Effect of carotenoids and monogalactosyl diglyceride on bacteriochlorophyll c aggregates in aqueous buffer: implications for the self-assembly of chlorosomes

Superradiance of bacteriochlorophyll c aggregates in chlorosomes of green photosynthetic bacteria

Chlorosomes are the main light-harvesting complexes of green photosynthetic bacteria that are adapted to a phototrophic life at low-light conditions. They contain a large number of bacteriochlorophyll c, d, or e molecules organized in self-assembling aggregates. Tight packing of the pigments results in strong excitonic interactions between the monomers, which leads to a redshift of the absorption spectra and excitation delocalization. Due to the large amount of disorder present in chlorosomes, the extent of delocalization is limited and further decreases in time after excitation. In this work we address the question whether the excitonic interactions between the bacteriochlorophyll c molecules are strong enough to maintain some extent of delocalization even after exciton relaxation. That would manifest itself by collective spontaneous emission, so-called superradiance. We show that despite a very low fluorescence quantum yield and short excited state lifetime, both caused by the aggregation, chlorosomes indeed exhibit superradiance. The emission occurs from states delocalized over at least two molecules. In other words, the dipole strength of the emissive states is larger than for a bacteriochlorophyll c monomer. This represents an important functional mechanism increasing the probability of excitation energy transfer that is vital at low-light conditions. Similar behaviour was observed also in one type of artificial aggregates, and this may be beneficial for their potential use in artificial photosynthesis.

Amphiphilic block copolymers as flexible membrane materials generating structural and functional mimics of green bacterial antenna complexes

We describe the ability of a short-chain amphiphilic block copolymer to self-assemble to form an artificial supramolecular light-harvesting system. Specifically, we demonstrate that the 2.5 kDa, poly(ethylene oxide)-block-poly(butadiene) (PEO-b-PBD), exhibits sufficient morphological flexibility as a membrane material and enables generation of mimics of three-dimensional chlorosomes as well as supported membrane bilayers containing energy acceptors. This overall architecture replicates green bacterial light-harvesting function whereby these assemblies exhibit long-range order and three-dimensional morphology similar to native chlorosomes and are capable of energy transfer internally and to external acceptors, located in a supporting biomimetic polymer membrane. Unlike native green bacterial systems that use multiple lipids as a matrix to generate the appropriate environment for chlorosome assembly and function, the described system matrix is comprised entirely of a single polymer amphiphile. This work demonstrates the potential of short-chain amphiphilic block copolymers in generating self-assembled, bio-mimetic membrane architectures, and in doing so, generates scalable, spatial-energetic landscapes for photonic applications. Finally, the results presented provide evidence of minimal requirements to induce chlorosome-like organization and function.

Excited state lifetimes and energies of okenone and chlorobactene, exemplary keto and non-keto aryl carotenoids

Photophysical properties of two typical aryl carotenoids, okenone and chlorobactene, were studied with application of femtosecond and microsecond time-resolved absorption spectroscopies.

Bacteriochlorophyll aggregates self-assembled on functionalized gold nanorod cores as mimics of photosynthetic chlorosomal antennae: a single molecule study

We prepare artificial aggregates that mimic the structure and function of natural chlorosomal light harvesting complexes of green photosynthetic bacteria. Gold nanorods functionalized with hydroxyl groups and immobilized on a substrate serve as cores for the growth of bacteriochlorophyll (BChl) aggregates from a buffer solution. The BChl pigments form large self-assembled aggregate particles with sizes more than twice that of natural chlorosomes. The size is controllable by the aggregation time. The aggregates are characterized on a single-particle level by atomic force microscopy, electron microscopy, and single-molecule spectroscopy. The absorption and fluorescence spectral properties which reflect the molecular level arrangement of the BChl aggregates closely resemble those of the natural chlorosomes of the photosynthetic bacterium Chlorobaculum tepidum. On the other hand, the results of linear dichroism and circular dichroism are different from those of the chlorosomes and indicate a different mesoscopic structure for the artificial aggregates. These results emphasize the structural role played by the baseplate pigment-protein complex in natural chlorosomes.

