Supramolecular structure in extramembraneous antennae of green photosynthetic bacteria

Energy, Charge, and Spin Transport in Molecules and Self-Assembled Nanostructures Inspired by Photosynthesis

Electron transfer in biological molecules provides both insight and inspiration for developing chemical systems having similar functionality. Photosynthesis is an example of an integrated system in which light harvesting, photoinduced charge separation, and catalysis combine to carry out two thermodynamically demanding processes, the oxidation of water and the reduction of carbon dioxide. The development of artificial photosynthetic systems for solar energy conversion requires a fundamental understanding of electron-transfer reactions between organic molecules. Since these reactions most often involve single-electron transfers, the spin dynamics of photogenerated radical ion pairs provide important information on how the rates and efficiencies of these reactions depend on molecular structure. Given this knowledge, the design and synthesis of large integrated structures to carry out artificial photosynthesis is moving forward. An important approach to achieving this goal is the development of small, functional building blocks, having a minimum number of covalent bonds, which also have the appropriate molecular recognition sites to facilitate self-assembly into a complete, functional artificial photosynthetic system.

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.

Light-harvesting energy transfer and subsequent electron transfer of cationic porphyrin complexes on clay surfaces

A novel energy-transfer system involving nonaggregated cationic porphyrins adsorbed on an anionic-type clay surface and the electron-transfer reaction that occurs after light harvesting are described. In the clay-porphyrin complexes, photochemical energy transfer from excited singlet zinc porphyrins to free-base porphyrins proceeds. The photochemical electron-transfer reaction from an electron donor in solution (hydroquinone) to the adsorbed porphyrin in the excited singlet state was also examined. Because the electron-transfer rate from the hydroquinone to the excited singlet free-base porphyrin is larger than that to the excited singlet zinc porphyrin, we conclude that the energy transfer accelerates the overall electron-transfer reaction.

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.

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.

Time-dependent self-assembly of 31-epimerically pure and mixed zinc methyl bacteriopheophorbides-d in an aqueous THF solution

The self-aggregation process of 3(1)-epimerically pure and mixed zinc methyl bacteriopheophorbides-d (ZMBPhes-d) was examined by stopped-flow technique. A 33(v/v)% tetrahydrofuran (THF) - water solution of ZMBPhe-d was rapidly mixed with a 7(v/v)% THF - water solution to form a chlorosome-type aggregate with a red-shifted Qy band around 700 nm. We observed a rapid autocatalytic aggregation in a subsecond time scale. Aggregates of the 3(1)R epimer increased with a change in the Qy absorption maximum from 698 to 705 nm, suggesting that small aggregates formed as intermediate species. In addition, the rate of aggregation was dependent on the stereochemistry at the 3(1)-position of ZMBPhe-d; the 3(1)R epimer self-aggregated more rapidly than the 3(1)S epimer.

Different sensitivities to oxygen between two strains of the photosynthetic green sulfur bacterium Chlorobium vibrioforme NCIB 8327 with bacteriochlorophyll c and d

Two sub-strains of the anoxygenic photosynthetic green sulfur bacterium Chlorobium vibrioforme NCIB 8327 were derived from the same clone and could be discriminated only by their possession of either bacteriochlorophyll (BChl) c or d as the major pigment in the peripheral light-harvesting antenna system, chlorosome (Saga Y et al. (2003) Anal Sci 19: 1575-1579). In the presence of a proper amount of oxygen in the initial culture medium, the BChl d strain showed longer retardation on its growth initiation than the BChl c strain, indicating that the latter was advantageous for survival under aerobic light conditions which produced reactive oxygen species in vivo. The result would be ascribable to the difference of the midpoint potentials between two kinds of chlorosomes formed by self-aggregates of BChl c and d as measured by their fluorescence quenching.

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.

