Structures of chlorosomes and aggregated BChlc inChlorobium tepidum from solid state high resolution CP/MAS(13)C NMR

Self-Assemblies of Zinc Bacteriochlorophyll-d Analogues Having Amide, Ester, and Urea Groups as Substituents at 17-Position and Observation of Lamellar Supramolecular Nanostructures

Chlorosomes are unique light-harvesting apparatuses in photosynthetic green bacteria. Single chlorosomes contain a large number of bacteriochlorophyll (BChl)-c, -d, -e, and -f molecules, which self-assemble without protein assistance. These BChl self-assemblies involving specific intermolecular interactions (Mg⋅⋅⋅O3² -H⋅⋅⋅O=C13¹ and π-π stacks of chlorin skeletons) in a chlorosome have been reported to be round-shaped rods (or tubes) with diameters of 5 or 10 nm, or lamellae with a layer spacing of approximately 2 nm. Herein, the self-assembly of synthetic zinc BChl-d analogues having ester, amide, and urea groups in the 17-substituent is reported. Spectroscopic analyses indicate that the zinc BChl-d analogues self-assemble in a nonpolar organic solvent in a similar manner to natural chlorosomal BChls with additional assistance by hydrogen-bonding of secondary amide (or urea) groups (CON-H⋅⋅⋅O=CNH). Microscopic analyses of the supramolecules of a zinc BChl-d analogue bearing amide and urea groups show round- or square-shaped rods with widths of about 65 nm. Cryogenic TEM shows a lamellar arrangement of the zinc chlorin with a layer spacing of 1.5 nm inside the rod. Similar thick rods are also visible in the micrographs of self-assemblies of zinc BChl-d analogues with one or two secondary amide moieties in the 17-substituent.

Hierarchal Light-Harvesting Aggregates and Their Potential for Solar Energy Applications

The tunable optical properties of self-assembled chromophores are exploited by photosynthetic organisms to optimize their ability to harvest a broad range of the solar spectrum. Similarly, the efficiency of solar photovoltaic and photoelectrochemical devices depends strongly on the coincidence of the absorption spectrum of the photoactive components with the spectrum of the sun. While the possibility of borrowing ideas about light-harvesting aggregates from nature in order to improve the efficiency of solar energy conversion is quite attractive, progress to date is hindered by incomplete understanding of aggregate internal structure and its relation to excitonic states. In this Perspective, we describe our recent work on the hierarchal structure of self-assembled porphyrin aggregates that are similar to light-harvesting complexes of photosynthetic bacteria. We address the question of whether aggregation can be beneficial to dye-sensitized solar energy conversion and present promising results for a solar cell based on an abundant plant pigment that displays signatures of aggregation when adsorbed on TiO2.

Anisotropic organization and microscopic manipulation of self-assembling synthetic porphyrin microrods that mimic chlorosomes: bacterial light-harvesting systems

Being able to control in time and space the positioning, orientation, movement, and sense of rotation of nano- to microscale objects is currently an active research area in nanoscience, having diverse nanotechnological applications. In this paper, we demonstrate unprecedented control and maneuvering of rod-shaped or tubular nanostructures with high aspect ratios which are formed by self-assembling synthetic porphyrins. The self-assembly algorithm, encoded by appended chemical-recognition groups on the periphery of these porphyrins, is the same as the one operating for chlorosomal bacteriochlorophylls (BChl's). Chlorosomes, rod-shaped organelles with relatively long-range molecular order, are the most efficient naturally occurring light-harvesting systems. They are used by green photosynthetic bacteria to trap visible and infrared light of minute intensities even at great depths, e.g., 100 m below water surface or in volcanic vents in the absence of solar radiation. In contrast to most other natural light-harvesting systems, the chlorosomal antennae are devoid of a protein scaffold to orient the BChl's; thus, they are an attractive goal for mimicry by synthetic chemists, who are able to engineer more robust chromophores to self-assemble. Functional devices with environmentally friendly chromophores-which should be able to act as photosensitizers within hybrid solar cells, leading to high photon-to-current conversion efficiencies even under low illumination conditions-have yet to be fabricated. The orderly manner in which the BChl's and their synthetic counterparts self-assemble imparts strong diamagnetic and optical anisotropies and flow/shear characteristics to their nanostructured assemblies, allowing them to be manipulated by electrical, magnetic, or tribomechanical forces.

