BChl Aggregates Research Articles

Towards first-principles calculation of electronic excitations in the ring of the protein-bound bacteriochlorophylls

Modeling electronic excitation of bacteriochlorophyll (BChl) molecules in light-harvesting (LH) antennae from photosynthetic centers presents a challenge for the quantum theory. We report on a quantum chemical study of the ring of 32 BChl molecules from the bacterial core complex LH1-RC. Diagonal and off-diagonal elements of the excitonic Hamiltonian matrices are estimated in quantum chemical calculations of relevant fragments using the TD-DFT and CIS approaches. The deviation of the computed excitation energy of this BChl system from the experimental data related to the Qy band maximum of this LH1-RC complex is about 0.2 eV. We demonstrate that corrections due to improvement in modeling of an individual BChl molecule and due to contributions from the protein environment are in the range of the obtained discrepancy between theory and experiment. Differences between results of the excitonic model and direct quantum chemical calculations of BChl aggregates fall in the same range.

Near-infrared absorption bacteriochlorophyll derivatives as biomaterial electron donor for organic solar cells

Abstract In this paper, we investigate two bacteriochlorophyll (BChl) derivatives, namely methyl (pyro)bacteriopheophorbide a [BChl-(2)1], with excellent absorption in near-infrared regions as donor materials for organic solar cells (OSCs). In the meantime, we prepare two methyl (pyro)pheophorbide a [Chl-(2)1] as reference to make a comparison. Compared to the small red-shifts of absorption bands of Chls-1/2 after forming the thin film through spin coating of the homogeneous solution, the absorption spectra of BChls-1/2 exhibited larger red-shifts from the absorption peak at 750 nm in the solution to the absorption peak at 860 nm after forming the solid film. This significant absorption shift is attributed to the formation of J -type aggregates by intermolecular π-π stacking among the BChl-1/2 molecules. What’s more the absorbed sunlight of the BChls-1/2 aggregates at the near-infrared (NIR) region can be transferred into photocurrent which can be proved from the EQE indicating the energy and electron transferring between BChl aggregates are efficient. The optimized power conversion efficiency (PCE) value of 1.43% under standard AM1.5 mW/cm 2 sunlight was achieved with the BChl-1: C 70 planar-heterojunction (PHJ) cells.

Impact of esterified bacteriochlorophylls on the biogenesis of chlorosomes in Chloroflexus aurantiacus.

A chlorosome is an antenna complex located on the cytoplasmic side of the inner membrane in green photosynthetic bacteria that contains tens of thousands of self-assembled bacteriochlorophylls (BChls). Green bacteria are known to incorporate various esterifying alcohols at the C-17 propionate position of BChls in the chlorosome. The effect of these functional substitutions on the biogenesis of the chlorosome has not yet been fully explored. In this report, we address this question by investigating various esterified bacteriochlorophyll c (BChl c) homologs in the thermophilic green non-sulfur bacterium Chloroflexus aurantiacus. Cultures were supplemented with exogenous long-chain alcohols at 52°C (an optimal growth temperature) and 44°C (a suboptimal growth temperature), and the morphology, optical properties and exciton transfer characteristics of chlorosomes were investigated. Our studies indicate that at 44°C Cfl. aurantiacus synthesizes more carotenoids, incorporates more BChl c homologs with unsaturated and rigid polyisoprenoid esterifying alcohols and produces more heterogeneous BChl c homologs in chlorosomes. Substitution of phytol for stearyl alcohol of BChl c maintains similar morphology of the intact chlorosome and enhances energy transfer from the chlorosome to the membrane-bound photosynthetic apparatus. Different morphologies of the intact chlorosome versus in vitro BChl aggregates are suggested by small-angle neutron scattering. Additionally, phytol cultures and 44°C cultures exhibit slow assembly of the chlorosome. These results suggest that the esterifying alcohol of BChl c contributes to long-range organization of BChls, and that interactions between BChls with other components are important to the assembly of the chlorosome. Possible mechanisms for how esterifying alcohols affect the biogenesis of the chlorosome are discussed.