ZnII-Salphen Complexes as Versatile Building Blocks for the Construction of Supramolecular Box Assemblies

Zn(II)-salphen complexes are readily accessible and interesting supramolecular building blocks with a large structural diversity. Higher-order supramolecular assemblies, such as molecular boxes based on a bis-Zn(II)-salphen building block and various ditopic bipyridine ligands, have been constructed by means of supramolecular, coordinative Zn(II)-N(pyr) interactions. The use of bipyridine ligands of differing sizes enables the construction of structures with predefined box diameters. The features of the 2:2 box assemblies were investigated in detail by (variable temperature) NMR spectroscopy, UV-visible spectroscopy, NMR titrations, and X-ray crystallographic studies. The spectroscopic studies reveal a high association constant for the Zn(II)-salphen-pyridyl motif, which lies in the range 10(5)-10(6) M(-1). The strong interaction between the Zn(II) center and pyridine donors was supported by PM3 calculations that showed a relatively high Lewis acid character of the metal center in the salphen complex. Titration curves monitored by UV-visible show a cooperative effect between the two bipyridine ligands upon complexation to the bis-Zn(II) template, suggesting the formation of 2:2 complexes. The crystal structures of two supramolecular boxes have been determined. In both examples such a 2:2 assembly is present in the solid state, and the box size is different because they consist of different building blocks. Interestingly, the box assemblies line up in the solid state to form porous channels that are potentially useful in a number of applications.

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.

In vitro activity of C-20 methyltransferase, BchU, involved in bacteriochlorophyllcbiosynthetic pathway in green sulfur bacteria

The activity of a methyltransferase, BchU, which catalyzes methylation at the C-20 position of chlorin ring in the biosynthetic pathway of bacteriochlorophyll c, was investigated in vitro. The bchU gene derived from the photosynthetic green sulfur bacterium, Chlorobium tepidum, was overexpressed in Escherichia coli as a His-tagged protein (His(6)-BchU), and the enzyme was purified. In the presence of S-adenosylmethionine, His(6)-BchU methylated zinc bacteriopheophorbide d at the C-20 position to give zinc bacteriopheophorbide c. Metal-free bacteriopheophorbide d could not be methylated by the BchU, indicating that the central metal in the chlorin should be required for the recognition by the BchU.

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.

Self-Aggregation of Synthetic Zinc Oxo-Bacteriochlorins Bearing Substituents Characteristic of Chlorosomal Chlorophylls

We prepared novel zinc 8-ethyl-8-methyl-7-oxo- and 7-ethyl-7-methyl-8-oxo-bacteriochlorins 1 and 2 possessing substituents characteristic of chlorosomal chlorophylls, exclusively observed in extramembraneous light-harvesting antennas of photosynthetic green bacteria. The electronic absorption spectra of monomeric 1 and 2 in THF were obviously different: the Q(y) maximum of the former was 724 and that of the latter was 683 nm. This observed spectral difference was clearly explained by theoretical ZINDO/S calculation of their energetically minimized molecules. The optical properties of monomeric 1/2 were controlled by the electronic effect of the 7/8-oxo groups. Specific spectral changes in the electronic, CD, and FT-IR absorption spectra by dilution of the monomeric THF solutions of 1/2 with a 100/200-fold volume of cyclohexane showed the formation of chlorosomal self-aggregation species constructed by 13-C=O...H-O(3(1))...Zn and pi-pi stacking. Especially, the red-shift values in the Q(y) band of 1/2 by self-aggregation were 2450/1970 cm(-1), indicating that exciton interaction among the composite molecules in the self-aggregation of 1 was stronger than in those of 2. Molecular model calculations for dodecamers of 1/2 based on a parallel chain arrangement gave partially different supramolecular structures; the specific hydrogen-bonding distances in 2-dodecamer were larger than those of 1-dodecamer, while both coordinations gave the same Zn-O distance. These modeling results showed that 1 was more tightly packed in the self-aggregates to give a larger red-shift value in the Q(y) band by self-aggregation than 2. The difference in the supramolecular structures is mainly ascribable to the steric effect of 8/7-dialkyl groups in self-aggregates of 1/2.

