In vitro demetalation of central magnesium in various chlorophyll derivatives using Mg-dechelatase homolog from the chloroflexi Anaerolineae.

In the metabolic pathway of chlorophylls (Chls), an enzyme called STAY-GREEN or SGR catalyzes the removal of the central magnesium ion of Chls and their derivatives to their corresponding free bases, including pheophytins. The substrate specificity of SGR has been investigated through in vitro reactions using Chl-related molecules. However, information about the biochemical properties and reaction mechanisms of SGR and its substrate specificity remains elusive. In this study, we synthesized various Chl derivatives and investigated their in vitro dechelations using an SGR enzyme. Chl-a derivatives with the C3-vinyl group on the A-ring, which is commonly found as a substituent in natural substrates, and their analogs with ethyl, hydroxymethyl, formyl, and styryl groups at the C3-position were prepared as substrates. In vitro dechelatase reactions of these substrates were performed using an SGR enzyme derived from an Anaerolineae bacterium, allowing us to investigate their specificity. Reactivity was reduced for substrates with an electron-withdrawing formyl or sterically demanding styryl group at the C3-position. Furthermore, the Chl derivative with the C8-styryl group on the B-ring was less reactive for SGR dechelation than the C3-styryl substrate. These results indicate that the SGR enzyme recognizes substituents on the B-ring of substrates more than those on the A-ring.

Supramolecular chirality in self-assembly of zinc protobacteriochlorophyll-d analogs possessing enantiomeric esterifying groups.

Zinc 3-hydroxymethyl-pyroprotopheophorbides-a esterified with a chiral secondary alcohol at the 17-propionate residue were prepared as bacteriochlorophyll-d analogs. The synthetic zinc 31-hydroxy-131-oxo-porphyrins self-aggregated in an aqueous Triton X-100 micellar solution to give red-shifted and broadened Soret and Qy absorption bands in comparison with their monomeric bands. The intense, exciton-coupled circular dichroism spectra of their self-aggregates were dependent on the chirality of the esterifying groups. The observation indicated that the self-aggregates based on the J-type stacking of the porphyrin cores were sensitive to the peripheral 17-propionate residues. The supramolecular structures of the present J-aggregates as models of bacteriochlorophyll aggregates in natural chlorosomes were remotely regulated by the esterifying groups.

Determination of N-centered stereochemistry in N22-methylated chlorophyll-a derivatives and their epimer-dependent optical spectra.

An N-centered epimeric mixture of chlorophyll-a derivatives methylated at the inner nitrogen atom was separated by reverse-phase high-performance liquid chromatography. Circular dichroism (CD) spectroscopic analyses of the epimerically pure N22-methyl-chlorins revealed that the minor first-eluted and major second-eluted stereoisomers were (22S)- and (22R)-configurations, respectively. Their visible absorption and CD spectra in solution were dependent on the N22-stereochemistry. The epimer-dependent spectral changes were independent of the substituents at the peripheral 3-position of the core chlorin chromophore.

Remodeling Zinc Deposition via Multisite Zincophilic Chlorophyll for Powerful Aprotic Zinc Batteries.

The organic electrolyte can resolve the hurdle of hydrogen evolution in aqueous electrolytes but suffers from sluggish electrochemical reaction kinetics due to a compromised mass transfer process. Herein, we introduce a chlorophyll, zinc methyl 3-devinyl-3-hydroxymethyl-pyropheophorbide-a (Chl), as a multifunctional electrolyte additive for aprotic zinc batteries to address the related dynamic problems in organic electrolyte systems. The Chl exhibits multisite zincophilicity, which significantly reduces the nucleation potential, increases the nucleation sites, and induces uniform nucleation of Zn metal with a nucleation overpotential close to zero. Furthermore, the lower LUMO of Chl contributes to a Zn-N-bond-containing SEI layer and inhibits the decomposition of the electrolyte. Therefore, the electrolyte enables repeated zinc stripping/plating up to 2000 h (2 Ah cm-2 cumulative capacity) with an overpotential of only 32 mV and a high Coulomb efficiency of 99.4%. This work is expected to enlighten the practical application of organic electrolyte systems.