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.

Energy transfer in aggregates of bacteriochlorophyll c self-assembled with azulene derivatives

Bacteriochlorophyll (BChl) c is the main light-harvesting pigment of certain photosynthetic bacteria. It is found in the form of self-assembled aggregates in the so-called chlorosomes. Here we report the results of co-aggregation experiments of BChl c with azulene and its tailored derivatives. We have performed spectroscopic and quantum chemical characterization of the azulenes, followed by self-assembly experiments. The results show that only azulenes with sufficient hydrophobicity are able to induce aggregation of BChl c. Interestingly, only azulene derivatives possessing a conjugated phenyl ring were capable of efficient (∼50%) excitation energy transfer to BChl molecules. These aggregates represent an artificial light-harvesting complex with enhanced absorption between 220 and 350 nm compared to aggregates of pure BChl c. The results provide insight into the principles of self-assembly of BChl aggregates and suggest an important role of the π-π interactions in efficient energy transfer.

Temperature and Carbon Assimilation Regulate the Chlorosome Biogenesis in Green Sulfur Bacteria

Green photosynthetic bacteria adjust the structure and functionality of the chlorosome—the light-absorbing antenna complex—in response to environmental stress factors. The chlorosome is a natural self-assembled aggregate of bacteriochlorophyll (BChl) molecules. In this study, we report the regulation of the biogenesis of the Chlorobaculum tepidum chlorosome by carbon assimilation in conjunction with temperature changes. Our studies indicate that the carbon source and thermal stress culture of C. tepidum grows slower and incorporates fewer BChl c in the chlorosome. Compared with the chlorosome from other cultural conditions we investigated, the chlorosome from the carbon source and thermal stress culture displays (a) smaller cross-sectional radius and overall size, (b) simplified BChl c homologs with smaller side chains, (c) blue-shifted Qy absorption maxima, and (d) a sigmoid-shaped circular dichroism spectra. Using a theoretical model, we analyze how the observed spectral modifications can be associated with structural changes of BChl aggregates inside the chlorosome. Our report suggests a mechanism of metabolic regulation for chlorosome biogenesis.

Identification of a Gene Essential for the First Committed Step in the Biosynthesis of Bacteriochlorophyll c

Bacteriochlorophylls (BChls) c, d, and e are the major chlorophylls in chlorosomes, which are the largest and one of the most efficient antennae produced by chlorophototrophic organisms. In the biosynthesis of these three BChls, a C-13(2)-methylcarboxyl group found in all other chlorophylls (Chls) must be removed. This reaction is postulated to be the first committed step in the synthesis of these BChls. Analyses of gene neighborhoods of (B)Chl biosynthesis genes and distribution patterns in organisms producing chlorosomes helped to identify a gene (bciC) that appeared to be a good candidate to produce the enzyme involved in this biochemical reaction. To confirm that this was the case, a deletion mutant of an open reading frame orthologous to bciC, CT1077, was constructed in Chlorobaculum tepidum, a genetically tractible green sulfur bacterium. The CT1077 deletion mutant was unable to synthesize BChl c but still synthesized BChl a and Chl a. The deletion mutant accumulated large amounts of various (bacterio)pheophorbides, all of which still had C-13(2)-methylcarboxyl groups. A C. tepidum strain in which CT1077 was replaced by an orthologous gene, Cabther_B0081 [corrected] from "Candidatus Chloracidobacterium thermophilum" was constructed. Although the product of Cabther_B0081 [corrected] was only 28% identical to the product of CT1077, this strain synthesized BChl c, BChl a, and Chl a in amounts similar to wild-type C. tepidum cells. To indicate their roles in the first committed step of BChl c, d, and e biosynthesis, open reading frames CT1077 and Cabther_B0081 [corrected] have been redesignated bciC. The potential mechanism by which BciC removes the C-13(2)-methylcarboxyl moiety of chlorophyllide a is discussed.

