The formation and characterization of the in vitro polymeric aggregates of bacteriochlorophyllc homologs fromChlorobium limicola in aqueous suspension in the presence of monogalactosyl diglyceride

Efficient two-step excitation energy transfer in artificial light-harvesting antenna based on bacteriochlorophyll aggregates

Chlorosomes of green photosynthetic bacteria are large light-harvesting complexes enabling these organisms to survive at extremely low-light conditions. Bacteriochlorophylls found in chlorosomes self-organize and are ideal candidates for use in biomimetic light-harvesting in artificial photosynthesis and other applications for solar energy utilization. Here we report on the construction and characterization of an artificial antenna consisting of bacteriochlorophyll c co-aggregated with β-carotene, which is used to extend the light-harvesting spectral range, and bacteriochlorophyll a, which acts as a final acceptor for excitation energy. Efficient energy transfer between all three components was observed by means of fluorescence spectroscopy. The efficiency varies with the β-carotene content, which increases the average distance between the donor and acceptor in both energy transfer steps. The efficiency ranges from 89 to 37% for the transfer from β-carotene to bacteriochlorophyll c, and from 93 to 69% for the bacteriochlorophyll c to bacteriochlorophyll a step. A significant part of this study was dedicated to a development of methods for determination of energy transfer efficiency. These methods may be applied also for study of chlorosomes and other pigment complexes.

A nanoscale bio-inspired light-harvesting system developed from self-assembled alkyl-functionalized metallochlorin nano-aggregates

Self-assembled supramolecular organization of nano-structured biomimetic light-harvesting modules inside solid-state nano-templates can be exploited to develop excellent light-harvesting materials for artificial photosynthetic devices. We present here a hybrid light-harvesting system mimicking the chlorosomal structures of the natural photosynthetic system using synthetic zinc chlorin units (ZnChl-C6, ZnChl-C12 and ZnChl-C18) that are self-aggregated inside the anodic aluminum oxide (AAO) nano-channel membranes. AAO nano-templates were modified with a TiO2 matrix and functionalized with long hydrophobic chains to facilitate the formation of supramolecular Zn-chlorin aggregates. The transparent Zn-chlorin nano-aggregates inside the alkyl-TiO2 modified AAO nano-channels have a diameter of ∼120 nm in a 60 μm length channel. UV-Vis studies and fluorescence emission spectra further confirm the formation of the supramolecular ZnChl aggregates from monomer molecules inside the alkyl-functionalized nano-channels. Our results prove that the novel and unique method can be used to produce efficient and stable light-harvesting assemblies for effective solar energy capture through transparent and stable nano-channel ceramic materials modified with bio-mimetic molecular self-assembled nano-aggregates.

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.

Self-assembly and energy transfer in artificial light-harvesting complexes of bacteriochlorophyll c with astaxanthin

Chlorosomes, the light-harvesting antennae of green photosynthetic bacteria, are based on large aggregates of bacteriochlorophyll molecules. Aggregates with similar properties to those in chlorosomes can also be prepared in vitro. Several agents were shown to induce aggregation of bacteriochlorophyll c in aqueous environments, including certain lipids, carotenes, and quinones. A key distinguishing feature of bacteriochlorophyll c aggregates, both in vitro and in chlorosomes, is a large (>60 nm) red shift of their Q(y) absorption band compared with that of the monomers. In this study, we investigate the self-assembly of bacteriochlorophyll c with the xanthophyll astaxanthin, which leads to the formation of a new type of complexes. Our results indicate that, due to its specific structure, astaxanthin molecules competes with bacteriochlorophylls for the bonds involved in the aggregation, thus preventing the formation of any significant red shift compared with pure bacteriochlorophyll c in aqueous buffer. A strong interaction between both the types of pigments in the developed assemblies, is manifested by a rather efficient (~40%) excitation energy transfer from astaxanthin to bacteriochlorophyll c, as revealed by fluorescence excitation spectroscopy. Results of transient absorption spectroscopy show that the energy transfer is very fast (<500 fs) and proceeds through the S(2) state of astaxanthin.

