3-vinyl Group Research Articles

Synthesis of C3-substituted chlorophyll-a derivatives fused by a π-conjugated E-ring and their physical properties with unique near-infrared absorption

Chlorophylls (Chls) are one of the most important pigments for the light-harvesting systems in photosynthesis. Methyl pheophorbide-a derivative fused by a π-conjugated E-ring with the C131=C132 double bond was found as a synthetic example of the unique Chl-a derivatives, which showed split Soret absorption bands in the purple and ultraviolet-A regions and a broad absorption band at a near-infrared (NIR) region as its monomeric state, but not its self-aggregated species. The 3-vinyl group of the Chl-a derivative was chemically transformed into ethyl, formyl, hydroxymethyl, and dialkoxymethyl groups. All the resulting chlorins exhibited unique wide bands at a NIR region. The substitution effect on the peaks of their wide NIR bands was similar to that on the sharp redmost Qy maxima at a visible region of the corresponding pheophorbides with the C131–C132 single bond. Protons at the C5-, C10-, and C20-positions as well as of the peripheral C2-, C3-, C7-, and C8-substituents of methyl 3-substituted 131,132-didehydro-131-deoxo-pheophorbides-a were upfield-shifted in 1H nuclear magnetic resonance, compared with those of intact pheophorbides. The shifts were ascribed to the less ring current of the cyclic tetrapyrroles by π-conjugation of the E-ring. The non-fluorescent C3-substituted Chl-a derivatives could be useful as new materials absorbing NIR wavelength light.

Synthesis of 31-aminopyridinio–chlorophyll-a derivatives and their optical properties

Oxidative reactions of isomeric amino-, methylamino-, and dimethylamino-pyridines with the 3-vinyl group of methyl pyropheophorbide-[Formula: see text], a chlorophyll-[Formula: see text] derivative, afforded 3-{1-[[Formula: see text]/[Formula: see text]/[Formula: see text]-(non/mono/di-methyl-amino)pyridinio]ethenyl}chlorins bearing the C31–N+ bond. Although all the [Formula: see text]/[Formula: see text]-substituents gave almost the same visible absorption spectra in acetonitrile, the [Formula: see text]-methylation effect on the Qy maxima was observed in the [Formula: see text]-substituted analogs with blue-shifted absorption bands. The substitution with most of the amino moieties suppressed the intramolecular electron transfer from the photoexcited chlorin core to the relatively electron-rich pyridinio periphery to enhance the fluorescence emission in acetonitrile, compared with the unsubstituted pyridinio analog. By contrast, the [Formula: see text]-NH2 substituted pyridine–chlorin conjugate was less fluorescent than any other aminated analogs as well as the unsubstituted counterpart. The electron-donating ability of the [Formula: see text]-NH2 group was recovered by the addition of alcohols to give highly fluorescent species.

Cationic 31-pyridylation of a chlorophyll-a derivative using Barluenga reagents

The 3-vinyl group in methyl pyropheophorbide-a was transformed into the cationic 1-pyridinioethenyl group by the action of a commercially available Barluenga reagent, bis(pyridine)iodonium(I) tetrafluoroborate, via the dehydroiodation of the 1-pyridinio-2-iodoethenyl adduct. Similar addition–elimination reactions with synthetic Barluenga reagent analogs possessing p-methyl, methoxy, and amino groups on the pyridine ligand afforded the 31-(p-substituted pyridinio) counterparts. Using another Barluenga reagent analog comprised of sterically demanded 2,4,6-collidine, the above chlorophyll-a derivative was reacted with p-(un)substituted pyridines to give the corresponding 31-pyridinio substitutes.

Spectral Properties of Chlorophyll f in the B800 Cavity of Light-harvesting Complex 2 from the Purple Photosynthetic Bacterium Rhodoblastus acidophilus.