Absorption linear dichroism measured directly on a single light-harvesting system: the role of disorder in chlorosomes of green photosynthetic bacteria

Chlorosomes are light-harvesting antennae of photosynthetic bacteria containing large numbers of self-aggregated bacteriochlorophyll (BChl) molecules. They have developed unique photophysical properties that enable them to absorb light and transfer the excitation energy with very high efficiency. However, the molecular-level organization, that produces the photophysical properties of BChl molecules in the aggregates, is still not fully understood. One of the reasons is heterogeneity in the chlorosome structure which gives rise to a hierarchy of structural and energy disorder. In this report, we for the first time directly measure absorption linear dichroism (LD) on individual, isolated chlorosomes. Together with fluorescence-detected three-dimensional LD, these experiments reveal a large amount of disorder on the single-chlorosome level in the form of distributions of LD observables in chlorosomes from wild-type bacterium Chlorobaculum tepidum . Fluorescence spectral parameters, such as peak wavelength and bandwidth, are measures of the aggregate excitonic properties. These parameters obtained on individual chlorosomes are uncorrelated with the observed LD distributions and indicate that the observed disorder is due to inner structural disorder along the chlorosome long axis. The excitonic disorder that is also present is not manifested in the LD distributions. Limiting values of the LD parameter distributions, which are relatively free of the effect of structural disorder, define a range of angles at which the excitonic dipole moment is oriented with respect to the surface of the two-dimensional aggregate of BChl molecules. Experiments on chlorosomes of a triple mutant of Chlorobaculum tepidum show that the mutant chlorosomes have significantly less inner structural disorder and higher symmetry, compatible with a model of well-ordered concentric cylinders. Different values of the transition dipole moment orientations are consistent with a different molecular level organization of BChl's in the mutant and wild-type chlorosomes.

Probing the rate of hole transfer in oxidized synthetic chlorin dyads via site-specific (13)C-labeling

Understanding electronic communication among interacting constituents of multicomponent molecular architectures is important for rational design in diverse fields including artificial photosynthesis and molecular electronics. One strategy for examining ground-state hole/electron transfer in an oxidized tetrapyrrolic array relies on analysis of the hyperfine interactions observed in the EPR spectrum of the pi-cation radical. This strategy has been previously employed to probe the hole/electron-transfer process in oxidized multiporphyrin arrays of normal isotopic composition, wherein (1)H and (14)N serve as the hyperfine "clocks", and in arrays containing site-specific (13)C-labels, which serve as additional hyperfine clocks. Herein, the hyperfine-clock strategy is applied to dyads of dihydroporphyrins (chlorins). Chlorins are more closely related structurally to chlorophylls than are porphyrins. A de novo synthetic strategy has been employed to introduce a (13)C label at the 19-position of the chlorin macrocycle, which is a site of large electron/hole density and is accessible synthetically beginning with (13)C-nitromethane. The resulting singly (13)C-labeled chlorin was coupled with an unlabeled chlorin to give a dyad wherein a diphenylethyne linker spans the 10-positions of the two zinc chlorins. EPR studies of the monocations of both the natural abundance and (13)C-labeled zinc chlorin dyads and benchmark zinc chlorin monomers reveal that the time scale for hole/electron transfer is in the 4-7 ns range, which is 5-10-fold longer than that in analogous porphyrin arrays. The slower hole/electron transfer rate observed for the chlorin versus porphyrin dyads is attributed to the fact that the HOMO is a(1u)-like for the chlorins versus a(2u)-like for the porphyrins; the a(1u)-like orbital exhibits little (or no) electron/hole density at the site of linker attachment whereas the a(2u)-like orbital exhibits significant electron/hole density at this site. Collectively, the studies of the chlorin and porphyrin dyads provide insights into the structural features that influence the hole/electron-transfer process.

