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Abstract: Green photosynthetic bacteria have unique light-harvesting antenna systems called chlorosomes. Chlorobaculum tepidum, a model organism of the bacteria, biosynthesized monogalactosyl- and rhamnosylgalactosyldiacylglycerides possessing a methylene-bridged palmitoleyl group characterized by a cis-substituted cyclopropane ring as the dominant glycolipids of its chlorosome surface. The formation of the cyclopropane ring was chemically inhibited by supplementation of sinefungin, an analog of S-adenosyl-l-methionine, into the bacterial cultivation. The presence of the cyclopropane ring reinforced acid resistance of the light-harvesting chlorosomes and suppressed acidic demetalation (pheophytinization) of bacteriochlorophyll-c pigments constructing the core part of chlorosomes. The ring-formation would represent direct and post-synthetic modifications of chlorosome membrane properties and was tolerant of acidic environments. [Copyright &y& Elsevier]

Abstract: The mutant lacking the enzyme BciA (renamed CT1063), which catalyzed reduction of the 8-vinyl group of a porphyrinoid-type 3,8-divinyl-(proto)chlorophyllide-a [DV-(P)Chlide-a] in the green sulfur bacterium Chlorobaculum (Cba.) tepidum, was reconstructed on the basis of the previous study reported by Chew and Bryant [J. Biol. Chem. 2007, 282, 2967–2975]. Cba. tepidum biosynthesizes the following three different types of chlorophylls (Chls) through their common precursory DV-(P)Chlide-a as its photosynthetically active pigments: bacteriochlorophyll(BChl)-c and Chl-a with the partially reduced 17,18-trans-dihydroporphyrin and BChl-a with the further reduced 7,8-trans-17,18-trans-tetrahydroporphyrin. The structures of Chls thus produced were characterized in detail by various spectroscopic techniques. In the mutant, both BChl-c and Chl-a possessing the alkyl group at the 8-position were exclusively replaced by their 8-vinylated derivatives, whereas BChl-a possessed the original 8-ethyl group. The present observations were inconsistent with the previous report. However, it was apparently confirmed that the enzyme BciA was responsible for the reduction of DV-(P)Chlide-a to produce BChl-c and Chl-a. Noteworthily, exclusive accumulation of the reduced (8-ethylated) form of BChl-a, not its 8-vinylated derivative, in the mutant indicates the presence of another enzyme catalyzing the 8-vinyl reduction as yet unidentified or any other reduction mechanism using a known enzyme to yield BChl-a. [Copyright &y& Elsevier]

Abstract: Chlorophyll(Chl)-c pigments in algae, diatoms and some prokaryotes are characterized by the fully conjugated porphyrin π-system as well as the acrylate residue at the 17-position. The precise structural characterization of Chl-c 3 from the haptophyte Emiliania huxleyi was performed. The conformations of the π-conjugated peripheral substituents, the 3-/8-vinyl, 7-methoxycarbonyl and 17-acrylate moieties were evaluated, in a solution, using nuclear Overhauser enhancement correlations and molecular modeling calculations. The rotation of the 17-acrylate residue was considerably restricted, whereas the other three substituents readily rotated at ambient temperature. Moreover, the stereochemistry at the 132-position was determined by combination of chiral high-performance liquid chromatography (HPLC) with circular dichroism (CD) spectroscopy. Compared with the CD spectra of the structurally related, synthetic (132 R)- and (132 S)-protochlorophyllide(PChlide)-a, naturally occurring Chl-c 3 had exclusively the (132 R)-configuration. To elucidate this natural selection of a single enantiomer, we analyzed the three major Chl-c pigments (Chl-c 1, c 2 and c 3) in four phylogenetically distinct classes of Chl-c containing algae, i.e., heterokontophyta, dinophyta, cryptophyta and haptophyta using chiral HPLC. All the photosynthetic organisms contained only the (132 R)-enantiomerically pure Chls-c, and lacked the corresponding enantiomeric (132 S)-forms. Additionally, Chl-c 2 was found in all the organisms as the common Chl-c. These results throw a light on the biosynthesis as well as photosynthetic function of Chl-c pigments: Chl-c 2 is derived from 8-vinyl-PChlide-a by dehydrogenation of the 17-propionate to acrylate residues as generally proposed, and the (132 R)-enantiomers of Chls-c function as photosynthetically active, light-harvesting pigments together with the principal Chl-a and carotenoids. [Copyright &y& Elsevier]