The interactions of chlorophyll (Chl) and bacteriochlorophyll (BChl) pigments with the polypeptides in photosynthetic light-harvesting proteins are responsible for controlling the absorption energy of (B)Chls in protein matrixes. The binding pocket of B800 BChl a in LH2 proteins, which are peripheral light-harvesting proteins in purple photosynthetic bacteria, is useful for studying such structure-property relationships. We report the reconstitution of Chl f, which has the formyl group at the 2-position, in the B800 cavity of LH2 from the purple bacterium Rhodoblastus acidophilus. The Qy absorption band of Chl f in the B800 cavity was shifted by 14 nm to longer wavelength compared to that of the corresponding five-coordinated monomer in acetone. This redshift was larger than that of Chl a and Chl b. Resonance Raman spectroscopy indicated hydrogen bonding between the 2-formyl group of Chl f and the LH2 polypeptide. These results suggest that this hydrogen bonding contributes to the Qy redshift of Chl f. Furthermore, the Qy redshift of Chl f in the B800 cavity was smaller than that of Chl d. This may have arisen from the different patterns of hydrogen bonding between Chl f and Chl d and/or from the steric hindrance of the 3-vinyl group in Chl f.

Synthesis of deuterated chlorophyll derivatives and their physical properties

The 3-vinyl group of methyl pyropheophorbide-a, one of the chlorophyll-a derivatives, was chemically transformed into partially or fully deuterated vinyl, 1-hydroxyethyl, acetyl, formyl, and hydroxymethyl groups via oxidation, oxidative cleavage, reduction, dehydration, or Grignard reaction. Using commercially available sodium borodeuteride and trideuteromethyl iodide as deuterium sources, a variety of regioselectively deuterated pyropheophorbides were obtained with either no or a slight loss of the deuteration degree compared with those of the starting deuterated materials. The deuteration affected largely the related infrared vibrational bands, slightly the chemical shifts of specific proton signals, and no change of visible absorption bands in a solution.

Synthesis of 20-substituted chlorophyll derivatives with F-ring and optical properties of their less distorted chlorin π-systems

Under acidic conditions, methyl bacteriopheophorbides-d bearing a variety of substituents at the 20-position were dehydrated in a 1-hydroxyethyl group at the 3-position to give methyl 20-substituted pyropheophorbides-a. In the same pot, the resulting 3-vinyl group was successively cyclized at the 5-position to afford the corresponding 3,5-ethano-chlorins as analogs of sedimentary porphyrins with two exo-five-membered rings. The 20-substitution is essential to the cyclization to produce the additionally fused F-ring. After the ring closure, the 20-bromine atom and acetyl group were removed by the action of the same acid to give the 20-unsubstituted product. The steric size of the 20-substituents affected the cyclization as well as the electronic absorption and emission of the 3,5-ethano-chlorins in a solution. The F-ring fusion suppressed the conformational distortion of the chlorin π-plane, partially regulating the optical properties: decrease of red shifts in Qy maxima by the 20-substitution and reduction of almost all Stokes shifts.

Distribution and functional analysis of the two types of 8-vinyl reductase involved in chlorophyll biosynthesis in marine cyanobacteria.

In the chlorophyll biosynthesis pathway, the 8-vinyl group of the chlorophyll precursor is reduced to an ethyl group by 8-vinyl reductase. Two isozymes of 8-vinyl reductase have been described in oxygenic photosynthetic organisms: one encoded by BciA and another by BciB. Only BciB contains an [Fe-S] cluster and most cyanobacteria harbor this form; whereas a few contain BciA. Given this disparity in distribution, cyanobacterial BciA has remained largely overlooked, which has limited understanding of chlorophyll biosynthesis in these microorganisms. Here, we reveal that cyanobacterial BciA encodes a functional 8-vinyl reductase, as evidenced by measuring the in vitro activity of recombinant Synechococcus and Acaryochloris BciA. Genomic comparison revealed that BciB had been replaced by BciA during evolution of the marine cyanobacterium Synechococcus, and coincided with replacement of Fe-superoxide dismutase (SOD) with Ni-SOD. These findings imply that the acquisition of BciA confers an adaptive advantage to cyanobacteria living in low-iron oceanic environments.