The lamellar spacing in self-assembling bacteriochlorophyll aggregates is proportional to the length of the esterifying alcohol

Chlorosomes from green photosynthetic bacteria are large photosynthetic antennae containing self-assembling aggregates of bacteriochlorophyll c, d, or e. The pigments within chlorosomes are organized in curved lamellar structures. Aggregates with similar optical properties can be prepared in vitro, both in polar as well as non-polar solvents. In order to gain insight into their structure we examined hexane-induced aggregates of purified bacteriochlorophyll c by X-ray scattering. The bacteriochlorophyll c aggregates exhibit scattering features that are virtually identical to those of native chlorosomes demonstrating that the self-assembly of these pigments is fully encoded in their chemical structure. Thus, the hexane-induced aggregates constitute an excellent model to study the effects of chemical structure on assembly. Using bacteriochlorophyllides transesterified with different alcohols we have established a linear relationship between the esterifying alcohol length and the lamellar spacing. The results provide a structural basis for lamellar spacing variability observed for native chlorosomes from different species. A plausible physiological role of this variability is discussed. The X-ray scattering also confirmed the assignments of peaks, which arise from the crystalline baseplate in the native chlorosomes.

Zinc chlorins for artificial light-harvesting self-assemble into antiparallel stacks forming a microcrystalline solid-state material

We introduce a concept to solve the structure of a microcrystalline material in the solid-state at natural abundance without access to distance constraints, using magic angle spinning (MAS) NMR spectroscopy in conjunction with X-ray powder diffraction and DFT calculations. The method is applied to a novel class of materials that form (semi)conductive 1D wires for supramolecular electronics and artificial light-harvesting. The zinc chlorins 3-devinyl-3(1)-hydroxymethyl-13(2)-demethoxycarbonylpheophorbide a (3',5'-bis-dodecyloxy)benzyl ester zinc complex 1 and 3-devinyl-3(1)-methoxymethyl-13(2)-demethoxycarbonylpheophorbide a (3',5'-bis-dodecyloxy)benzyl ester zinc complex 2, self-assemble into extended excitonically coupled chromophore stacks. (1)H-(13)C heteronuclear dipolar correlation MAS NMR experiments provided the (1)H resonance assignment of the chlorin rings that allowed accurate probing of ring currents related to the stacking of macrocycles. DFT ring-current shift calculations revealed that both chlorins self-assemble in antiparallel pi-stacks in planar layers in the solid-state. Concomitantly, X-ray powder diffraction measurements for chlorin 2 at 80 degrees C revealed a 3D lattice for the mesoscale packing that matches molecular mechanics optimized aggregate models. For chlorin 2 the stacks alternate with a periodicity of 0.68 nm and a 3D unit cell with an approximate volume of 6.28 nm(3) containing 4 molecules, which is consistent with space group P2(1)22(1).

Alternating syn-anti bacteriochlorophylls form concentric helical nanotubes in chlorosomes

Chlorosomes are the largest and most efficient light-harvesting antennae found in nature, and they are constructed from hundreds of thousands of self-assembled bacteriochlorophyll (BChl) c, d, or e pigments. Because they form very large and compositionally heterogeneous organelles, they had been the only photosynthetic antenna system for which no detailed structural information was available. In our approach, the structure of a member of the chlorosome class was determined and compared with the wild type (WT) to resolve how the biological light-harvesting function of the chlorosome is established. By constructing a triple mutant, the heterogeneous BChl c pigment composition of chlorosomes of the green sulfur bacteria Chlorobaculum tepidum was simplified to nearly homogeneous BChl d. Computational integration of two different bioimaging techniques, solid-state NMR and cryoEM, revealed an undescribed syn-anti stacking mode and showed how ligated BChl c and d self-assemble into coaxial cylinders to form tubular-shaped elements. A close packing of BChls via pi-pi stacking and helical H-bonding networks present in both the mutant and in the WT forms the basis for ultrafast, long-distance transmission of excitation energy. The structural framework is robust and can accommodate extensive chemical heterogeneity in the BChl side chains for adaptive optimization of the light-harvesting functionality in low-light environments. In addition, syn-anti BChl stacks form sheets that allow for strong exciton overlap in two dimensions enabling triplet exciton formation for efficient photoprotection.