The isocyclic ring (E-ring) is a common structural feature of chiorophylls. The E-ring is formed by two structurally unrelated Mg-protoporphyrin IX monomethylester (MPE) cyclase systems, oxygen-dependent (AcsF), and oxygen-independent (BchE) systems, which involve incorporation of an oxygen atom from molecular oxygen and water into the C-13¹ position of MPE, respectively. Which system operates in cyanobacteria that can thrive in a variety of anaerobic environments remains an open question. The cyanobacterium Synechocystis sp. PCC 6803 has two acsF-like genes, sll1214 (chlA1) and sll1874 (chlA11), and three bchE-like genes, slr0905, sll1242, and slr0309. Five mutants lacking one of these genes were isolated. The ΔchlA1 mutant failed to grow under aerobic conditions with anomalous accumulation of a pigment with fluorescence emission peak at 595 nm, which was identified 3,8-divinyl MPE by high-performance liquid chromatography-mass spectrometry analysis. The growth defect of ΔchlA1 was restored by the cultivation under oxygen-limited (micro-oxic) conditions. MPE accumulation was also detected in ΔchlA11 grown under microoxic conditions, but not in any of the bchE mutants. The phenotype was consistent with the expression pattern of two chlA genes: chlA11 was induced under micro-oxic conditions in contrast to the constitutive expression of chlA1 . These findings suggested that ChlA11 is the sole MPE cyclase system under aerobic conditions and that the induced ChlA1, operates together with ChlA1 under micro-oxic conditions. In addition, the accumulation of 3,8-divinyl MPE in the ΔchlA mutants suggested that the reduction of 8-vinyl group occurs after the formation of E-ring in Synechocystis sp. PCC 6803. [ABSTRACT FROM AUTHOR]

In most photosynthetic organisms, the chlorin ring structure of chlorophyll a is formed by the reduction of the porphyrin D-ring by the dark-operative nitrogenase-like enzyme, protochlorophyllide reductase (DPOR). Subsequently, the chlorin B-ring is reduced in bacteriochlorophyll biosynthesis to form a bacteriochlorin ring structure. Phenotypic analysis of mutants lacking one of three genes, bchX, bchY, or bchZ, which show significant sequence similarity to the structural genes of nitrogenase, suggests that a second nitrogenase-like enzyme is involved in the chlorin B-ring reduction. However, there is no biochemical evidence for this. Here, we report the reconstitution of chlorophyllide a reductase (COR) with purified proteins. Two Rhodobacter capsulatus strains that overexpressed Strep-tagged BchX and BchY were isolated. Strep-tagged BchX was purified as a single polypeptide, and BchZ was co-purified with Strep-tagged BchY. When BchX and BchY-BchZ components were incubated with chlorophyllide a, ATP, and dithionite under anaerobic conditions, chlorophyllide a was converted to a new pigment with a Qy band of longer wavelength at 734 nm (P734) in 80% acetone. The formation of P734 was dependent on ATP and dithionite. High performance liquid chromatography and mass spectroscopic analysis indicated that P734 is 3-vinyl bacteriochlorophyllide a, which is formed by the B-ring reduction of chlorophyllide a. These results demonstrate that the B-ring of chlorin is reduced by a second nitrogenase-like enzyme and that the sequential actions of two nitrogenase-like enzymes, DPOR and COR, convert porphyrin to bacteriochlorin. The evolutionary implications of nitrogenase-like enzymes to determine the ring structure of (bacterio)chlorophyll pigments are discussed. [ABSTRACT FROM AUTHOR]

Abstract: Gallium(III) chlorins possessing a series of axial ligands were synthesized as model compounds of natural chlorophylls. A pair of diastereomers arising from the fifth axial coordination onto the asymmetric chlorin π-macrocycle could be discriminated in a solution by both 1H- and 13C NMR spectroscopies. [Copyright &y& Elsevier]

Abstract: Zinc chlorins having a bridged moiety between 3- or 8- and 17-positions were designed as protected chlorophylls at one of π-faces, aiming at the investigation of asymmetric coordination ability towards the central metal. These strapped chlorins were synthesized by the cyclization of the dihydroxylated chlorins with dicarboxylic acid dichlorides under highly diluted conditions. The synthetic zinc chlorins complexed with pyridine as an axial ligand in benzene to form 5-coordinated species. The 1:1 binding constants determined by UV–vis titration method were smaller than those of the corresponding acyclic (unstrapped) compounds. [Copyright &y& Elsevier]

We prepared bacteriochlorophyll(BChl)-b and g models by Diels–Alder reactions of 8-vinyl-chlorophylls with tetracyanoethylene. The resulting 8-alkylidene-bacteriochlorins with various substituent groups at the 3-position had the same π-conjugate as BChls-b/g. While the natural pigments isomerized by addition of an acid to afford the corresponding chlorins, the synthetic models were stable under the acidic conditions due to dialkylation at the 7-position. These BChl-b/g models are useful for investigating the optical properties of relatively unstable BChls-b/g. [Copyright &y& Elsevier]