Stacking of Bacteriochlorophyll c Macrocycles in Chlorosome from Chlorobium limicola As Revealed by Intermolecular 13C Magnetic-Dipole Correlation, X-Ray Diffraction, and Quadrupole Coupling in 25Mg NMR

The stacking of the bacteriochlorophyll (BChl) c macrocycles and the role of water in forming an aggregate sheet, in chlorosome, were examined by means of (13)C NMR spectroscopy, the measurement of the X-ray diffraction pattern, and (25)Mg NMR spectroscopy. (1) The stacking of the macrocycles, i.e., weakly overlapped dimers forming displaced layers, was selected out of six different kinds of stacking so far identified in the aggregates of isomeric BChl c in solution and in the solid aggregate of an isomeric mixture of BChl c extracted from Chlorobium limicola. The selection was based on the comparison of the intermolecular (13)C...(13)C magnetic-dipole correlations with the nearest-neighbor carbon-to-carbon close contacts simulated for the above six different stackings. It has turned out that the stacking of the macrocycles in chlorosome is basically the same as that in the in vitro solid aggregate. (2) The crucial role of water in stabilizing the aggregate structure in chlorosome was shown by tracing the dehydration processes and by comparison with the solid aggregate using the X-ray diffraction pattern. Possible binding sites of water molecules were located, by structural simulation, based on the particular stacking structure. (3) The dimer-based stacking of the macrocycles was evidenced by (25)Mg NMR spectroscopy, which exhibited a pair of signals showing different quadrupole coupling, due to the presence or absence of a water molecule in the axial position.

Effect of quinones on formation and properties of bacteriochlorophyll c aggregates

Chlorosomes of green photosynthetic bacterium Chlorobium tepidum contain aggregates of bacteriochlorophyll c (BChl c) with carotenoids and isoprenoid quinones. BChl aggregates with very similar optical properties can be prepared also in vitro either in non-polar solvents or in aqueous buffers with addition of lipids and/or carotenoids. In this work, we show that the aggregation of BChl c in aqueous buffer can be induced also by quinones (vitamin K(1 )and K(2)), provided they are non-polar due to a hydrophobic side-chain. Polar vitamin K(3, )which possess the same functional group as K(1 )and K(2), does not induce the aggregation. The results confirm a principal role of the hydrophobic interactions as a driving force for the aggregation of chlorosomal BChls. The chlorosomal quinones play an important role in a redox-dependent excitation quenching, which may protect the cells against damage under oxygenic conditions. We found that aggregates of BChl c with vitamin K(1 )and K(2) exhibit an excitation quenching as well. The amplitude of the quenching depends on quinone concentration, as determined from fluorescence measurements. No lipid is necessary to induce the quenching, which therefore originates mainly from interactions of BChl c with quinones incorporated in the aggregate structure. In contrast, only a weak quenching was observed for dimers of BChl c in buffer (either with or without vitamin K(3)) and also for BChl c aggregates prepared with a lipid (lecithin). Thus, the weak quenching seems to be a property of BChl c itself.

The Effects of Epimerization at the 31-position of Bacteriochlorophylls c on their Aggregation in Chlorosomes of Green Sulfur Bacteria. Control of the Ratio of 31 Epimers by Light Intensity ‡