Spectroscopic properties and bacteriochlorophyll c isomer composition of extramembranous light-harvesting complexes in the green sulfur photosynthetic bacterium Chlorobium tepidum and its CT0388-deleted mutant under vitamin B12-limited conditions

The effects of exogenous vitamin B12 on the green sulfur photosynthetic bacterium Chlorobium (Chl.) tepidum were examined. Wild-type cells and mutant cells lacking a gene CT0388 (denoted as VB0388) of Chl.tepidum were grown in liquid cultures containing different concentrations of vitamin B12. The VB0388 cells hardly grew in vitamin B12-limited media, indicating that the product of CT0388 actually played an important role in vitamin B12 biosynthesis in Chl. tepidum. Both wild-type and VB0388 cells in vitamin B12-limited media exhibited absorption bands and CD signals at the Qy region that were shifted to a shorter wavelength than those of cells grown in normal media. BChl c isomers that had S-stereochemistry at the 3(1)-position tended to increase in Chl. tepidum grown in vitamin B12-limited media.

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.

Bacteriochlorophyllide c C-8(2) and C-12(1) methyltransferases are essential for adaptation to low light in Chlorobaculum tepidum

Bacteriochlorophyll (BChl) c is the major photosynthetic pigment in the green sulfur bacterium Chlorobaculum tepidum, in which it forms protein-independent aggregates that function in light harvesting. BChls c, d, and e are found only in chlorosome-producing bacteria and are unique among chlorophylls because of methylations that occur at the C-8(2) and C-12(1) carbons. Two genes required for these methylation reactions were identified and designated bchQ (CT1777) and bchR (CT1320). BchQ and BchR are members of the radical S-adenosylmethionine (SAM) protein superfamily; each has sequence motifs to ligate a [4Fe-4S] cluster, and we propose that they catalyze the methyl group transfers. bchQ, bchR, and bchQ bchR mutants of C. tepidum were constructed and characterized. The bchQ mutant produced BChl c that was not methylated at C-8(2), the bchR mutant produced BChl c that was not methylated at C-12(1), and the double mutant produced [8-ethyl, 12-methyl]-BChl c that lacked methylation at both the C-8(2) and C-12(1) positions. Compared to the wild type, the Qy absorption bands for BChl c in the mutant cells were narrower and blue shifted to various extents. All three mutants grew slower and had a lower cellular BChl c content than the wild type, an effect that was especially pronounced at low light intensities. These observations show that the C-8(2) and C-12(1) methylations of BChl c play important roles in the adaptation of C. tepidum to low light intensity. The data additionally suggest that these methylations also directly or indirectly affect the regulation of the BChl c biosynthetic pathway.

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.

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.

Bacteriochlorophyll-c homolog composition in green sulfur photosynthetic bacterium Chlorobium vibrioforme dependent on the concentration of sodium sulfide in liquid cultures

Green sulfur photosynthetic bacteria Chlorobium (Chl.) vibrioforme (DSM 263 strain and NCIB 8327 substrain possessing BChl-c) and Chl. tepidum (ATCC 49652) were photoautotrophically grown in liquid cultures containing different concentrations of sodium sulfide (Na2S). BChl-c homologs possessing a methyl group at the 12-position tended to increase in cells of the two strains of Chl. vibrioforme cultured under high Na2S concentrations. In contrast, the Na2S concentration in liquid cultures did not affect the relative composition of BChl-c homologs in Chl. tepidum. 8-Propyl-12-methyl([P,M])-BChl-c homolog, which has been little observed in usual cultivations, could be isolated by reverse-phase high-performance liquid chromatography from the cells of Chl. vibrioforme grown under high Na2S contents. The [P,M]-BChl-c homolog has the R-configuration at the 3(1)-position, which was determined by 1H-NMR analyses.