The supramolecular organization of self-assembling chlorosomal bacteriochlorophyll c, d, or e mimics

Bacteriochlorophylls (BChls) c, d, and e are the main light-harvesting pigments of green photosynthetic bacteria that self-assemble into nanostructures within the chlorosomes forming the most efficient antennas of photosynthetic organisms. All previous models of the chlorosomal antennae, which are quite controversially discussed because no single crystals could be grown so far from these organelles, involve a strong hydrogen-bonding interaction between the 3(1) hydroxyl group and the 13(1) carbonyl group. We have synthesized different self-assemblies of BChl c mimics having the same functional groups as the natural counterparts, that is, a hydroxyethyl substituent, a carbonyl group and a divalent metal atom ligated by a tetrapyrrole. These artificial BChl mimics have been shown by single crystal x-ray diffraction to form extended stacks that are packed by hydrophobic interactions and in the absence of hydrogen bonding. Time-resolved photoluminescence proves the ordered nature of the self-assembled stacks. FT-IR spectra show that on self-assembly the carbonyl frequency is shifted by approximately 30 cm(-1) to lower wavenumbers. From the FT-IR data we can infer the proximal interactions between the BChls in the chlorosomes consistent with a single crystal x-ray structure that shows a weak electrostatic interaction between carbonyl groups and the central zinc atom.

The length of esterifying alcohol affects the aggregation properties of chlorosomal bacteriochlorophylls

Chlorosomes, the main light-harvesting complexes of green photosynthetic bacteria, contain bacteriochlorophyll (BChl) molecules in the form of self-assembling aggregates. To study the role of esterifying alcohols in BChl aggregation we have prepared a series of bacteriochlorophyllide c (BChlide c) derivatives differing in the length of the esterifying alcohol (C(1), C(4), C(8) and C(12)). Their aggregation behavior was studied both in polar (aqueous buffer) and nonpolar (hexane) environments and the esterifying alcohols were found to play an essential role. In aqueous buffer, hydrophobic interactions among esterifying alcohols drive BChlide c derivatives with longer chains into the formation of dimers, while this interaction is weak for BChlides with shorter esterifying alcohols and they remain mainly as monomers. All studied BChlide c derivatives form aggregates in hexane, but the process slows down with longer esterifying alcohols due to competing hydrophobic interactions with hexane molecules. In addition, the effect of the length of the solvent molecules (n-alkanes) was explored for BChl c aggregation. With an increasing length of n-alkane molecules, the hydrophobic interaction with the farnesyl chain becomes stronger, leading to a slower aggregation rate. The results show that the hydrophobic interaction is the driving force for the aggregation in an aqueous environment, while in nonpolar solvents it is the hydrophilic interaction.

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.

Isotopic Replacement of Pigments and a Lipid in Chlorosomes fromChlorobium limicola: Characterization of the Resultant Chlorosomes