Synthesis of Cationic Pyridinium–Chlorin Conjugates with Various Counter Anions and Effects of the Anions on Their Photophysical Properties

Abstract Cationic pyridinium pending chlorophyll-a derivatives with several counter anions were synthesized by Ag(I)-induced oxidation reactions of methyl pyropheophorbide-a possessing the 3-vinyl group with pyridine. The high water-solubility of the synthetic 31-pyridinio-3-vinyl-chlorin cation with a chloride anion resulted in efficient anion exchange reactions conducted in a biphasic system of H2O and CH2Cl2. Using either or both of the two synthetic procedures, ten salts were successfully synthesized with SbF6−, PF6−, BF4−, ClO4−, (CF3SO2)2N−, CF3SO3−, NO3−, I−, Br−, and Cl−. The effects of the counter anions on the physical properties of the cationic chlorophyll-a derivatives were discussed using their 1H NMR, visible absorption, and fluorescence emission data as well as theoretical model calculations. Although the visible absorption and fluorescence emission spectra were changed faintly, the fluorescence quantum yields and lifetimes were dependent on the counter anions, showing that the anions affected the pyridinium group acting as an electron-acceptor and the fluorescence quenching by intramolecular charge transfer.

Cinchona Alkaloids-Derivatives and Applications.

Major Cinchona alkaloids quinine, quinidine, cinchonine, and cinchonidine are available chiral natural compounds (chiral pool). Unlike many other natural products, these alkaloids are available in multiple diastereomeric forms which are separated on an industrial scale. The introduction discusses in short conformational equilibria, traditional separation scheme, biosynthesis, and de novo chemical syntheses. The second section concerns useful chemical applications of the alkaloids as chiral recognition agents and effective chiral catalysts. Besides the Sharpless ethers and quaternary ammonium salts (chiral PTC), the most successful bifunctional organocatalysts are based on 9-amino derivatives: thioureas and squaramides. The third section reports the main transformations of Cinchona alkaloids. This covers reactions of the 9-hydroxyl group with the retention or inversion of configuration. Specific Cinchona rearrangements enlarging [2.2.2]bicycle of quinuclidine to [3.2.2] products are connected to the 9-OH substitution. The syntheses of numerous esterification and etherification products are described, including many examples of bi-Cinchona alkaloid ethers. Further derivatives comprise 9-N-substituted compounds. The amino group is introduced via an azido function with the inversion of configuration at the stereogenic center C9. The 9-epi-amino-alkaloids provide imines, amides, imides, thioureas, and squaramides. The syntheses of 9-carbon-, 9-sulfur-, and 9-selenium-substituted derivatives are discussed. Oxidation of the hydroxyl group of any alkaloid gives ketones, which can be selectively reduced, reacted with Grignard reagents, or subjected to the Corey-Chaykovsky reaction. The alkaloids were also partially degraded by splitting C4'-C9 or N1-C8 bonds. In order to immobilize Cinchona alkaloids the transformations of the 3-vinyl group were often exploited. Finally, miscellaneous functionalizations of quinuclidine, quinoline, and examples of various metal complexes of the alkaloids are considered.

C31-Selective substitution of cationic N-heteroaromatic groups into a 3-vinylated chlorophyll-a derivative.

A variety of N-containing heteroarenes were site-selectively introduced at the C31-position of a chlorophyll-a derivative possessing a 3-vinyl group through a C-N+ bond via oxidative reactions using iodine in the presence of a silver(i) salt. Electron-rich N-heteroarenes were effectively substituted, while electron-withdrawing or bulky nitrogen-neighboring substituents suppressed the reactivities. The cationic products formed were characterized by 1D/2D NMR, and their optical properties were also investigated.

