Self-aggregates of bacteriochlorophylls-c, d and e in a light-harvesting antenna system of green photosynthetic bacteria: Effect of stereochemistry at the chiral 3-(1-hydroxyethyl) group on the supramolecular arrangement of chlorophyllous pigments

Synthesis of ¹⁸O-labeled photosynthetically active chlorophylls at the 3- or 7-carbonyl group with high regioselectivity

The 3- and 7-formyl groups of chlorophyll-d (Chl-d) and bacteriochlorophyll-e (BChl-e), respectively, were regioselectively labeled with an isotopically stable oxygen-18 (¹⁸O) atom to give 3¹-¹⁸O-labeled Chl-d and 7¹-¹⁸O-labeled BChl-e (ca. 90% ¹⁸O) by exchanging the carbonyl oxygen atoms in the presence of acidic H₂ ¹⁸O (ca. 95% ¹⁸O). Another photosynthetically active chlorophyll, BChl-a possessing the 3-acetyl group was treated under similar acidic conditions to afford a trace amount of 3¹-¹⁸O-labeled BChl-a and further demetallated compound, the corresponding 3¹-¹⁸O-labeled bacteriopheophytin-a as the major product with 55% ¹⁸O-degree. The FT-IR spectra of ¹⁸O-(un)labeled chlorophylls in the solution and the solid states showed that the 3- and 7-carbonyl stretching vibration modes moved to about a 30-cm⁻¹ lower wavenumber by ¹⁸O-labeling at the 3¹- and 7¹-oxo moieties. In artificial chlorosome-like self-aggregates of BChl-e, the ¹⁸O-labeled 7-carbonyl stretching mode was completely resolved from the specially hydrogen-bonded 13-C=O stretching mode, evidently indicating no interaction of the 7-CHO with other functional groups in the supramolecules.

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.

Mimics of the self-assembling chlorosomal bacteriochlorophylls: regio- and stereoselective synthesis and stereoanalysis of acyl(1-hydroxyalkyl)porphyrins

Diacylation of copper 10,20-bis(3,5-di-tert-butylphenylporphyrin) using Friedel-Crafts conditions at short reaction times, high concentrations of catalyst, and 0-4 degrees C affords only the 3,17-diacyl-substituted porphyrins, out of the 12 possible regioisomers. At longer reaction times and higher temperatures, the 3,13-diacyl compounds are also formed, and the two isomers can be conveniently separated by normal chromatographic techniques. Monoreduction of these diketones affords in good yields the corresponding acyl(1-hydroxyalkyl)porphyrins, which after zinc metalation are mimics of the natural chlorosomal bacteriochlorophyll (BChl) d. Racemate resolution by HPLC on a variety of chiral columns was achieved and further optimized, thus permitting easy access to enantiopure porphyrins. Enantioselective reductions proved to be less effective in this respect, giving moderate yields and only 79% ee in the best case. The absolute configuration of the 3(1)-stereocenter was assigned by independent chemical and spectroscopic methods. Self-assembly of a variety of these zinc BChl d mimics proves that a collinear arrangement of the hydroxyalkyl substituent with the zinc atom and the carbonyl substituent is not a stringent requirement, since both the 3,13 and the 3,17 regioisomers self-assemble readily as the racemates. Interestingly, the separated enantiomers self-assemble less readily, as judged by absorption, fluorescence, and transmission electron microscopy studies. Circular dichroism spectra of the self-assemblies show intense Cotton effects, which are mirror-images for the two 3(1)-enantiomers, proving that the supramolecular chirality is dependent on the configuration at the 3(1)-stereocenter. Upon disruption of these self-assemblies with methanol, which competes with zinc ligation, only very weak monomeric Cotton effects are present. The favored heterochiral self-assembly process may also be encountered for the natural BChls. This touches upon the long-standing problem of why both 3(1)-epimers are encountered in BChls in ratios that vary with the illumination and culturing conditions.