R- and S-epimerization at the 31 position of bacteriochlorophyll (BChl) c and the formation of rod-like aggregates in chlorosomes of green sulfur bacteria were markedly affected in Chlorobium (Cb.) tepidum and Cb.limicola by cultivation under various light intensities (photon fluence rate). The stronger the light, the higher the ratio of the S-epimer to the R-epimer for each homolog of BChl c in the bacteria. S[P,E] BChl cF and S[I,E] BChl cF were found to be the major S-epimers in Cb. tepidum and Cb. limicola, respectively. R[P,E] BChl cF decreased markedly compared to R[E,E] BChl cF in Cb. tepidum, whereas no observable change in the ratio of R[P,E]/R[E,E] was detected for Cb. limicola. With increase in light intensity the Qy absorption maximum of the bacteria shifted to shorter wavelengths. In vitro spectroscopic studies of the aggregates showed a marked difference in the formation of aggregates from R- and S-epimers of BChl c; the S-epimers formed aggregates much more slowly than did the R-epimers. These results suggest that the ratio of the epimers of BChl c might significantly affect the aggregation of BChl in the chlorosome. We propose different roles for the R- and S-epimers in chlorosomes of Cb. limicola and Cb. tepidum.

Self-Assembly of Natural Light-Harvesting Bacteriochlorophylls of Green Sulfur Photosynthetic Bacteria in Silicate Capsules as Stable Models of Chlorosomes

Naturally occurring bacteriochlorophyll(BChl)s-c, -d, and -e from green sulfur photosynthetic bacteria were self-assembled in an aqueous solution in the presence of octadecyltriethoxysilane and tetraethoxysilane, followed by polycondensation of the alkoxysilanes by incubation for 50 h at 25 degrees C. The resulting BChl self-assemblies in silicate capsules exhibited visible absorption and circular dichroism spectra similar to the corresponding natural light-harvesting systems (chlorosomes) of green sulfur bacteria. Dynamic light scattering measurements indicated that the silicate capsules had an average hydrodynamic diameter of several hundred nanometers. BChl self-aggregates in silicate capsules were significantly stable to a nonionic surfactant Triton X-100, which was apt to decompose the BChl aggregates to their monomeric form, compared with conventional micelle systems. BChls in silicate capsules were more tolerant to demetalation of the central magnesium under acidic conditions than the natural systems.

Single Supramolecule Spectroscopy of Natural and Alkaline-Treated Chlorosomes from Green Sulfur Photosynthetic Bacteria

Fluorescence emission properties of intact and alkaline-treated chlorosomes containing bacteriochlorophyll(BChl)-c, d, and e, which were isolated from four species of green sulfur photosynthetic bacteria, were successfully studied at the single-unit level using a total internal reflection fluorescence microscope. Single intact chlorosomes containing BChl-c from Chlorobium (Chl.) tepidum exhibited heterogeneous emission bands of BChl-c self-aggregates. In contrast, fluorescence spectra of chlorosomal BChl self-aggregates in single intact chlorosomes from the other three Chlorobium species were less heterogeneous than those from Chi. tepidum. Removal of energy-accepting BChl-a/protein complexes called baseplates from the intact chlorosomes by treatments with alkaline media hardly changed spectral shapes of BChl aggregates and their peak distributions at the single-chlorosome level. The similarity of spectral properties at the single-unit level between intact and alkaline-treated chlorosomes of four Chlorobium species clearly indicated that the removal of base-plates from intact chlorosomes by the alkaline-treatment did not affect BChl self-aggregates inside single chlorosomes.

Comparative Study of the Energy Transfer Kinetics in Artificial BChleAggregates Containing a BChlaAcceptor and BChle-Containing Chlorosomes ofChlorobium phaeobacteroides

Chlorosomes are the light-harvesting organelles of green bacteria, containing mainly special bacteriochlorophylls (BChls) carrying a 3(1)-hydroxy side chain. Artificial aggregates of BChl c, d, and e have been shown to resemble the native chlorosomes in many respects. They are therefore seen as good model systems for understanding the spectroscopic properties of these antenna systems. We have investigated the excitation energy transfer in artificial aggregates of BChl e, containing small amounts of BChl a as an energy acceptor, using steady-state and time-resolved fluorescence. Global analysis of the kinetic data yields two lifetimes attributable to energy transfer: a fast one of 12-20 ps and a slower one of approximately 50 ps. For comparison, BChl e-containing native chlorosomes of Chlorobium phaeobacteroides and chlorosomes in which the energy acceptor had been degraded by alkaline treatment were also studied. A similar behavior is seen in both the artificial and the natural systems. The results suggest that the artificial aggregates of BChls have a potential as antenna systems in future artificial photonic devices.

