaculum tepidum and their supramolecular nanostructures

Abstract

Chlorosomes are major light-harvesting antenna apparatuses in green photosynthetic bacteria. Chlorosomes contain a large amount of bacteriochlorophyll(BChl)-c, d, e and f molecules and the supramolecular nanostructures of their self-assemblies are suggested to be rods with 5-nm or 10-nm diameters and/or lamellas with approximately 2-nm spacing. BChls-c were extracted, isolated and purified from cultured cells of a green sulfur photosynthetic bacterium, Chlorobaculum (Cba.) tepidum. Self-assemblies of the natural composite BChls-c as well as each pure homolog/epimer were prepared in a hydrophobic hexane-based solution and the resulting self-assembled solids were investigated by spectroscopic and microscopic techniques. Visible and near-infrared absorption and circular dichroism spectra of the in vitro self-assemblies of the natural composite BChls-c on a quartz substrate were closely similar to those in cells of Cba. tepidum in an aqueous buffer solution. Self-assemblies of (31R)-epimerically pure BChl-c molecules showed a wider Qy absorption band with increase in the steric bulkiness of hydrocarbon substituents at the 8- and 12-positions, which were effective for capturing wider wavelength light. (31S)-BChls-c gave similar self-assembly bands, but more monomeric and/or dimeric species than (31R)-BChls-c. Atomic force microscopic images of in vitro self-assemblies of natural composite BChls-c showed a rod with a 5-nm height and bundles of fibrils with 1–2-nm heights. Spectroscopic and microscopic analysis suggested that the 82-methylation enhanced π–π stacking of self-assemblies by hydrophobic interaction, but the 121-methylation slightly weakened the intermolecular interactions due to its steric bulkiness.