Pigments including bacteriochlorophyll (BChl) c, carotenoids, and a trace of BChl a together with a lipid, monogalactosyl diglyceride (MGDG), were extracted with chloroform/methanol (1:1 v/v) from an aqueous suspension (50 mM Tris-HCl, pH 8.0) of chlorosomes from Chlorobium limicola; other lipids and proteins were left behind in the aqueous layer by funnel separation. The chloroform layer was dried by purging N2 gas, dissolved in methanol, and rapidly injected into the aqueous layer to reassemble chlorosomes. This technique has been developed to replace one-half of the inherent 12C-BChl c by 13C-BChl c to identify the intermolecular 13C...13C magnetic dipole correlation peaks (that are supposed to reduce their intensities to one-fourth by reducing the 13C-BChl c concentration into one-half) and to determine the structure of BChl c aggregates in the rod elements by means of solid-state NMR spectroscopy. The isotopically replaced chlorosomes were characterized (1) by sucrose density gradient centrifugation, zeta potential measurement, electron microscopy, and dynamic light scattering measurement to determine the morphology of chlorosomes, (2) by 13C NMR spectroscopy, electronic absorption and circular dichroism spectroscopies, and low-angle X-ray diffraction to determine the pigment assembly in the rod elements, and (3) by subpicosecond time-resolved absorption spectroscopy to determine the excited-state dynamics in the pigment assembly. The results characterized the reassembled chlorosomes to have (1) similar but longer morphological structures, (2) almost the same pigment assembly in the rod elements, and (3) basically the same excited-state dynamics in the pigment assembly.

Triplet exciton formation as a novel photoprotection mechanism in chlorosomes of Chlorobium tepidum

Chlorosomes comprise thousands of bacteriochlorophylls (BChl c, d, or e) in a closely packed structure surrounded by a lipid-protein envelope and additionally contain considerable amounts of carotenoids, quinones, and BChl a. It has been suggested that carotenoids in chlorosomes provide photoprotection by rapidly quenching triplet excited states of BChl via a triplet-triplet energy transfer mechanism that prevents energy transfer to oxygen and the formation of harmful singlet oxygen. In this work we studied triplet energy transfer kinetics and photodegradation of chlorosomes isolated from wild-type Chlorobium tepidum and from genetically modified species with different types of carotenoids and from a carotenoid-free mutant. Supporting a photoprotective function of carotenoids, carotenoid-free chlorosomes photodegrade approximately 3 times faster than wild-type chlorosomes. However, a significant fraction of the BChls forms a long-lived, triplet-like state that does not interact with carotenoids or with oxygen. We propose that these states are triplet excitons that form due to triplet-triplet interaction between the closely packed BChls. Numerical exciton simulations predict that the energy of these triplet excitons may fall below that of singlet oxygen and triplet carotenoids; this would prevent energy transfer from triplet BChl. Thus, the formation of triplet excitons in chlorosomes serves as an alternative photoprotection mechanism.

Structure of the light-harvesting bacteriochlorophyll c assembly in chlorosomes from Chlorobium limicola determined by solid-state NMR

We have determined the atomic structure of the bacteriochlorophyll c (BChl c) assembly in a huge light-harvesting organelle, the chlorosome of green photosynthetic bacteria, by solid-state NMR. Previous electron microscopic and spectroscopic studies indicated that chlorosomes have a cylindrical architecture with a diameter of approximately 10 nm consisting of layered BChl molecules. Assembly structures in huge noncrystalline chlorosomes have been proposed based mainly on structure-dependent chemical shifts and a few distances acquired by solid-state NMR, but those studies did not provide a definite structure. Our approach is based on (13)C dipolar spin-diffusion solid-state NMR of uniformly (13)C-labeled chlorosomes under magic-angle spinning. Approximately 90 intermolecular C C distances were obtained by simultaneous assignment of distance correlations and structure optimization preceded by polarization-transfer matrix analysis. It was determined from the approximately 90 intermolecular distances that BChl c molecules form piggyback-dimer-based parallel layers. This finding rules out the well known monomer-based structures. A molecular model of the cylinder in the chlorosome was built by using this structure. It provided insights into the mechanisms of efficient light harvesting and excitation transfer to the reaction centers. This work constitutes an important advance in the structure determination of huge intact systems that cannot be crystallized.