Substituent effects of porphyrins on structures and photophysical properties of amphiphilic porphyrin aggregates

Substituent effects of porphyrin on the structures and photophysical properties of the J-aggregates of protonated 5-(4-alkoxyphenyl)-10,15,20-tris(4-sulfonatophenyl)porphyrin have been examined for the first time. Selective formation of the porphyrin J-aggregate was attained when suitable length of the alkoxy group was employed for the amphiphilic porphyrin. Namely, a regular leaflike structure was observed for the J-aggregates of protonated 5-(4-octyloxyphenyl)-10,15,20-tris(4-sulfonatophenyl)porphyrin, which was consistent with the results obtained by using the UV-visible absorption and dynamic light-scattering measurements. A bilayer structure in which the hydrophobic alkoxyl groups facing inside the bilayer are interdigitated to each other, whereas the hydrophilic porphyrin moieties are exposed outside, was proposed to explain the unique porphyrin J-aggregate. Fast energy migration and efficient quenching by defect site in the J-aggregates were suggested to rationalize the short lifetimes of the excited J-aggregates.

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.

Self-aggregation behavior of synthetic zinc 3-hydroxymethyl-13/15-carbonyl-chlorins as models of main light-harvesting components in photosynthetic green bacteria

Zinc complexes of 3-hydroxymethyl-13/15-carbonyl-chlorins having a six-membered lactone as the E-ring were prepared by modifying purpurin-18 as models of bacteriochlorophyll-d, one of the chlorophyllous pigments in the main light-harvesting antenna systems (chlorosomes) of green photosynthetic bacteria. The synthetic 13-carbonylated compound self-aggregated in 1%(v/v) tetrahydrofuran and hexane to give large oligomers possessing red-shifted and broadened electronic absorption bands and intense circular dichroism bands at the shifted Q ( y ) region, indicating that the supramolecular structure of the resulting self-aggregate was similar to those of natural and artificial chlorosomal aggregates. The red-shift value observed here was smaller than the reported values in chlorosomal pigments having a five-membered keto-ring, which was ascribable to a weaker intermolecular hydrogen-bonding of 13-C=O with 3(1)-OH in a supramolecule of the former self-aggregate and suppression of the pi-pi interaction among the composite chlorins. On the other hand, the isomeric 15-carbonylated molecule was monomeric even in the nonpolar organic solvent, confirming the reported proposal that the linear orientation of three interactive moieties, OH, C=O and Zn, in a molecule is requisite for its chlorosomal self-aggregation.

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.

Self-assembled single- and double-stack pi-aggregates of chlorophyll derivatives on highly ordered pyrolytic graphite

The highly efficient, light-harvesting antennae systems present in chlorosomes of green phototrophic bacteria, such as Chloroflexus aurantiacus, serve as a model for the design of bioinspired nanostructured materials. A semisynthetic zinc chlorin, derived from natural chlorophyll a, is used as a building block that self-assembles into excitonically coupled chromophore stacks. Temperature-dependent UV/Vis and circular dichroism spectroscopic measurements show the reversible formation of soluble chiral aggregates of this new zinc chlorin dye. High-resolution scanning tunneling microscopy and atomic force microscopy studies reveal the formation of two types of well-ordered pi-stacked aggregates on highly ordered pyrolytic graphite.

Self-aggregation of synthetic zinc chlorophyll derivatives possessing multi-perfluoroalkyl chains in perfluorinated solvents

Zinc 3(1)-hydroxy-13(1)-oxo-chlorins possessing two, three, four and six perfluorooctyl chains were synthesized from naturally occurring chlorophyll-a. Only the synthetic zinc chlorin possessing six perfluorooctyl chains was directly dissolved in perfluorinated solvents due to its high fluorine content in molecular weight (over 50%). In this solution, visible absorption spectra gave a red-shifted Q(y) band at 723 nm (compared to 648 nm in THF) and showed the formation of well-ordered self-aggregates. No monomeric form was observed in the solution from any fluorescence emission spectra from visible absorption spectra. In the aggregate solution, no precipitation occurred during either standing for a long period or heating at 70 degrees C. This showed that the supramolecular structure was stabilized by F-F interactions on its surface among the perfluorooctyl chains on the 17-position and perfluorinated solvents. The core part of the supramolecular structure was constructed by a special intramolecular bond of Zn ... O3(2)-H ... O=C13(1), which was confirmed from resonance Raman spectral analysis.