Structural and functional roles of carotenoids in chlorosomes

Chlorosomes are large light-harvesting complexes found in three phyla of anoxygenic photosynthetic bacteria. Chlorosomes are primarily composed of self-assembling pigment aggregates. In addition to the main pigment, bacteriochlorophyll c, d, or e, chlorosomes also contain variable amounts of carotenoids. Here, we use X-ray scattering and electron cryomicroscopy, complemented with absorption spectroscopy and pigment analysis, to compare the morphologies, structures, and pigment compositions of chlorosomes from Chloroflexus aurantiacus grown under two different light conditions and Chlorobaculum tepidum. High-purity chlorosomes from C. aurantiacus contain about 20% more carotenoid per bacteriochlorophyll c molecule when grown under low light than when grown under high light. This accentuates the light-harvesting function of carotenoids, in addition to their photoprotective role. The low-light chlorosomes are thicker due to the overall greater content of pigments and contain domains of lamellar aggregates. Experiments where carotenoids were selectively extracted from intact chlorosomes using hexane proved that they are located in the interlamellar space, as observed previously for species belonging to the phylum Chlorobi. A fraction of the carotenoids are localized in the baseplate, where they are bound differently and cannot be removed by hexane. In C. tepidum, carotenoids cannot be extracted by hexane even from the chlorosome interior. The chemical structure of the pigments in C. tepidum may lead to π-π interactions between carotenoids and bacteriochlorophylls, preventing carotenoid extraction. The results provide information about the nature of interactions between bacteriochlorophylls and carotenoids in the protein-free environment of the chlorosome interior.

Structure of chlorosomes from the green filamentous bacterium Chloroflexus aurantiacus

The green filamentous bacterium Chloroflexus aurantiacus employs chlorosomes as photosynthetic antennae. Chlorosomes contain bacteriochlorophyll aggregates and are attached to the inner side of a plasma membrane via a protein baseplate. The structure of chlorosomes from C. aurantiacus was investigated by using a combination of cryo-electron microscopy and X-ray diffraction and compared with that of Chlorobi species. Cryo-electron tomography revealed thin chlorosomes for which a distinct crystalline baseplate lattice was visualized in high-resolution projections. The baseplate is present only on one side of the chlorosome, and the lattice dimensions suggest that a dimer of the CsmA protein is the building block. The bacteriochlorophyll aggregates inside the chlorosome are arranged in lamellae, but the spacing is much greater than that in Chlorobi species. A comparison of chlorosomes from different species suggested that the lamellar spacing is proportional to the chain length of the esterifying alcohols. C. aurantiacus chlorosomes accumulate larger quantities of carotenoids under high-light conditions, presumably to provide photoprotection. The wider lamellae allow accommodation of the additional carotenoids and lead to increased disorder within the lamellae.

Hexanol-induced order-disorder transitions in lamellar self-assembling aggregates of bacteriochlorophyll c in Chlorobium tepidum chlorosomes

Chlorosomes are light-harvesting complexes of green photosynthetic bacteria. Chlorosomes contain bacteriochlorophyll (BChl) c, d, or e aggregates that exhibit strong excitonic coupling. The short-range order, which is responsible for the coupling, has been proposed to be augmented by pigment arrangement into undulated lamellar structures with spacing between 2 and 3 nm. Treatment of chlorosomes with hexanol reversibly converts the aggregated chlorosome chlorophylls into a form with spectral properties very similar to that of the monomer. Although this transition has been extensively studied, the structural basis remains unclear due to variability in the obtained morphologies. Here we investigated hexanol-induced structural changes in the lamellar organization of BChl c in chlorosomes from Chlorobium tepidum by a combination of X-ray scattering, electron cryomicroscopy, and optical spectroscopy. At a low hexanol/pigment ratio, the lamellae persisted in the presence of hexanol while the short-range order and exciton interactions between chlorin rings were effectively eliminated, producing a monomer-like absorption. The result suggested that hexanol hydroxyls solvated the chlorin rings while the aliphatic tail partitioned into the hydrophobic part of the lamellar structure. This partitioning extended the chlorosome along its long axis. Further increase of the hexanol/pigment ratio produced round pigment-hexanol droplets, which lost all lamellar order. After hexanol removal the spectral properties were restored. In the samples treated under the high hexanol/pigment ratio, lamellae reassembled in small domains after hexanol removal while the shape and long-range order were irreversibly lost. Thus, all the interactions required for establishing the short-range order by self-assembly are provided by BChl c molecules alone. However, the long-range order and overall shape are imposed by an external structure, e.g., the proteinaceous chlorosome baseplate.