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

Aggregation of bacteriochlorophyll (BChl) c from chlorosomes, the main light-harvesting complex of green bacteria, has been studied in aqueous buffer. Unlike other chlorophyll-like molecules, BChl c is rather soluble in aqueous buffer, forming dimers. When BChl c is mixed with carotenoids (Car), the BChl c Qy transition is further redshifted, in respect to that of monomers and dimers. The results suggest that Car are incorporated in the aggregates and induce further aggregation of BChl c. The redshift of the BChl c Qy band is proportional to the Car concentration. In contrast, the mixture of bacteriochlorophyllide (BChlide) c, which lacks the nonpolar esterifying alcohol, does not form aggregates with Car in aqueous buffer or nonpolar solvents. Instead, the position of the BChlide c Qy transition remains unshifted in respect to that of the monomeric molecule, and Car precipitates with the course of time in aqueous buffer. Similar effects on both BChl c and BChlide c are also observed when monogalactosyl diglyceride (MGDG), which forms the monolayer envelope of chlorosomes, is used instead of (or together with) Car. The results show that the hydrophobic interactions of the BChl c esterifying alcohols with themselves and the nonpolar carbon skeleton of Car, or the fatty acid tails of MGDG, are essential driving forces for BChl aggregation in chlorosomes.

Assembly of a Mixture of Isomeric BChl c from Chlorobium limicola As Determined by Intermolecular 13C−13C Dipolar Correlations: Coexistence of Dimer-Based and Pseudo-Monomer-Based Stackings

A mixture of bacteriochlorophyll (BChl) c isomers was extracted from the cells of Chlorobium limicola that were grown in the media of 13C-enriched and natural-abundance isotopic compositions. The magic-angle spinning 13C NMR proton-driven spin-diffusion spectra were recorded with mixing times of 50, 100, and 250 ms for two different kinds of in vitro aggregates, one consisting of pure [13C]BChl c and the other consisting of a 1:1 mixture of [13C]BChl c and [12C]BChl c; those peaks whose intensities were reduced to approximately 1/4 by this dilution were assigned to intermolecular 13C-13C dipolar correlation peaks. On the other hand, the nearest-neighbor intermolecular carbon-carbon close contacts with distances of 4-6 A were simulated, to predict observed correlation peaks, for six different models of BChl c assembly. They include weakly overlapped monomers forming structure 1 and structure 2, strongly overlapped dimers forming straight and inclined columns, and weakly overlapped dimers forming aligned and displaced layers. Comparison between the observed correlation peaks and the predicted carbon-carbon close contacts, for both the macrocycles and the side chains, led us to a conclusion that the weakly overlapped dimers forming displaced layers are most likely the assembly of the BChl c molecules in the aggregate.

Isolation and characterization of carotenosomes from a bacteriochlorophyll c-less mutant of Chlorobium tepidum

Chlorosomes are the light-harvesting organelles in photosynthetic green bacteria and typically contain large amounts of bacteriochlorophyll (BChl) c in addition to smaller amounts of BChl a, carotenoids, and several protein species. We have isolated vestigial chlorosomes, denoted carotenosomes, from a BChl c-less, bchK mutant of the green sulfur bacterium Chlorobium tepidum. The physical shape of the carotenosomes (86 +/- 17 nm x 66 +/- 13 nm x 4.3 +/- 0.8 nm on average) was reminiscent of a flattened chlorosome. The carotenosomes contained carotenoids, BChl a, and the proteins CsmA and CsmD in ratios to each other comparable to their ratios in wild-type chlorosomes, but all other chlorosome proteins normally found in wild-type chlorosomes were found only in trace amounts or were not detected. Similar to wild-type chlorosomes, the CsmA protein in the carotenosomes formed oligomers at least up to homo-octamers as shown by chemical cross-linking and immunoblotting. The absorption spectrum of BChl a in the carotenosomes was also indistinguishable from that in wild-type chlorosomes. Energy transfer from the bulk carotenoids to BChl a in carotenosomes was poor. The results indicate that the carotenosomes have an intact baseplate made of remarkably stable oligomeric CsmA-BChl a complexes but are flattened in structure due to the absence of BChl c. Carotenosomes thus provide a valuable material for studying the biogenesis, structure, and function of the photosynthetic antennae in green bacteria.