Comparison between chlorosomes containing bacteriochlorophyll-c and chlorosomes containing bacteriochlorophyll-d isolated from two substrains of green sulfur photosynthetic bacterium Chlorobium vibrioforme NCIB 8327

Chlorosomes containing bacteriochlorophyll(BChl)-c and those containing BChl-d were isolated from two substrains of Chlorobium vibrioforme f. sp. thiosulfatophilum NCIB 8327, respectively. The two types of chlorosomes were investigated from the following aspect, what kinds of effects the molecular structure of chlorosomal BChls had on structural and spectroscopic properties of in vivo self-aggregates in chlorosomes without alteration of the other components such as chlorosomal proteins and lipids; both chlorosomes were expected to have the same components except for light-harvesting BChls. In their visible absorption spectra, the differences of Soret and Q(y) peak positions between BChl-c containing and BChl-d containing chlorosomes were similar to the differences between monomeric BChl-c and d. An inverse S-shaped CD signal in the Q(y) region of BChl-d containing chlorosomes was 1.4 times larger than that of BChl-c containing chlorosomes, when the Q(y) absorbance of the two chlorosomes was almost the same. This implies that the excitonic interaction of BChl-d is larger than that of BChl-c in natural chlorosomes. Resonance Raman spectroscopy showed that BChl self-assemblies in both chlorosomes were essentially formed by the same local structural interaction among 3(1)-hydroxy group, 13-keto group, and central magnesium. BChl-d self-aggregates in chlorosomes were more tolerant of 1-hexanol than in vivo BChl-c aggregates, suggesting that the molecular structure of BChl-d provided more stable self-assemblies than BChl-c in natural chlorosomes.

Exciton theory for supramolecular chlorosomal aggregates: 1. Aggregate size dependence of the linear spectra

The interior of chlorosomes of green bacteria forms an unusual antenna system organized without proteins. The steady-spectra (absorption, circular dichroism, and linear dichroism) have been modeled using the Frenkel Hamiltonian for the large tubular aggregates of bacteriochlorophylls with geometries corresponding to those proposed for Chloroflexus aurantiacus and Chlorobium tepidum chlorosomes. For the Cf. aurantiacus aggregates we apply a structure used previously (V. I. Prokhorenko., D. B. Steensgaard, and A. R. Holzwarth, Biophys: J. 2000, 79:2105-2120), whereas for the Cb. tepidum aggregates a new extended model of double-tube aggregates, based on recently published solid-state nuclear magnetic resonance studies (B.-J. van Rossum, B. Y. van Duhl, D. B. Steensgaard, T. S. Balaban, A. R. Holzwarth, K. Schaffner, and H. J. M. de Groot, Biochemistry 2001, 40:1587-1595), is developed. We find that the circular dichroism spectra depend strongly on the aggregate length for both types of chlorosomes. Their shape changes from "type-II" (negative at short wavelengths to positive at long wavelengths) to the "mixed-type" (negative-positive-negative) in the nomenclature proposed in K. Griebenow, A. R. Holzwarth, F. van Mourik, and R. van Grondelle, Biochim: Biophys. Acta 1991, 1058:194-202, for an aggregate length of 30-40 bacteriochlorophyll molecules per stack. This "size effect" on the circular dichroism spectra is caused by appearance of macroscopic chirality due to circular distribution of the transition dipole moment of the monomers. We visualize these distributions, and also the corresponding Frenkel excitons, using a novel presentation technique. The observed size effects provide a key to explain many previously puzzling and seemingly contradictory experimental data in the literature on the circular and linear dichroism spectra of seemingly identical types of chlorosomes.

The role of carotenoids in the photoadaptation of the brown-colored sulfur bacterium Chlorobium phaeobacteroides