Self-aggregation of zinc chlorophylls possessing perfluoroalkyl chains in fluorous solvents: Selective extraction of the self-aggregates with fluorous phase and accelerated formation of the ordered supramolecules in this phase

Zinc 3(1)-hydroxy-13(1)-oxo-chlorins possessing various perfluorooctyl groups in the 17-propionate were prepared by modifying naturally occurring chlorophyll-a. The synthetic compound having four perfluorooctyl groups readily self-assembled to form large J-aggregates, which were dissolved in fluorous phases with more ordered supramolecular structures than in a hydrocarbon solvent.

Regioisomerically Controlled Self-Aggregation of Zinc 3-Hydroxymethyl-13-formyl-chlorin/Porphyrin and Their 3,13-Inverted Pigments

Zinc 3-hydroxymethyl-13-formyl-chlorin, 1, and its 3,13-inverted (3-formyl-13-hydroxymethyl) regioisomer, 2, and their corresponding 17,18-dehydrogenated porphyrins, 3 and 4, were synthesized for models of natural bacteriochlorophylls-c/d/e possessing 3(1)-OH and 13-C=O groups which self-aggregate in main light-harvesting antenna systems of green photosynthetic bacteria. Zinc chlorins 1 and 2 were monomers in neat THF and gave an obvious difference in their visible absorption spectra, indicating that sole inversion of the 3- and 13-substituents in a chlorin chromophore controlled their optical properties. In an aqueous Triton X-100 solution (a nonionic surfactant), zinc 3(1)-OH-13-CHO-chlorin 1 and porphyrin 3 self-aggregated as do natural bacteriochlorophylls, while zinc 3-CHO-13(1)-OH chlorin 2 and porphyrin 4 (the 3,13-inverted regioisomers of 1 and 3) hardly formed such large oligomers, showing that the inversion of the peripheral 3,13-substituents made their oligomerization unfavorable. FT-IR spectra of aggregated 1-4 in the solid film and their molecular modeling calculations suggested that the 17(2)-C=O moiety in inverted 2/4 interacted with its own 13(1)-OH group to disturb further aggregation.

Artificial photosynthetic systems: assemblies of slipped cofacial porphyrins and phthalocyanines showing strong electronic coupling

This paper reviews selected types of structurally well defined assemblies of porphyrins and phthalocyanines with strong electronic coupling. Face-to-face, head-to-tail, slipped cofacial, and non-parallel dimeric motifs constructed by covalent and non-covalent bonds are compared in the earlier sections. Their molecular orientation, electronic overlap, and absorption and fluorescence properties are discussed with a view towards the development of artificial photosynthetic systems and molecular electronics. Complementary coordination dimers are fully satisfactory in terms of structural stability, orientation factor, pi-electronic overlap, and zero fluorescence quenching. In later sections, several polymeric and macrocyclic porphyrin assemblies constructed by a combination of covalent bonds and complementary coordination bonds are discussed from the viewpoint of light-harvesting antenna functions.

Low-temperature fluorescence from single chlorosomes, photosynthetic antenna complexes of green filamentous and sulfur bacteria

Fluorescence spectra of single chlorosomes isolated from a green filamentous bacterium (Chloroflexus (Cfl.) aurantiacus) and a green sulfur bacterium (Chlorobium (Cb.) tepidum) were measured by using a confocal laser microscope at 13 K. Chlorosomes were frozen either in a liquid solution (floating chlorosome) or on a quartz plate after being adsorbed (adsorbed chlorosome). Fluorescence peak wavelengths were shorter for the adsorbed single chlorosomes than for the floating ones. Single floating Cfl. chlorosomes showed a distribution of fluorescence peak positions having a center at 759.0 nm with a full width at half maximum of 6.3 nm. Single floating Cb. chlorosomes showed a 782.7 nm center with a full width at half-maximum of 3.4 nm. The distribution shifted to the blue and became wider with increasing temperature, especially in Cb. chlorosomes, suggesting a large excitonic density of states just above the lowest level. Energy transfer from BChl-c aggregates to BChl-a molecules in the baseplate proteins was observed in the floating chlorosomes but not in the adsorbed ones. A positive correlation was found between the peak wavelength of BChl-c fluorescence and the intensity of BChl-a fluorescence in single Cfl. chlorosomes. The results suggest that the BChl-c aggregates with longer wavelengths of the fluorescence peaks have a more efficient Förster-type energy transfer to the baseplate BChl-a.