Lamellar organization of pigments in chlorosomes, the light harvesting complexes of green photosynthetic bacteria

Chlorosomes of green photosynthetic bacteria constitute the most efficient light harvesting complexes found in nature. In addition, the chlorosome is the only known photosynthetic system where the majority of pigments (BChl) is not organized in pigment-protein complexes but instead is assembled into aggregates. Because of the unusual organization, the chlorosome structure has not been resolved and only models, in which BChl pigments were organized into large rods, were proposed on the basis of freeze-fracture electron microscopy and spectroscopic constraints. We have obtained the first high-resolution images of chlorosomes from the green sulfur bacterium Chlorobium tepidum by cryoelectron microscopy. Cryoelectron microscopy images revealed dense striations approximately 20 A apart. X-ray scattering from chlorosomes exhibited a feature with the same approximately 20 A spacing. No evidence for the rod models was obtained. The observed spacing and tilt-series cryoelectron microscopy projections are compatible with a lamellar model, in which BChl molecules aggregate into semicrystalline lateral arrays. The diffraction data further indicate that arrays are built from BChl dimers. The arrays form undulating lamellae, which, in turn, are held together by interdigitated esterifying alcohol tails, carotenoids, and lipids. The lamellar model is consistent with earlier spectroscopic data and provides insight into chlorosome self-assembly.

Isolation and structure determination of a complete set of bacteriochlorophyll-d homologs and epimers from a green sulfur bacterium Chlorobium vibrioforme and their aggregation properties in hydrophobic solvents

Eight bacteriochlorophyll (BChl)-d homologs and epimers were isolated from a strain of the green sulfur bacterium Chlorobium vibrioforme. By a combination of mass spectrometry and 1H-NMR spectroscopy using the chemical shifts of meso- and 3(1)-protons and 1H-1H NOE correlations, the molecular structures were determined as (3(1)R)-8-ethyl-12-methyl, (3'R)-8-ethyl-12-ethyl, (3(1)R)-8-propyl-12-methyl, (3(1)S)-8-propyl-12-methyl, (3(1)R)-8-propyl-12-ethyl, (3(1)S)-8-propyl-12-ethyl, (3(1)S)-8-isobutyl-12-methyl and (3(1)S)-8-isobutyl-12-ethyl. The aggregation behavior of the epimerically pure BChls-d in hydrophobic organic solvents was examined to investigate the absolute configuration of the 3-(1-hydroxyethyl) group as well as the bulkiness of the C8 and C12 side-chains by using electronic-absorption and fluorescence-emission spectroscopies At high concentration of the BChls-d in CH2Cl2, the absolute configuration of the 3-(1-hydroxyethyl) group governed the formation of a subunit as a building block for the subsequent higher assembly. Upon dilution of the resulting subunit with hexane, the bulkiness of the C8 and C12 side-chains were found to affect the association of the subunits differently: the bulkiness of the C8 side-chain acted as a promoter for the association due to a stabilized hydrophobic interaction among the relevant larger side-chain, whereas the bulkiness of the C12 side-chain acted as an inhibitor for that association due to introduction of a particular steric-hindrance around the side-chain in the aggregates.