The brown-colored sulfur bacterium Chlorobium (Cb.) phaeobacteroides 1549 (new name, Chlorobaculum limnaeum 1549) contains many kinds of carotenoids as well as bacteriochlorophyll (BChl) e. These carotenoids were identified with C18-high-performance liquid chromatography, absorption, mass and proton nuclear magnetic resonance spectroscopies and were divided into two groups: the first is carotenoid with one or two phi-end groups such as isorenieratene and beta-isorenieratene and the second is carotenoid with one or two beta-end groups such as p-zeacarotene, beta-carotene and 7,8-dihydro-beta-carotene. The latter 7,8-dihydro-beta-carotene was found to be a novel carotenoid in nature. OH-gamma-Carotene glucoside laurate and OH-chlorobactene glucoside laurate were also found as minor components. The distribution of BChl e homologs in Cb. phaeobacteroides cultivated under various light intensities did not change, but the carotenoid to BChl e ratio changed markedly: carotenoid with the phi-end group maintained the same ratio to BChl e, whereas that with the beta-end group increased with increasing light intensity. The cells cultured under low-light intensity contained more phi-end carotenoids than beta-end. In Cb. phaeobacteroides the wavelength of the Qy band of BChl e aggregates did not change. We suggested that Cb. phaeobacteroides photoadapts to light intensity by changing the carotenoid composition.

Coaggregate of amphiphilic zinc chlorins with synthetic surfactants in an aqueous medium to an artificial supramolecular light-harvesting system

Aqueous assemblies of zinc chlorins possessing a nonionic (oligo)oxyethylene, a cationic quaternary ammonium or an anionic sulfonate group were prepared in the presence of a synthetic surfactant. The nonionic zinc chlorin formed aggregates when admixed with a nonionic surfactant such as Triton X-100 to give a highly ordered oligomeric J-aggregate similarly as natural bacteriochlorophyll-c or d does in a chlorosome. In addition, the coassemblies of the cationic zinc chlorin with an anionic surfactant and of the anionic zinc chlorin with a cationic surfactant gave large oligomers of these chlorophyllous pigments. The structures of hydrophilic groups in both the zinc chlorin and surfactant molecules controlled their aqueous coassemblies.

Facile synthesis of chlorophyll analog possessing a disulfide bond and formation of self-assembled monolayer on gold surface

A chlorophyll analog forming self-assembled monolayers (SAMs) on a gold surface was synthesized for the first time. 13(2)-(Demethoxycarbonyl)pheophorbide-a, which was converted from naturally occurring chlorophyll-a, was condensed with 2-hydroxyethyl disulfide to give a chlorin dyad linked by a disulfide bond. The chlorin analog was spontaneously immobilized on a gold substrate by soaking in an acetone solution of the dyad for 24 h. The resulting gold plate exhibited a visible absorption spectrum with about 420- and 675-nm maxima as the Soret and Qy peaks, respectively, indicating that chlorin pi-conjugates were modified on the gold substrate through Au-S bonding. Both visible absorption and fluorescence emission bands of the chlorin chromophores on the gold substrate were red-shifted compared with those of the synthesized chlorin dyad in a homogeneous acetone solution. The measured absorbance at the Soret maximum suggests that the chlorin chromophores on the gold plate were densely packed on a gold surface to form a SAM. Cathodic photocurrents were generated from SAMs of the chlorins on a gold substrate with irradiation of visible-lights above 400 nm. Photoinduced electron transfer from chlorins on the gold substrate to oxygen molecules in an electrolyte solution were attributed to the cathodic photocurrent generation.

Presence of Exclusively Bacteriochlorophyll-c Containing Substrain in the Culture of Green Sulfur Photosynthetic Bacterium Chlorobium vibrioforme Strain NCIB 8327 Producing Bacteriochlorophyll-d

The light-dependent composition change of light harvesting bacteriochlorophyll(BChl)s in the present culture of a green sulfur photosynthetic bacterium Chlorobium (Chl.) vibrioforme f. sp. thiosulfatophilum strain NCIB 8327 was investigated by visible absorption spectroscopy and HPLC analyses. When the culture was repeatedly grown in liquid media under a low light condition, both the Soret and Qy absorption bands of the in vivo spectrum were shifted to longer wavelengths. Analysis of the extracted pigments by HPLC revealed that the ratio of the amount of BChl-c to that of BChl-d molecules gradually increased during repeated cultivation. In contrast, when the culture grown under a low light intensity was transferred to a high light condition and continued to be grown, the absorption bands were shifted to shorter wavelengths and the ratio of BChls-c/d decreased finally to the almost original value. Colonies were prepared on solid agar media from the liquid culture containing both BChls-c and d, which was grown under a low light intensity. Each colony obtained was found to contain either BChl-c or d, but not both of them. Two types of cells isolated in this study were derived from the same clone, judged from their genetic analyses. The variation of pigment composition in our liquid culture observed here could be ascribed to the difference of growth rates between two substrains containing BChl-c and BChl-d, respectively, depending on light conditions.