Light-dependent accumulation of new bacteriochlorophyll-e bearing a vinyl group at the 8-position in the green sulfur bacterium Chlorobaculum limnaeum

The green sulfur bacterium Chlorobaculum limnaeum producing bacteriochlorophyll(BChl)-e was anaerobically cultured by illumination using red LED light at around 660nm. When the bacterium was cultured at the intensity of approximately 10,000lux (high-light conditions), a pair of new chlorophyll pigments showing the identical electronic absorption and mass spectra was produced at remarkable amounts. The pigments exhibited the Soret and Qy bands at absorption maxima of 480 and 656nm, respectively, in tetrahydrofuran. These maxima were red-shifted compared with those of the original BChl-e at 467 and 651nm, indicating that the vinyl group was substituted at the 8-position. The pigments were identified as epimers at the 31-position, and the molecular structures, including the stereo-configurations, were determined to be (31R/S)-8-vinyl-12-ethyl-BChl-e esterified with a C15 farnesyl moiety as the 17-propionate residue using mass spectrometry and 1H NMR spectroscopy as well as chemical modification. The accumulation of 8-vinylated BChl-e supports the idea that irradiation with intense red light suppresses the activity of divinyl reductase (BciA), which is responsible for hydrogenation of the 8-vinyl group of precursory divinyl-chlorophyllide-a during the biosynthesis of BChl-e.

In vitro enzymatic assays of photosynthetic bacterial 3-vinyl hydratases for bacteriochlorophyll biosyntheses.

A chlorosome is a large and efficient light-harvesting antenna system found in some photosynthetic bacteria. This system comprises self-aggregates of bacteriochlorophyll (BChl) c, d, or e possessing a chiral 1-hydroxyethyl group at the 3-position, which plays a key role in the formation of the supramolecule. Biosynthesis of chlorosomal pigments involves stereoselective conversion of 3-vinyl group to 3-(1-hydroxyethyl) group facilitated by a 3-vinyl hydratase. This 3-vinyl hydration also occurs in BChl a biosynthesis, followed by oxidation that introduces an acetyl group at the 3-position. Herein, we present in vitro enzymatic assays of paralogous 3-vinyl hydratases derived from green sulfur bacteria, Chlorobaculum tepidum and Chlorobaculum limnaeum, the filamentous anoxygenic phototroph Chloroflexus aurantiacus, and the chloracidobacterium Chloracidobacterium thermophilum. All the hydratases showed hydration activities. The biosynthetic pathway of BChl a and other chlorosomal pigments is discussed considering the substrate specificity and stereoselectivity of the present hydratases.

Synthesis of chlorophyll-a derivatives methylated in the 3-vinyl group and their intrinsic site energy

Wittig reaction of methyl pyropheophorbide-d possessing the 3-formyl group gave readily methyl pyropheophorbides-a bearing a variety of 3-alkenyl groups as semi-synthetic models of chlorophyll-a. The 3-substituents rotated around the C3–C31 bond from the coplanar conformation with the chlorin π-system, moving the redmost visible absorption maxima to a shorter wavelength. The model experiments showed that natural chlorophyll-a carrying the 3-vinyl group would take a similar rotamer to control its intrinsic site energy.

In vitro stereospecific hydration activities of the 3-vinyl group of chlorophyll derivatives by BchF and BchV enzymes involved in bacteriochlorophyll c biosynthesis of green sulfur bacteria.