Predicted Structure of the BciC Enzyme Catalyzing the Removal of the C132-Methoxycarbonyl Group for Biosynthesis of Chlorosomal Chlorophylls: A Mechanism for Dual Catalytic Functions of Hydrolysis and Decarboxylation inside Its Active Site.

Green photosynthetic bacteria, one of the phototrophs, have the largest and most efficient light-harvesting antenna systems, called chlorosomes. The core part of chlorosomes consists of unique bacteriochlorophyll c/d/e molecules. In the biosynthetic pathway of these molecules, a BciC enzyme catalyzes the removal of the C132-methoxycarbonyl group of chlorophyllide a. Two sequential reactions have been proposed for the BciC enzymatic demethoxycarbonylation: the BciC enzyme would catalyze the hydrolysis of the C132-methoxycarbonyl group, and the resulting carboxylic acid would be rapidly decarboxylated to generate pyrochlorophyllide a. In this study, we computationally predicted the three-dimensional structure of the BciC protein. Its active site was proposed based on structural analysis using docking simulation. In vitro enzymatic reaction assays of mutated BciC supported the prediction. The BciC enzymatic hydrolysis would be an aspartic/glutamic acid hydrolase, which involves the amino residues E85 and D180. Furthermore, Y58 and H126 might depend on stabilization and/or recognition with the substrate. Most importantly, H137 would protonate 13-C═O or deprotonate C132-COOH in the hydrolyzed product to promote decarboxylation. In conclusion, the BciC enzyme has the dual functions of hydrolysis and decarboxylation.

Self-aggregation behavior of dimeric chlorophyll-a derivatives linked with ethynylene and m-phenylene moieties.

Chlorophyll(Chl)-a derivatives inserting an ethynylene-m-phenylene group between a zinc chlorin ring and a hydroxymethyl group, in which various substituents were introduced on the benzene spacer, were prepared as model compounds for the light-harvesting antennae (chlorosomes) of photosynthetic green bacteria. These compounds were synthesized from a C3-ethynylated Chl-a derivative via sequential Sonogashira cross-coupling reaction, and the effects of the substituents on the phenylene linker on their self-aggregation behaviors were investigated by electronic absorption, circular dichroism, and infrared absorption spectroscopic measurements. These studies exhibited that some compounds gave the disordered self-assemblies including several species; however, the zinc complex of the dimeric Chl-a derivative primarily allowed a single J-aggregate species in an aqueous Triton X-100 micellar solution. Additional control experiments revealed that its self-assembly was constructed through the hydrogen and coordination bonding involving the hydroxymethyl group on benzene ring, keto-carbonyl group at C13-position, and central zinc atom, and this is consistent with a conventional chlorosomal manner.

Incomplete Hydrogenation by Geranylgeranyl Reductase from a Proteobacterial Phototroph Halorhodospira halochloris, Resulting in the Production of Bacteriochlorophyll with a Tetrahydrogeranylgeranyl Tail.