Self-assembly of synthetic zinc chlorins in a silicate micelle prepared by sol-gel process

Zinc methyl 3-devinyl-3-hydroxymethyl-pyropheophorbide-a (1), a good model compound of light-harvesting pigments of green photosynthetic bacteria, formed self-aggregates in the presence of octadecyltriethoxysilane and tetraethoxysilane in an aqueous solution to exhibit visible absorption spectra similar to the natural antenna. Base-catalyzed cross-linked polymerization of the additive Si-ORs (R=ethyl and H) afforded the formation of a siloxane network (Si-O-Si) on the surface of the self-assemblies of 1. The resulting microcapsules were stable to tolerate the deaggregation to monomeric 1 by addition of surfactant Triton X-100 more largely than the corresponding micelles before polymerization.

Spectral heterogeneity in single light-harvesting chlorosomes from green sulfur photosynthetic bacterium chlorobium tepidum

The fluorescence emission properties of single chlorosomes from the green sulfur photosynthetic bacterium Chlorobium (Chl.) tepidum are studied for the first time, using a total internal reflection fluorescence microscope. The fluorescence peak positions of bacteriochlorophyll (BChl)-c self-aggregates in a single chlorosome of Chl. tepidum were widely distributed in the wavelength region between 750 and 768 nm, and the standard deviation (s.d. = 4.1 nm, n = 51) was larger than that of single chlorosomes of Chloroflexus (Cfl.) (s.d. = 1.9 nm, n = 50). The spectral heterogeneity among single chlorosomes from Chl. tepidum was in sharp contrast to those from Cfl. aurantiacus. The difference of chlorosomal spectral properties between Chl. tepidum and Cfl. aurantiacus at the single-unit level would be ascribed to the homolog composition of BChl-c--chlorosomes of Chl. tepidum have BChl substituted with various alkyl groups at both the 8- and 12-positions, whereas light-harvesting BChl-c molecules in Cfl. chlorosomes have the same substituents at the 8- (ethyl group) and 12- (methyl group) positions.

Spectroscopic studies on self-aggregation of bacteriochlorophyll-e in nonpolar organic solvents: effects of stereoisomeric configuration at the 3(1)-position and alkyl substituents at the 8(1)-position

Self-aggregation of naturally occurring bacteriochlorophyll (BChl)-e in nonpolar organic solvents was investigated by visible absorption, fluorescence emission and circular dichroism spectra. Cultured brown-colored photosynthetic bacteria have several BChl-e as light-harvesting antenna pigments. Three major BChl-e homologs were separated from the extracts of the culture by reverse-phase high-performance liquid chromatography (HPLC) and characterized by 1H-NMR and fast-atom bombardment mass spectroscopy: 8-ethyl-12-ethyl ([E,E])-, 8-propyl-12-ethyl- and 8-isobutyl-12-ethyl-BChl-e farnesyl esters. All the homologs consisted of a mixture of the 3(1)-epimers, and epimerically pure BChl-e were also given by HPLC separation. All the separated BChl-e epimers, the epimeric mixtures and the homologous mixtures formed self-aggregates in 2% dichloromethane/hexane, giving visible absorption spectra similar to that of the whole cells, which showed two peaks (or shoulders) around 430-450 and 520 nm at the Soret region as well as a red-shifted Qy band relative to the monomeric. The spectral properties of the Soret band were basically unchanged among the epimers or epimeric/homologous mixtures. In contrast, the Qy band of aggregates of epimeric mixtures (except [E,E]) and homologous mixtures red-shifted and broadened compared with the epimerically pure. The red-shift and broadening of the Qy band are advantageous for efficient energy transfer from BChl-e aggregates to BChl-a in a baseplate in chlorosomes because their spectral overlap increases.