Transmission electron microscopic study on supramolecular nanostructures of bacteriochlorophyll self-aggregates in chlorosomes of green photosynthetic bacteria

Supramolecular nanostructures of bacteriochlorophyll (BChl) self-aggregates in major light-harvesting complexes (chlorosomes) of green photosynthetic bacteria were successfully observed by freeze-fracture transmission electron microscope. Rod-shaped nanostructures with approximately 10 nm in diameter could be visualized in three types of green sulfur bacteria (Chlorobium). Diameter of the rod-shaped nanostructures in Chlorobium chlorosomes was independent of the molecular structures of their light-harvesting pigments, namely BChl-c or d. In contrast, chlorosomes of the green filamentous bacterium Chloroflexus aurantiacus had rod-shaped nanostructures with approximately 5 nm in diameter. The present results support that BChl self-aggregates in chlorosomes form rod-shaped nanostructures called rod-elements with approximately 10- and 5-nm diameters for Chlorobium and Chloroflexus, respectively.

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.

Crystal structures of BchU, a methyltransferase involved in bacteriochlorophyll c biosynthesis, and its complex with S-adenosylhomocysteine: implications for reaction mechanism

BchU plays a role in bacteriochlorophyll c biosynthesis by catalyzing methylation at the C-20 position of cyclic tetrapyrrole chlorin using S-adenosylmethionine (SAM) as a methyl source. This methylation causes red-shifts of the electronic absorption spectrum of the light-harvesting pigment, allowing green photosynthetic bacteria to adapt to low-light environments. We have determined the crystal structures of BchU and its complex with S-adenosylhomocysteine (SAH). BchU forms a dimer and each subunit consists of two domains, an N-terminal domain and a C-terminal domain. Dimerization occurs through interactions between the N-terminal domains and the residues responsible for the catalytic reaction are in the C-terminal domain. The binding site of SAH is located in a large cavity between the two domains, where SAH is specifically recognized by many hydrogen bonds and a salt-bridge. The electron density map of BchU in complex with an analog of bacteriochlorophyll c located its central metal near the SAH-binding site, but the tetrapyrrole ring was invisible, suggesting that binding of the ring to BchU is loose and/or occupancy of the ring is low. It is likely that His290 acts as a ligand for the central metal of the substrate. The orientation of the substrate was predicted by simulation, and allows us to propose a mechanism for the BchU directed methylation: the strictly conserved Tyr246 residue acts catalytically in the direct transfer of the methyl group from SAM to the substrate through an S(N)2-like mechanism.

Synthesis and Optical Properties of Bacteriochlorophyll-a Derivatives Having Various C3 Substituents on the Bacteriochlorin π-System

Methyl bacteriopyropheophorbide-a derivatives having a series of substituents at the C3 position were prepared and their optical properties were compared with the corresponding chlorin analogues. Two kinds of oxidation reaction (C3-vinyl --> formyl --> carboxy group) were found to be applicable with a little alteration of the free-base bacteriochlorin macrocycles. The Qx and Qy electronic absorption peak positions of synthetic bacteriochlorins in CH2Cl2 were affected by the C3 substituents and found to be more sensitive than those of the chlorins. The observed Qx/Qy peaks in their monomeric states were shifted to a longer wavelength in the order of 1-hydroxyethyl < hydroxymethyl < acetoxymethyl < vinyl < acetyl < carboxy < formyl < 2,2-dicyanoethynyl group. Zinc complex with the C3-hydroxymethyl group formed self-aggregates in a nonpolar organic solvent, which showed the largest red-shift of the Qy band (2380 cm(-1), 726 nm in THF to 878 nm in 1% THF-cyclohexane) among those of the synthetic self-aggregative (bacterio)chlorins examined.