Signature pigments of green sulfur bacteria in lower Pleistocene deposits from the Banyoles lacustrine area (Spain)

Signature pigments of photosynthetic green sulfur bacteria (GSB) were found in ancient sediments collected from an abandoned clay quarry located in the Banyoles lacustrine area (Spain). The sediments belong to the Interglacial Waalian of the lower Pleistocene (0.7–1.5 millions years old) and were deposited after a marshy event occurring during that geologic period. Reverse-phase high performance liquid chromatography (RP-HPLC) analyses of acetone:methanol sediment extracts revealed that the main pigments found were carotenoids of both eukaryotic and prokaryotic photosynthetic organisms. In particular, isorenieratene (Isr) and β-isorenieratene (β-Isr) constituted the larger bacterial fraction (35–40% of the total carotenoid content), whereas okenone, a signature pigment of purple sulfur bacteria, accounted for less than 2%. Xanthophylls from oxygenic photosynthetic organisms accounted for the remaining carotenoids. Likewise, traces of degradation products of both bacteriochlorophyll (BChl) and chlorophyll (Chl) were also detected. The low concentration of Chl degradation products made proper identification of these compounds impossible. In contrast, degradation products of BChl-e such as bacteriochlorophyllide-e and bacteriopheophorbide-e were identified in the HPLC analyses, suggesting that chemical degradation of naturally occurring BChl aggregates of GSB was slower in the clay quarry sediments. The presence of signature pigments of brown-colored species of GSB (Isr, β-Isr and degradation products of BChl-e) in the sediments suggests an ancient aquatic environment where GSB were present and where episodes of sulfide-rich anoxia reaching the photic zone could be envisaged. Similarly, the large percentage of algal xanthophylls supports that algae could be an important part of the microbial photosynthetic community in this ancient ecosystem.

Effect of Carotenoids and Monogalactosyl Diglyceride on Bacteriochlorophyll c Aggregates in Aqueous Buffer: Implications for the Self‐assembly of Chlorosomes¶

AbstractAggregation 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.

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.

Diastereoselective Control of Bacteriochlorophyll e Aggregation. 3-S-BChl e Is Essential for the Formation of Chlorosome-Like Aggregates

We have investigated supramolecular bacteriochlorophyll (BChl) e aggregates in four different solvent systems. Aggregates of HPLC-purified BChl e species, 31-R-8-ethyl-12-ethyl BChl e, 31-R-8-propyl-12 ethyl BChl e, 31-S-8-propyl-12-ethyl BChl e, and 31-S-8-isobutyl-12-ethyl BChl e, in cyclohexane, n-hexane, H2O/monogalactosyldiglyceride, or H2O/lecithin, indicate two different aggregate structures, as judged from absorption, circular dichroism, infrared, and resonance Raman spectroscopies. Aggregates of 31-R-BChl (R-BChl) e were characterized by a 706 nm absorption band and broad C-131 carbonyl stretching bands at 1650−1680 cm-1. In contrast, aggregates of 31-S-BChl (S-BChl) e displayed a Qy band maximum at 717 nm and a carbonyl stretching band at 1650 cm-1 only. All measurements indicated that the aggregates of S-8-isobutyl-12-ethyl BChl (S[IE]BChl) e mimicked the supramolecular aggregates in intact chlorosomes of Chlorobium phaeobacteroides much better than did the R-8-ethyl-12-ethyl BChl (R[EE]BChl) e...

The effects of epimerization at the 3(1)-position of bacteriochlorophylls c on their aggregation in chlorosomes of green sulfur bacteria. Control of the ratio of 3(1) epimers by light intensity.