Supramolecular Polymerization and Gel Formation of Bis(Merocyanine) Dyes Driven by Dipolar Aggregation

Two highly dipolar merocyanine dyes were tethered by a rigid tris(n-dodecyloxy)xylylene unit that preorganizes the dyes for a supramolecular polymerization process through intermolecular aggregation of the dyes. UV/vis spectroscopy revealed a solvent dependent equilibrium between monomeric dyes and two different types of dye aggregates that are characterized by hypsochromically shifted D- and H-type absorption bands. Taking into account the ditopic nature of the supramolecular building blocks, the occurrence of the D-band indicates the formation of an oligomeric/polymeric supramolecular chain whereas the observation of the H-band suggests a higher order assembly. For the H-aggregated dyes, intrinsic viscosities exceed 0.65 L g(-1) in methylcyclohexane, values typically found for macromolecular solutions. At higher concentration, further association of these aggregates takes place by entanglement of the alkyl groups leading to a substantial increase in viscosity and gelation. Rheology studies show linear viscoelastic behavior which was attributed to the formation of an entangled dynamic network. AFM and cryo-TEM studies of the gel reveal long and stiff rod-type assemblies. X-ray diffraction studies for a solid film show columnar mesomorphism. Based on these results, a structural model is proposed in which six helically preorganized strands of the supramolecular polymer intertwine to form a rod with a diameter of about 5 nm. Within these rods all dyes are tightly aggregated in a tubular fashion giving rise to delocalized excitonic states, and the pi-conjugated tube is jacketed by the tridodecyloxy groups.

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.

Determination of 31-stereochemistry in synthetic bacteriochlorophyll-d homologues and self-aggregation of their zinc complexes

Zinc complex of methyl 3(1)-octadecyl-bacteriopheophorbide-d was prepared from modification of naturally occurring chlorophyll-a. The 3(1)-epimerically pure samples were obtained by HPLC separation and their stereochemistry including the absolute configuration at the secondary alcoholic 3(1)-position was determined by combination of esterification to methoxy(trifluoromethyl)phenylacetate and NMR spectroscopy (Mosher's method). Both the epimers were monomeric in a polar organic solvent and self-aggregated in a non-polar solvent to give oligomers as well as dimers possessing red-shifted visible absorption bands. Visible spectra of the non-polar organic solutions were dependent upon the 3(1)-chirality and such a diastereoselective control on the self-aggregation led to the formation of self-aggregates with different supramolecular structures.

Excited-state energy-transfer dynamics of self-assembled imine-linked porphyrin dyads

Toward the development of new strategies for the synthesis of multiporphyrin arrays, we have prepared and characterized (electrochemistry and static/time-resolved optical spectroscopy) a series of dyads composed of a zinc porphyrin and a free base porphyrin joined via imine-based linkers. One dyad contains two zinc porphyrins. Imine formation occurs under gentle conditions without alteration of the porphyrin metalation state. Five imine linkers were investigated by combination of formyl, benzaldehyde, and salicylaldehyde groups with aniline and benzoic hydrazide groups. The imine-linked dyads are quite stable to routine handling. The excited-state energy-transfer rate from zinc to free base porphyrin ranges from (70 ps)(-)(1) to (13 ps)(-)(1) in toluene at room temperature depending on the linker employed. The energy-transfer yield is generally very high (>97%), with low yields of deleterious hole/electron transfer. Collectively, this work provides the foundation for the design of multiporphyrin arrays that self-assemble via stable imine linkages, have predictable electronic properties, and have comparable or even enhanced energy-transfer characteristics relative to those of other types of covalently linked systems.

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.

