Structural Variations in Chlorosomes from Wild-Type and a bchQR Mutant of Chlorobaculum tepidum Revealed by Single-Molecule Spectroscopy

Abstract

Green sulfur bacteria can grow photosynthetically by absorbing only a few photons per bacteriochlorophyll molecule per day. They contain chlorosomes, perhaps the most efficient light-harvesting antenna system found in photosynthetic organisms. Chlorosomes contain supramolecular structures comprising hundreds of thousands of bacteriochlorophyll molecules, which are properly positioned with respect to one another solely by self-assembly and not by using a protein scaffold as a template for directing the mutual arrangement of the monomers. These two features-high efficiency and self-assembly-have attracted considerable attention for developing light-harvesting systems for artificial photosynthesis. However, reflecting the heterogeneity of the natural system, detailed structural information at atomic resolution of the molecular aggregates is not yet available. Here, we compare the results for chlorosomes from the wild type and two mutants of Chlorobaculum tepidum obtained by polarization-resolved, single-particle fluorescence-excitation spectroscopy and theoretical modeling with results previously obtained from nuclear-magnetic resonance spectroscopy and cryo-electron microscopy. Only the combination of information obtained from all of these techniques allows for an unambiguous description of the molecular packing of bacteriochlorophylls within chlorosomes. In contrast to some suggestions in the literature, we find that, for the chlorosomes from the wild type as well as for those from mutants, the dominant secondary structural element features tubular symmetry following a very similar construction principle. Moreover, the results suggest that the various options for methylation of the bacteriochlorophyll molecules, which are a primary source of the structural (and spectral) heterogeneity of wild-type chlorosome samples, are exploited by nature to achieve improved spectral coverage at the level of individual chlorosomes.

Limitations of Linear Dichroism Spectroscopy for Elucidating Structural Issues of Light-Harvesting Aggregates in Chlorosomes

“…This protocol relies on the assumption that the incorporation of the secondary structural elements into the individual chlorosomes follows roughly the same overall organizational scheme. This is supported by data from cryo-electron microscopy [4,7], where it appears that the tubular structures are typically aligned along the long direction of the chlorosomes, which is also in line with the observation of the strong modulations of the fluorescence intensity as a function of the polarization [12,23,28,29,43,44]. As will be shown below the presence of such an organization is consistent with our analysis for the mutants and to a lesser extent for the WT.…”

“…tepidum [43,44] have led to debates about structural models [45] designed using ensemble LD spectra reported in the literature for benchmarking [12,17,18,22,42]. Here, we reconstruct both ensemble and single-chlorosome LD spectra from the polarization-resolved fluorescenceexcitation spectra reported in [43,44] for comparison with results from available ensemble LD spectroscopy. This tends us to conclude that LD spectroscopy is not suited to discriminate between different structural models of hierarchically organized supramolecular assemblies, such as chlorosomes.…”

“…This paper is organized as follows. In Section 2, motivated by the fact that tubular assemblies seem to be the dominant secondary structures in chlorosomes [43,44], we give the generic theoretical background of linear dichroism for molecular aggregates that feature a monomer arrangement with cylindrical symmetry. This is followed in Section 3 by the presentation of the reconstructed LD spectra.…”

Biosynthesis of the modified tetrapyrroles—the pigments of life

“…12). These two modifications allow these BChls to form protein-free, supramolecular aggregates in chlorosomes (314), the structures of which are now known (315)(316)(317)(318). These supramolecular aggregates have emergent absorption properties that are not observed for the monomeric BChls in organic solvents.…”

Interplay between structural hierarchy and exciton diffusion in artificial light harvesting

“…In reality, however, natural systems are notoriously challenging to work with as they suffer from sample degradation once extracted from their stabilizing environment and feature inherently heterogeneous structures 4,5 , which disguises relations between supramolecular morphology and excitonic properties. In this context, a class of multi-layered, supramolecular nanotubes holds promise as artificial light-harvesting systems owing to their intriguing optical properties and structural homogeneity paired with self-assembly capabilities and robustness 6–8 .…”