Light harvesting and charge separation are functions of chlorophyll and bacteriochlorophyll pigments. While most photosynthetic organisms use (bacterio)chlorophylls with a phytyl (2-phytenyl) group as the hydrophobic isoprenoid tail, Halorhodospira halochloris, an anoxygenic photosynthetic bacterium belonging to Gammaproteobacteria, produces bacteriochlorophylls with a unique 6,7,14,15-tetrahydrogeranylgeranyl (2,10-phytadienyl) tail. Geranylgeranyl reductase (GGR), encoded by the bchP gene, catalyzes hydrogenation at three unsaturated C=C bonds of a geranylgeranyl group, giving rise to the phytyl tail. In this study, we discovered that H. halochloris GGR exhibits only partial hydrogenation activities, resulting in the tetrahydrogeranylgeranyl tail formation. We hypothesized that the hydrogenation activity of H. halochloris GGR differed from that of Chlorobaculum tepidum GGR, which also produces a pigment with partially reduced hydrophobic tails (2,6-phytadienylated chlorophyll a). An engineered GGR was also constructed and demonstrated to perform only single hydrogenation, resulting in the dihydrogeranylgeranyl tail formation. H. halochloris original and variant GGRs shed light on GGR catalytic mechanisms and offer prospective bioengineering tools in the microbial production of isoprenoid compounds. IMPORTANCE Geranylgeranyl reductase (GGR) catalyzes the hydrogenation of carbon-carbon double bonds of unsaturated hydrocarbons of isoprenoid compounds, including α-tocopherols, phylloquinone, archaeal cell membranes, and (bacterio)chlorophyll pigments in various organisms. GGRs in photosynthetic organisms, including anoxygenic phototrophic bacteria, cyanobacteria, and plants perform successive triple hydrogenation to produce chlorophylls and bacteriochlorophylls with a phytyl chain. Here, we demonstrated that the GGR of a gammaproteobacterium Halorhodospira halochloris catalyzed unique double hydrogenation to produce bacteriochlorophylls with a tetrahydrogeranylgeranyl tail. We also constructed a variant enzyme derived from H. halochloris GGR that performs only single hydrogenation. The results of this study provide new insights into catalytic mechanisms of multiposition reductions by a single enzyme.

Characterization of regioisomeric diterpenoid tails in bacteriochlorophylls produced by geranylgeranyl reductase from Halorhodospira halochloris and Blastochloris viridis.

Geranylgeranyl reductase (GGR) encoded by the bchP gene catalyzes the reductions of three unsaturated C = C double bonds (C6 = C7, C10 = C11, and C14 = C15) in a geranylgeranyl (GG) group of the esterifying moiety in 17-propionate residue of bacteriochlorophyll (BChl) molecules. It was recently reported that GGR in Halorhodospira halochloris potentially catalyzes two hydrogenations, yielding BChl with a tetrahydrogeranylgeranyl (THGG) tail. Furthermore, its engineered GGR, in which N-terminal insertion peptides characteristic for H. halochloris were deleted, performed single hydrogenation, producing BChl with a dihydrogeranylgeranyl (DHGG) tail. In some of these enzymatic reactions, it remained unclear in which order the C = C double bond in a GG group was first reduced. In this study, we demonstrated that the (variant) GGR from H. halochloris catalyzed an initial reduction of the C6 = C7 double bond to yield a 6,7-DHGG tail. The intact GGR of H. halochloris catalyzed the further hydrogenation of the C14 = C15 double bonds to give a 6,7,14,15-THGG group, whereas deleting the characteristic peptide region from the GGR suppressed the C14 = C15 reduction. We also verified that in a model bacterium, Blastochloris viridis producing standard BChl-b, the reduction of a GG to phytyl group occurred via 10,11-DHGG and 6,7,10,11-THGG. The high-performance liquid chromatographic elution profiles of BChls-a/b employed in this study are essential for identifying the regioisomeric diterpenoid tails in the BChls of phototrophic bacteria distributed in nature and elucidating GGR enzymatic reactions.

Biohybrid Organic Heterostructure Based on Chlorophyll-Bacteriochlorophyll Aggregates for Ecofriendly Hydrogen Production.