A fluorescence study of tetraphenylporphyrinatozinc(II)/imidazolyl-linked porphyrinatoiron(III) systems

The fluorescence spectra of porphyrinatozinc(II)/iron(III) systems which consisted of tetraphenylporphyrinatozinc(II) and three kinds of imidazolyl-linked porphyrinatoiron(III) have been studied. An efficient fluorescence quenching of tetraphenylporphyrinatozinc(II) in the system was observed. Addition of a stronger organic base, such as piperdine, to the system can displace imidazolyl-linked porphyrinatoiron(III) and the fluorescence of the system restored partly. All these indicate the formation of porphyrinatozinc(II)/iron(III) supramolecular complex and coordination bonding formed by the coordination of imidazolyl group in imidazolyl-linked porphyrinatoiron(III) to Zn(II) in tetraphenyl-porphyrinatozinc(II) is the driving force of the supramolecular self-assembly. The association constants of the supramolecular complexes were calculated from the fluorescence spectroscopic titration data, and the differences among the association constants of the supramolecular complexes are discussed on the basis of the conformations which are dependent on the length of alkoxy chain linking imidazolyl group to porphyrinatoiron(III).

Synthesis of 18O-Labelled chlorophyll derivatives at carbonyl oxygen atoms by acidic hydrolysis of the ethylene ketal and acetal

The ethylene ketal of pyropheophorbides, chlorophylls possessing the 13-keto carbonyl group and lacking the 13(2)-methoxycarbonyl group, reacted with H(2)(18)O (ca. 95% 18O atom) by acidic hydrolysis to give efficiently and regioselectively 13(1)-18O-oxo-labelled compounds (ca. 92% 18O). The resulting 18O-labelled chlorin was modified by several chemical reactions to afford some derivatives with little loss of the 18O atom. Following the same procedures, 3(1),13(1)-doubly-18O-labelled pyrochlorophyll derivatives were also prepared. All the synthetic 18O-labelled compounds were identified by FAB-mass and vibrational spectra. Especially, in the vibrational spectroscopic results including IR and resonance Raman spectra, an about 30 cm(-1) wavenumber down-shift of the 3- and/or 13-C[double bond]O stretching vibrational bands was observed by exchanging 3(1)- or 13(1)-oxo-oxygen atom from 16O to 18O.

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

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

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.

Regio- and stereoisomeric control of the aggregation of zinc-chlorins possessing inverted interactive hydroxyl and carbonyl groups

As models for a self-aggregative, naturally occurring magnesium-chlorin bacteriochlorophyll-d possessing 3(1)-secondary alcoholic hydroxyl and 13(1)-oxo groups, zinc-chlorins were synthesized with 3(1)-oxo and 13(1)-secondary (1) or tertiary hydroxyl groups (2). Compared to the monomers in a tetrahydrofuran solution, diastereomers 13(1)R-1R and 13(1)S-1S gave red-shifted absorption maxima (643 --> 674 nm in 1R and 708 nm in 1S) in 1 v/v% CH(2)Cl(2)-hexane solution, indicating their self-aggregation. Therefore, the positioning of the two groups at 3(1)/13(1) or 13(1)/3(1) on the N21-N23 molecular (Q(y)) axis is not necessarily important for the self-aggregation. The (1)H NMR and CD spectroscopic studies showed that the 674 nm absorbing species of 1R was characterized as a face-to-face "closed" dimer, while the 708 nm absorbing species of 1S was a large oligomer constructed with aggregation of head-to-tail "open" dimers. This diastereomeric control over the aggregation of 1R and 1S is more pronounced than that observed in the regioisomerically 3(1)-secondary alcoholic R/S-diastereomers 3R and 3S. The difference is ascribable to the conformational fixation of the 13(1)-hydroxyl group of the exo five-membered ring in 1. In contrast to self-aggregative 3(1)-tertiary alcoholic 4, both 13(1)-epimers of 13(1)-tertiary alcoholic 2 were monomeric even in nonpolar organic media: the additional 13(1)-methyl group (1 --> 2) drastically suppressed the self-aggregation due to the interference of the methyl group in intermolecular pi-pi interaction.

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.

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

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