Pure and scrambled self-aggregates prepared with zinc analogues of bacteriochlorophylls c and d

Zinc analogues of bacteriochlorophylls c and d self-assembled in aqueous media with phospholipids. A methanol solution of zinc chlorin and alpha-lecithin was put in a cellulose tube and the inner methanol solvent was gradually replaced with water by dialysis to form the self-assembled oligomers. Visible absorption spectra of the aqueous solution showed that zinc chlorins formed J-aggregates within the hydrophobic core of alpha-lecithin assemblies and that the supramolecular structure of the aggregates depended upon the stereochemistry at the 3(1)-position and the alkyl substituents at the 8-, 12-, and 17(4)-positions of the zinc chlorin. When the aqueous aggregates were prepared with a mixture of 3(1)-epimers and/or 8-, 12-, or 17(4)-homologues of zinc 3(1)-hydroxy-13(1)-oxochlorins, the structurally distinct components coaggregated to make scrambled oligomers. However, during the dialysis, zinc 3(1)-hydroxy- and 7(1)-hydroxy-13(1)-oxochlorins slowly individually aggregated to give two structurally different oligomer units in the cellulose tube. In contrast, if the two zinc chlorin components rapidly self-assembled in an aqueous medium, these components coaggregated to form scrambled oligomers. The present study shows that both the molecular structure of the pigments and the speed of the oligomerization determine the molecular arrangement in chlorosome-type self-assembled oligomers.

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.

Association of bacteriochlorophyll a with the CsmA protein in chlorosomes of the photosynthetic green filamentous bacterium Chloroflexus aurantiacus

The protein assumed to be associated with bacteriochlorophyll (BChl) a in chlorosomes from the photosynthetic green filamentous bacterium Chloroflexus aurantiacus was investigated by alkaline treatment, proteolytic digestion and a new treatment using 1-hexanol, sodium cholate and Triton X-100. Upon alkaline treatment, only the 5.7 kDa CsmA protein was removed from the chlorosomes among six proteins detected by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) analysis, concomitantly with the disappearance of BChl a absorption at 795 nm. Trypsin treatment removed two proteins with molecular masses of 11 and 18 kDa (CsmN and CmsM), whereas the spectral properties of BChl a and BChl c were not changed. By the new hexanol-detergent (HD) treatment, most BChl c and all of the detected proteins except CsmA were removed from the chlorosomes without changing the BChl a spectral properties. Subsequent proteinase K treatment of these HD-treated chlorosomes caused digestion of CsmA and a simultaneous decrease of the BChl a absorption band. Based on these results, we suggest that CsmA is associated with BChl a in the chlorosomes. This suggestion was supported by the measured stoichiometric ratio of BChl a to CsmA in isolated chlorosomes, which was estimated to be between 1.2 and 2.7 by amino acid analysis of the SDS-PAGE-resolved protein bands.

The influence of the presence of lipid on the aggregation of 8,12-diethyl farnesyl bacteriochlorophyll c located in adsorbed layers and monolayers

The photoacoustic spectra and time-resolved delayed luminescence spectra in the microsecond time range were measured for layers of 8,12-diethyl farnesyl bacteriochlorophyll c adsorbed on quartz supports by solvent evaporation and as Langmuir-Blodgett monolayers. Both types of model system were also investigated with the addition of lipid. The data showed a very strong influence of lipid addition on pigment aggregation. In samples with synthetic and natural lipid addition, the pigments were found to be predominantly in the monomeric and dimeric states, whereas in the same type of sample without lipid, the pigments were aggregated to a higher degree. The influence of the presence of lipid on the aggregation of bacteriochlorophyll c in monolayers and adsorbed layers may also suggest that the contact of various pigment molecules with the lipids surrounding the chlorosome may influence the formation of various pigment aggregates in vivo. The synthetic lipid L-alpha-phosphatidylcholine dipalmitoyl and the natural lipid L-alpha-phosphatidylcholine type IVS from soy beans were used. In the latter case, only adsorbed layers were investigated. Our interpretation is preliminary as only one 8,12-diethyl farnesyl bacteriochlorophyll c homologue was present in our systems.