Selective detection and assignment of the solution NMR signals of bacteriochlorophyll a in a reconstituted subunit of a light-harvesting complex

High-resolution solution NMR spectra have been obtained for bacteriochlorophyll (BChl) a molecules in a biologically functional subunit of a bacterial core light-harvesting complex based on a modified reconstitution method. The reconstituted subunit of pigment-integral membrane polypeptides is stable and homogeneous at high concentrations at room temperature and exhibits a Q(y) absorption peak at 818 nm. (1)H and (13)C chemical shifts have been specifically assigned for BChl a using the fully and selectively (13)C-labeled pigments incorporated with natural abundance polypeptides in deuterated detergent solution. Remarkable signal broadening has been observed upon reconstitution, where the bacteriochlorin macrocycle is shown in a highly restricted molecular motion while the phytol side chain remains relatively mobile. Two sets of resonances are revealed for 3(2), 8(1), 10, 12(1), and 13(4) protons, and 8(2) methyl protons exhibit four resonances with large upfield complexation shifts. The result indicates a nonequivalent state for the two BChl a molecules in the subunit and can be best interpreted in terms of a parallel face-to-face configuration with partial overlap over the pyrrolic rings II, III, and V. In comparison with BChl a in acetone, 8(2), 13(2), and 13(4) protons are largely perturbed, and the propionic and phytol side chain may adopt a different conformation in the reconstituted subunit. The (13)C chemical shift of 3(1) carbonyl carbon shows a large change downfield, indicating strong hydrogen bonding for all the acetyl carbonyls. Carbonyl carbons at 13(1) give rise to two (13)C resonances with equal intensities, suggesting that the keto carbonyl in one BChl a molecule within a subunit forms a stronger hydrogen bond than that in another BChl a molecule.

Supramolecular structure of self-assembled synthetic zinc-13(1)-oxo-chlorins possessing a primary, secondary or tertiary alcoholic 3(1)-hydroxyl group: visible spectroscopic and molecular modeling studies

Zinc-chlorin 3 (see Fig. 2 in text) possessing a tertiary 3(1)-hydroxyl group and a 13-keto group was synthesized as a model for the antenna chlorophylls of green bacteria. Self-aggregation of 3 in nonpolar organic media was examined and compared to 1 and 2 possessing a primary and secondary 3(1)-hydroxyl group, respectively. Zinc-chlorin 3 self-aggregated in 1 vol% CH2Cl2-hexane to form oligomers and showed a red-shifted Qy maximum at 704 nm compared to the monomer (648 nm in CH2Cl2). This red-shift is larger than that of 2S (648-->697 nm) and comparable to that of 2R (648-->705 nm), but smaller than that of 1 (648-->740 nm), indicating that while a single 3(1)-methyl group (prim-OH-->sec-OH) suppressed close and/or higher aggregation, the additional 3(1)-methyl group (sec-OH-->tert-OH) did not further suppress aggregation. The relative stability of the aggregates was in the order 1 > 2R-3 > 2S as determined by visible spectral analyses. Molecular modeling calculations on dodecamers of zinc-chlorins 1, 2R and 3 gave similar well-ordered energy-minimized structures, while 1 stacked more tightly than 2R and 3. In contrast, 2S gave a relatively disordered (twisted) structure. The calculated dodecameric structures could explain the visible spectral data of 1-3 in nonpolar organic media.

Exciton dynamics in the chlorosomal antennae of the green bacteria Chloroflexus aurantiacus and Chlorobium tepidum

The energy transfer processes in isolated chlorosomes from green bacteria Chlorobium tepidum and Chloroflexus aurantiacus have been studied at low temperatures (1.27 K) by two-pulse photon echo and one-color transient absorption techniques with approximately 100 fs resolution. The decay of the coherence in both types of chlorosomes is characterized by four different dephasing times stretching from approximately 100 fs up to 300 ps. The fastest component reflects dephasing that is due to interaction of bacteriochlorophylls with the phonon bath, whereas the other components correspond to dephasing due to different energy transfer processes such as distribution of excitation along the rod-like aggregates, energy exchange between different rods in the chlorosome, and energy transfer to the base plate. As a basis for the interpretation of the excitation dephasing and energy transfer pathways, a superlattice-like structural model is proposed based on recent experimental data and computer modeling of the Bchl c aggregates (1994. Photosynth. Res. 41:225-233.) This model predicts a fine structure of the Q(y) absorption band that is fully supported by the present photon echo data.