Stereochemical determination of chlorophyll-d molecule from Acaryochloris marina and its modification to a self-aggregative chlorophyll as a model of green photosynthetic bacterial antennae

Acaryochloris marina is a unique photosynthetic prokaryote containing chlorophyll(Chl)-d as a major photoactive pigment (over 95%). The molecular structure of Chl-d is proposed as the 3-formyl analog of Chl-a. However, the stereochemistry of Chl-d at the 13(2)-, 17- and 18-positions has not yet been established unambiguously. In the first part of this paper, we describe the determination of their stereochemistries to be 13(2)-(R)-, 17-(S)- and 18-(S)-configurations by using 1H-1H NOE correlations in 1H-NMR and circular dichroism spectra as well as chemical modification of Chl-a to produce stereochemically defined Chl derivatives. In the second part of the paper, we report a facile synthesis of a self-aggregative Chl by modifying isolated Chl-d. Since Chl-d was characterized by its reactive 3-formyl group, the formyl group was reduced with t-BuNH2BH3 to afford the desirable Chl, 3-deformyl-3-hydroxymethyl-pyrochlorophyll-d (3(1)-OH-pyroChl-d). The synthetic 3(1)-OH-pyroChl-d molecules spontaneously self-organized to form well-ordered aggregates in a non-polar organic solvent. The self-aggregates are a good model of major light-harvesting antenna systems of green photosynthetic bacteria, chlorosomes, in terms of the following three findings. (1) Both the red-shifted electronic absorption band above 750 nm and its induced reverse S-shape CD signal around 750 nm were observed in 0.5% (v/v) THF-cyclohexane. (2) The stretching mode of the 13-carbonyl group was downshifted by about 35 cm(-1) from the wavenumber of its free carbonyl. (3) The self-aggregates were quite stable on titration of pyridine to the suspension, in comparison with those of natural chlorosomal bacteriochlorophyll-d possessing the 3-(1-hydroxyethyl) group.

Tunable interchromophore electronic interaction of a merocyanine dye in hydrogen-bonded supramolecular assemblies scaffolded by bismelamine receptors

The absorption and fluorescence properties of a barbiturate-type donor-pi-acceptor (D-pi-A) merocyanine dye are controlled by complexation with dimeric melamine receptors featuring different tether lengths.

Self-aggregation of synthetic zinc chlorophyll derivative possessing a perfluoroalkyl group in a fluorinated solvent

Zinc 3(1)-hydroxy-13(1)-oxo-chlorin possessing a perfluoroheptyl group on the 17-propionate was prepared by modifying chlorophyll a. The synthetic compound self-aggregated in 0.1% (v/v) THF and HCFC225cb (CClF2CF2CHClF) to give similar large oligomers to natural light-harvesting antennae of green photosynthetic bacteria and their models. Visible absorption, circular dichroism, and fluorescence emission spectra showed that F-F interaction between the perfluoroheptyl side chain and the fluorinated solvent (HCFC225cb) stabilized the supramolecular structure. The core part of the supramolecular structure was constructed by a special bond of Zn...O3(1)-H...O=C13(1) and pi-pi stacking, and the 17-propionates were at the peripheral part.

Self-aggregates of natural and modified chlorophylls as photosynthetic light-harvesting antenna systems: substituent effect on the B-ring

Extramembranous light-harvesting antennae called 'chlorosomes' are the main sunlight-absorbing and energy-migrating systems in photosynthetic green bacteria. In a chlorosome, specific chlorophyllous pigments self-aggregate in hydrophobic environments surrounded by a lipid monolayer to form large oligomers. The self-aggregates of chlorosomal chlorophylls possessing a chlorin pi-system absorb sunlight and can emit near-infrared light, which is transferred to a bacteriochlorin pigment situated in the chlorosomal surface membrane. In vivo and in vitro self-aggregates of natural chlorosomal chlorophylls and their models have been investigated by electronic absorption analysis. Here their self-aggregation is reviewed from the viewpoint of substituent effect on the pyrrolic B-ring. Substituents at the 7- and 8-positions did not disturb the formation of their self-aggregates but affected their absorption bands.