R- and S-epimerization at the 3(1) position of bacteriochlorophyll (BChl) c and the formation of rod-like aggregates in chlorosomes of green sulfur bacteria were markedly affected in Chlorobium (Cb.) tepidum and Cb. limicola by cultivation under various light intensities (photon fluence rate). The stronger the light, the higher the ratio of the S-epimer to the R-epimer for each homolog of BChl c in the bacteria. S[P,E] BChl cF and S[I,E] BChl cF were found to be the major S-epimers in Cb. tepidum and Cb. limicola, respectively. R[P,E] BChl cF decreased markedly compared to R[E,E] BChl cF in Cb. tepidum, whereas no observable change in the ratio of R[P,E]/R[E,E] was detected for Cb. limicola. With increase in light intensity the Qy absorption maximum of the bacteria shifted to shorter wavelengths. In vitro spectroscopic studies of the aggregates showed a marked difference in the formation of aggregates from R- and S-epimers of BChl c; the S-epimers formed aggregates much more slowly than did the R-epimers. These results suggest that the ratio of the epimers of BChl c might significantly affect the aggregation of BChl in the chlorosome. We propose different roles for the R- and S-epimers in chlorosomes of Cb. limicola and Cb. tepidum.

Electronic Excitations in Aggregates of Bacteriochlorophylls

Photosynthetic organisms have developed antenna systems to enlarge their cross section for capturing sunlight. These systems involve in some cases aggregates of bacteriochlorophylls (BChls). The structure of one such aggregate, a tightly coupled circular hexadecamer of BChls and a loosely coupled octamer of BChls, has recently been solved. In this paper we investigate the electronic excitations in these two aggregates as well as in monomeric BChl and BChl dimers by means of INDO/S-CI calculations. The results provide a detailed description of the properties of electronic states in the BChl aggregates (energies, dipole and transition dipole moments) that are relevant for the biological function of BChls, to absorb light and transfer energy on the subpicosecond time scale. The lower-energy excitations of the BChl hexadecamer are of exciton type, i.e., experiencing a strong coupling that leads to excitation delocalization over the entire aggregate. An effective Hamiltonian is provided which reproduces these ...

Studies of the location and function of isoprenoid quinones in chlorosomes from green sulfur bacteria

The chlorosome antenna of the green sulfur bacterium Chlorobium tepidum essentially consists of aggregated bacteriochlorophyll (BChl) c enveloped in a glycolipid monolayer. Small amounts of protein and the isoprenoid quinones chlorobiumquinone (CK) and menaquinone-7 (MK-7) are also present. Treatment of isolated chlorosomes from Cb. tepidum with sodium dodecyl sulfate (SDS) did not affect the quinones, demonstrating that these are located in a site which is inaccessible to SDS, probably in the interior of the chlorosomes. About half of the quinones were removed by Triton X-100. The non-ionic character of Triton probably allowed it to extract components from within the chlorosomes. MK-10 in chlorosomes from the green filamentous bacterium Chloroflexus aurantiacus was likewise found to be located in the chlorosome interior. The excitation transfer in isolated chlorosomes from Cb. tepidum is redox-regulated. We found a ratio of BChl c fluorescenceintensity under reducing conditions (Fred) to that under oxidizing conditions (Fox) of approximately 40. The chlorosomal BChl a fluorescence was also redox-regulated. When the chlorosomal BChl c–BChl c interactions were disrupted by 1-hexanol, the BChl c Fred/Fox ratiodecreased to approximately 3. When CK and MK-7 were extracted from isolated chlorosomes with hexane, the BChl c Fred/Fox ratio also decreased to approximately 3. A BChl c Fred/Fox ratio of 3–5 was furthermore observed in aggregates of pure BChl c and in chlorosomes from Cfx. aurantiacus which do not contain CK. We therefore suggest that BChl c aggregates inherently exhibit a small redox-dependent fluorescence (Fred/Fox ≈ 3) and that the large redox-dependent fluorescence observed in chlorobial chlorosomes (Fred/Fox ≈ 40) is CK-dependent.