The photosynthetic green sulfur bacterium Chlorobaculum (Cba.) tepidum produces bacteriochlorophyll (BChl) c pigments bearing a chiral 1-hydroxyethyl group at the 3-position, which self-aggregate to construct main light-harvesting antenna complexes, chlorosomes. The secondary alcoholic hydroxy group is requisite for chlorosomal aggregation and biosynthesized by hydrating the 3-vinyl group of their precursors. Using recombinant proteins of Cba. tepidum BchF and BchV, we examined in vitro enzymatic hydration of some 3-vinyl-chlorophyll derivatives. Both the enzymes catalyzed stereoselective hydration of zinc 3-vinyl-8-ethyl-12-methyl-bacteriopheophorbide c or d to the zinc 31 R-bacteriopheophorbide c or d homolog, respectively, with a slight amount of the 31 S-epimric species. A similar R-stereoselectivity was observed in the BchF-hydration of zinc 3-vinyl-8-ethyl- and propyl-12-ethyl-bacteriopheophorbides c, while their BchV-hydration gave a relatively larger amount of the 31 S-epimers. The in vitro stereoselective hydration confirmed the in vivo production of the S-epimeric species by BchV. The enzymatic hydration for the above 8-propylated substrate proceeded more slowly than that for the 8-ethylated, and the 8-isobutylated substrate was no longer hydrated. Based on these results, biosynthetic pathways of BChl c homologs and epimers are proposed.

Synthesis of chlorophyll-c derivatives by modifying natural chlorophyll-a

Chlorophyll-a (Chl-a) was extracted from cyanobacterial cells and modified to methyl pyropheophorbide-a. The 3-vinyl-chlorin was transformed to zinc complex of the corresponding 3-acetyl-porphyrin. The zinc porphyrin was oxidized to give cis-7,8- and 17,18-dihydroxy-chlorins as well cis-7,8-cis-17,18-tetrahydroxybacteriochlorin. After zinc-demetallation, the isolated cis-7,8- and 17,18-diols were reduced at the 3-acetyl group and triply dehydrated under acidic conditions to afford two regioisomeric 3-vinyl-porphyrins, methyl divinyl-pyroprotopheophorbide-a possessing the 8-vinyl group and 17-propionate residue (one of the divinyl-protoChl-a derivatives) and methyl pyropheophorbide-c 1 possessing the 8-ethyl group and 17-acrylate residue (one of the Chl-c 1 derivatives), respectively. The resulting 7,8,17,18-tetrol was reduced and then acidically treated, giving five-fold dehydrated free base porphyrin, methyl pyropheophorbide-c 2 possessing the 3,8-divinyl groups and 17-acrylate residue (one of the Chl-c 2 derivatives). The visible absorption and fluorescence emission spectra of the three semi-synthetic 3-vinyl-porphyrins in dichloromethane were compared with those of the corresponding 8-ethyl-porphyrin bearing the 17-propionate residue, methyl pyroprotopheophorbide-a (one of the protoChl-a derivatives). The Soret and Qy absorption maxima were shifted to longer wavelengths with an increase of π-conjugation in a molecule: protoChl-a (8-CH2CH3/17-CH2CH2COOCH3)<divinyl-protoChl-a (8-CH=CH2/17-CH2CH2COOCH3)<Chl-c 1 (8-CH2CH3/17-CH=CHCOOCH3)<Chl-c 2 derivatives (8-CH=CH2/17-CH=CHCOOCH3). The 17(1),17(2)-dehydrogenation broadened the absorption bands. The emission maxima were bathochromically shifted in the same order. The reaction mechanism of the present dehydration indicates that the biosynthetic pathway of Chls-c would include the hydroxylation of the 17-propionate reside at the 17(1)-position and successive dehydration to the 17-acrylate residue.

Dimeric Cinchona alkaloids.

Nature is full of dimeric alkaloids of various types from many plant families, some of them with interesting biological properties. However, dimeric Cinchona alkaloids were not isolated from any species but were products of designed partial chemical synthesis. Although the Cinchona bark is amongst the sources of oldest efficient medicines, the synthetic dimers found most use in the field of asymmetric synthesis. Prominent examples include the Sharpless dihydroxylation and aminohydroxylation ligands, and dimeric phase transfer catalysts. In this article the syntheses of Cinchona alkaloid dimers and oligomers are reviewed, and their structure and applications are outlined. Various synthetic routes exploit reactivity of the alkaloids at the central 9-hydroxyl group, quinuclidine, and quinoline rings, as well as 3-vinyl group. This availability of reactive sites, in combination with a plethora of linker molecules, contributes to the diversity of the products obtained.