Hydrogen energy is an abundant, clean, sustainable and environmentally friendly renewable energy source. Therefore, the production of hydrogen by photocatalytically splitting water on semiconductors has been considered in recent years as a promising and sustainable strategy for converting solar energy into chemical energy to replace conventional energy sources and to solve the growing problem of environmental pollution and the global energy crisis. However, highly efficient solar-driven photocatalytic hydrogen production remains a huge challenge due to the poor visible light response of available photocatalytic materials and the low efficiency of separation and transfer of photogenerated electron-hole pairs. In the present work, organic heterojunction structures based on bacteriochlorophyll (BChl) and chlorophyll (Chl) molecules were introduced and used for solar-driven photocatalytic hydrogen production from water under visible light. Also, noble metal-free photocatalyst was successfully constructed on Ti3 C2 Tx nanosheets by simple successive deposition of Chl and BChl, which was used for the photocatalytic splitting water to hydrogen evolution reaction (HER). The results show that the optimal BChl@Chl@Ti3 C2 Tx composite has a high HER performance with 114 µmol/h/gcat , which is much higher than the BChl@Ti3 C2 Tx and Chl@Ti3 C2 Tx composites.

Intramolecular axial α/ß-coordination of the 132-terminal pyridyl group to the central zinc atom in chlorophyll-a derivatives.

Chlorophyll(Chl)-a derivatives possessing a zinc center and a C132-alkanoate residue with a terminal pyridyl group were synthesized. Their C132-epimerically pure products were isolated by preparative reverse phase HPLC. The C132-stereochemistry resulted in two directed terminal pyridinyl groups that coordinate with the central zinc atom in each stereoisomeric molecule: α/ß-intramolecular axial coordination. The asymmetric axial coordinations mimic the immobilization manner of Chl-a in photosynthetically active proteins. The diastereomerically dependent conformers in solution were characterized by 1D/2D NMR, UV-visible absorption, and fluorescence emission as well as circular dichroism (CD) spectroscopy. The 1H NMR and DOSY spectra revealed that the stereoselectively intramolecular coordination occurred in less coordinative deuterated chloroform, whereas a highly coordinative deuterated pyridine molecule replaced the terminal pyridine moiety as the axial ligand to form a mixture of α/ß-coordinated species. Their optical spectra in pyridine were nearly independent of the C132-stereochemistry and the linkers in the C132-substituents. The CD bands of ß-coordinated species in chloroform were more intense than those of the corresponding α-coordinated stereoisomers, indicating that the former had a larger distortion of the chlorin π-plane than the latter. Therefore, the α-coordinated Chl complex is more conformationally stable than the ß-complex.

Excited-state dynamics of dipyrrolyldiketone difluoroboron complexes.

Anion-responsive photofunctional materials have been extensively studied because anions are important for biotic activity and constitute the building blocks of elegant supramolecular architectures. A number of fluorescent anion receptors that can probe anions in their environments have been reported, but the excited states of many of these molecules remain elusive. Studies on excited-state dynamics provide fruitful information for optimizing the emission properties, minimizing the photodegradation and photorelease of anions, and exploring novel photofunctions. In this study, we investigated the excited-state dynamics of an aryl-substituted dipyrrolyldiketone difluoroboron complex, a π-conjugated anion receptor, by time-resolved visible and infrared absorption spectroscopy and emission decay measurements combined with quantum chemical calculations. Anion binding was found to alter the radiative and nonradiative rate constants and the excited-state absorption of the anion receptor. In contrast, the molecular structures and binding abilities were similar in the S0 and S1 states.

Chlorophyll Derivative-Sensitized TiO2 Electron Transport Layer for Record Efficiency of Cs2AgBiBr6 Double Perovskite Solar Cells.

The power conversion efficiency (PCE) of Cs2AgBiBr6-based perovskite solar cells (PSCs) is still low owing to the inherent defects of Cs2AgBiBr6 films. Herein, we demonstrate a carboxy-chlorophyll derivative (C-Chl)-sensitized mesoporous TiO2 (m-TiO2) film as an electron transport layer (ETL) to enhance and extend the absorption spectrum of Cs2AgBiBr6-based PSCs. The C-Chl-based device achieves a significantly improved PCE, exceeding 3% for the first time, with an increase of 27% in short-circuit current density. Optoelectronic investigations confirm that the introduction of C-Chl reduces the defects, accelerates the electron extraction, and suppresses charge recombination at the interface of ETL/perovskite. Moreover, the unencapsulated PSCs display restrained hysteresis and great stability under ambient conditions.