Abstract Epimerically and homologously pure farnesyl bacteriochlorophylls- f (BChls- f F) were isolated from the cultured cells of the bchU gene-deleted mutant of Chlorobaculum limnaeum , which is a green sulfur bacterium, by reverse-phase HPLC. Homologously pure 8-ethyl-, propyl-, and isobutyl-BChls- f F with the 12-ethyl group were fully characterized by their VIS, 1H NMR, and MS spectra. All the 31-epimerically pure samples of each homolog showed nearly the same visible absorption and fluorescence emission spectra in THF, and their emission quantum yields and lifetimes were independent of the 31-stereochemistry and 82-methylation. Graphical abstract Image 1 Highlights • Epimerically and homologously pure farnesyl bacteriochlorophylls- f were isolated. • Homologously pure samples were fully characterized by VIS, 1H NMR, and MS spectra. • All epimers showed almost same VIS absorption and fluorescence emission spectra. • Emission quantum yields and lifetimes were independent on their substituents. [ABSTRACT FROM AUTHOR]

All regioisomeric di- and tetrahydrogeranylgeraniols possessing the C2 C3 double bond were prepared as authentic samples. The synthetic C 20 -isoprenoid alcohols were separated well by gas chromatography. Based on the chromatographic analysis, the enzymatic reduction pathway of a geranylgeranyl group was investigated to identify the last stage of (bacterio)chlorophyll biosynthesis in phototrophs. The geranylgeranyl group was triply reduced to the phytyl group through the first regio- and stereospecific hydrogenation of C10 C11 to C10H C11( S )H, the second of C6 C7 to C6H C7( S )H, and the third of C14 C15 to C14H C15H. The identification of the reduction sequence completes the biosynthetic pathways for naturally occurring chlorophyll- a and bacteriochlorophyll- a bearing a phytyl group as the esterifying moiety in the 17-propionate residues. [ABSTRACT FROM AUTHOR]

Abstract: 3,8-Divinyl-chlorophyll(Chl)-a possessing a phytyl ester was hydrogenated in acetone by rhodium catalyst on alumina to afford 3-vinyl-8-ethyl-, 3-ethyl-8-vinyl- and 3,8-diethyl-Chls. The ratio of produced 3-ethyl-8-vinyl- over 3-vinyl-8-ethyl-Chls was determined to be 1.2, indicating that the reactivity of the 3-vinyl group was slightly higher than that of the 8-vinyl group. Catalytic hydrogenation of divinyl-protochlorophyll-a possessing a porphyrin π-skeleton (C17ead of the above chlorin moiety (C17H–C18H) gave an equal amount of mono-reduced regioisomers. The slight (or no) selectivity is different from that in the enzymatic reduction of divinyl-(proto)chlorophyllides-a lacking a phytyl ester in the biosynthetic pathway of Chl-a where the sole 8-vinyl group is transformed to the ethyl group. [ABSTRACT FROM AUTHOR]

Abstract: Photochemical reaction of zinc 3-acetyl-chlorophyll derivative possessing a chlorin π-system (trans-17,18-dihydroporphyrin) in the presence of a reductant in a deaerated solution gave trans-2,3-dihydrogenated products in 94% yield based on consumed starting material. The resulting photoreduced products are chemically stable and provide useful samples for investigation of isobacteriochlorins, 2,3,17,18-tetrahydroporphyrinoids. [Copyright &y& Elsevier]

Abstract: A pair of zinc chlorins having a bridged moiety between the 3 and 132-positions on the front or back side of its π-face were synthesized, and their asymmetric coordination ability towards pyridine was determined in benzene. [Copyright &y& Elsevier]

Abstract: The magneto-crystalline uniaxial anisotropy constant, Ku , of β-FeOOH was estimated by analyzing temperature dependence of Mössbauer absorption spectra taking into account the thermal relaxation of the internal magnetic fields. It was deduced to be 2.1×103 J/m3, using a typical volume (9×10−24 m3) of synthesized β-FeOOH particles observed by TEM. The value, Ku of β-FeOOH is in the same order but twice as large as that of α-FeOOH, 1×103 J/m3. [Copyright &y& Elsevier]

Abstract: Photodegradation and demethylation reactions of methylene blue (MB) by a TiO2 photocatalytic film have been studied spectrophotometrically by estimating molar concentration changes of MB and demethylated species in an MB aqueous solution according to the Beer–Lambert law. On the other hand, MB was less demethylated by the reaction with an Au particles-TiO2 (Au–TiO2) composite film. [Copyright &y& Elsevier]