Evolution of a New Chlorophyll Metabolic Pathway Driven by the Dynamic Changes in Enzyme Promiscuous Activity

Organisms generate an enormous number of metabolites; however, the mechanisms by which a new metabolic pathway is acquired are unknown. To elucidate the importance of promiscuous enzyme activity for pathway evolution, the catalytic and substrate specificities of Chl biosynthetic enzymes were examined. In green plants, Chl a and Chl b are interconverted by the Chl cycle: Chl a is hydroxylated to 7-hydroxymethyl chlorophyll a followed by the conversion to Chl b, and both reactions are catalyzed by chlorophyllide a oxygenase. Chl b is reduced to 7-hydroxymethyl chlorophyll a by Chl b reductase and then converted to Chl a by 7-hydroxymethyl chlorophyll a reductase (HCAR). A phylogenetic analysis indicated that HCAR evolved from cyanobacterial 3,8-divinyl chlorophyllide reductase (DVR), which is responsible for the reduction of an 8-vinyl group in the Chl biosynthetic pathway. In addition to vinyl reductase activity, cyanobacterial DVR also has Chl b reductase and HCAR activities; consequently, three of the four reactions of the Chl cycle already existed in cyanobacteria, the progenitor of the chloroplast. During the evolution of cyanobacterial DVR to HCAR, the HCAR activity, a promiscuous reaction of cyanobacterial DVR, became the primary reaction. Moreover, the primary reaction (vinyl reductase activity) and some disadvantageous reactions were lost, but the neutral promiscuous reaction (NADH dehydrogenase) was retained in both DVR and HCAR. We also show that a portion of the Chl c biosynthetic pathway already existed in cyanobacteria. We discuss the importance of dynamic changes in promiscuous activity and of the latent pathways for metabolic evolution.

Ene-thiol reaction of C3-vinylated chlorophyll derivatives in the presence of oxygen: synthesis of C3-formyl-chlorins under mild conditions

Reactions of thiol with the C 3-vinyl group of various chlorophyll (Chl) derivatives were examined. The reactions resemble thiol-olefin co-oxidation, except that the vinyl C = C double bond was cleaved to afford a formyl group without any transition metal catalyst, and that the simple anti-Markovnikov adduct of thiol to olefin was obtained as a minor product. Peripheral substituents of Chl derivatives little affected the reaction, while the central metal atom of the chlorin macrocycle influenced the composition of the products. Oxygen and acid dissolved in the reaction mixture can facilitate the oxidation. Sufficiently mild conditions in this regioselective oxidation at the C 31-position are significant in bioorganic chemistry.

Characterization of chlorophyll pigments in the mutant lacking 8-vinyl reductase of green photosynthetic bacterium Chlorobaculum tepidum

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.

Molecular Cloning and Characterization Analysis of 3,8-Divinyl Protochlorophyllide a 8-Vinyl Reductase Gene from &lt;i&gt;Dunaliella parva&lt;/i&gt;

The vast majority of photosynthetic organisms utilize monovinyl chlorophyll for their photosynthetic reactions. For the biosynthesis of monovinyl chlorophyll, the reduction of the 8-vinyl group which is located on the B-ring of the macrocycle is essential. 3,8-Divinyl protochlorophyllide a 8-vinyl reductase (DVR) catalyzes the reduction of 8-vinyl group on the tetrapyrrole to an ethyl group, which is necessary for monovinyl chlorophyll (Chl) synthesis. The former studies indicated the DVR could enhance photosynthesis. The full-length cDNA encoding DVR was obtained from oleaginous microalgae Dunaliella parva, which include 1326 bp open reading frame (ORF), 22 bp 5′-untranslated sequence and 383 bp 3′-untranslated sequence. Dunaliella parva DVR showed the highest sequence similarity with the DVR from Chlamydomonas reinhardtii and Volvox carteri. The Dunaliella parva DVR also showed wide similarity with other species.