The 17-propionate function of (bacterio)chlorophylls: biological implication of their long esterifying chains in photosynthetic systems

Molecular structures of (bacterio)chlorophylls [= (B)Chls] in photosynthetic apparatus are surveyed, and a diversity of the ester groups of the 17-propionate substituent is particularly focused on in this review. In oxygenic photosynthetic species including green plants and algae, the ester of Chl molecules is limited to a phytyl group. Geranylgeranyl and farnesyl groups in addition to phytyl are observed in (B)Chl molecules inside photosynthetic proteins of anoxygenic bacteria. In main light-harvesting antennas of green bacteria (chlorosomes), a greater variety of ester groups including long straight chains are used in the composite BChl molecules. This diversity is ascribable to the fact that chlorosomal BChls self-aggregate to form a core part of chlorosomes without any specific interaction of oligopeptides. Biological significance of the long chains is discussed in photosynthetic apparatus, especially in chlorosomes.

X-ray scattering and electron cryomicroscopy study on the effect of carotenoid biosynthesis to the structure of Chlorobium tepidum chlorosomes

Chlorosomes, the main antenna complexes of green photosynthetic bacteria, were isolated from null mutants of Chlorobium tepidum, each of which lacked one enzyme involved in the biosynthesis of carotenoids. The effects of the altered carotenoid composition on the structure of the chlorosomes were studied by means of x-ray scattering and electron cryomicroscopy. The chlorosomes from each mutant strain exhibited a lamellar arrangement of the bacteriochlorophyll c aggregates, which are the major constituents of the chlorosome interior. However, the carotenoid content and composition had a pronounced effect on chlorosome biogenesis and structure. The results indicate that carotenoids with a sufficiently long conjugated system are important for the biogenesis of the chlorosome baseplate. Defects in the baseplate structure affected the shape of the chlorosomes and were correlated with differences in the arrangement of lamellae and spacing between the lamellar planes of bacteriochlorophyll aggregates. In addition, comparisons among the various mutants enabled refinement of the assignments of the x-ray scattering peaks. While the main scattering peaks come from the lamellar structure of bacteriochlorophyll c aggregates, some minor peaks may originate from the paracrystalline arrangement of CsmA in the baseplate.

Internal structure of chlorosomes from brown-colored chlorobium species and the role of carotenoids in their assembly

Chlorosomes are the main light harvesting complexes of green photosynthetic bacteria. Recently, a lamellar model was proposed for the arrangement of pigment aggregates in Chlorobium tepidum chlorosomes, which contain bacteriochlorophyll (BChl) c as the main pigment. Here we demonstrate that the lamellar organization is also found in chlorosomes from two brown-colored species (Chl. phaeovibrioides and Chl. phaeobacteroides) containing BChl e as the main pigment. This suggests that the lamellar model is universal among green sulfur bacteria. In contrast to green-colored Chl. tepidum, chlorosomes from the brown-colored species often contain domains of lamellar aggregates that may help them to survive in extremely low light conditions. We suggest that carotenoids are localized between the lamellar planes and drive lamellar assembly by augmenting hydrophobic interactions. A model for chlorosome assembly, which accounts for the role of carotenoids and secondary BChl homologs, is presented.