Chlorophyll-Based Organic Heterojunction on Ti3 C2 Tx MXene Nanosheets for Efficient Hydrogen Production.

The Z-scheme process is a photoinduced electron-transfer pathway in natural oxygenic photosynthesis involving electron transport from photosystem II (PSII) to photosystem I (PSI). Inspired by the interesting Z-scheme process, herein a photocatalytic hydrogen evolution reaction (HER) employing chlorophyll (Chl) derivatives, Chl-1 and Chl-2, on the surface of Ti3 C2 Tx MXene with two-dimensional accordion-like morphology, forming Chl-1@Chl-2@Ti3 C2 Tx composite, is demonstrated. Due to the frontier molecular orbital energy alignments of Chl-1 and Chl-2, sublayer Chl-1 is a simulation of PSI, whereas upper layer Chl-2 is equivalent to PSII, and the resultant electron transport can take place from Chl-2 to Chl-1. Under the illumination of visible light (>420 nm), the HER performance of Chl-1@Chl-2@Ti3 C2 Tx photocatalyst was found to be as high as 143 µmol h-1 gcat -1 , which was substantially higher than that of photocatalysts of either Chl-1@Ti3 C2 Tx (20 µmol h-1 g-1 ) or Chl-2@Ti3 C2 Tx (15 µmol h-1 g-1 ).

Detection of 132-carboxy-chlorin produced by the in vitro BciC enzymatic hydrolysis of zinc chlorophyllide.

Green photosynthetic bacteria with an efficient light-harvesting system contain special chlorophyll molecules, called bacteriochlorophylls c, d, e, in their main antennae. In the biosynthetic pathway, a BciC enzyme is proposed to catalyze the hydrolysis of the C132-methoxycarbonyl group of chlorophyllide a, but the resulting C132-carboxy group has not been detected yet because it is spontaneously removed due to the instability of the ß-keto-carboxylic acid. In this study, the in vitro BciC enzymatic reactions of zinc methyl (131R/S)-hydroxy-mesochlorophyllides a were examined and a carboxylic acid possessing the C132S-OH was first observed as the hydrolyzed product of the C132-COOCH3.

Exciton delocalization length in chlorosomes investigated by lineshape dynamics of two-dimensional electronic spectra.

A chlorosome, a photosynthetic light-harvesting complex found in green sulfur bacteria, is an aggregate of self-assembled pigments and is optimized for efficient light harvesting and energy transfer under dim-light conditions. In this highly-disordered aggregate, the absorption and transfer of photoexcitation energy are governed by the degree of disorder. To describe the disorder, the number of molecules forming excitons, which is termed exciton delocalization length (EDL), is a relevant parameter because the EDL sensitively changes with the disorder of the constituent molecules. In this work, we determined the EDL in chlorosomes using two-dimensional electronic spectroscopy (2D-ES). Since spectral features correlated with EDL are spread out in the two-dimensional (2D) electronic spectra, we were able to determine the EDL accurately without the effects of homogeneous and inhomogeneous line broadening. In particular, by taking advantage of the multi-dimensionality and the time evolution of 2D spectra, we not only determined the excitation frequency dependence of EDL but also monitored the temporal change of EDL. We found that the EDL is ∼7 at 77 K and ∼6 at 298 K and increases with the excitation frequency, with the maximum located well above the maximum of the absorption spectrum of chlorosomes. The spectral profile of EDL changes rapidly within 100 fs and becomes flat over time due to dephasing of initial exciton coherence. From the coherent oscillations superimposed on the decay of EDL, it was learned that high-frequency phonons are more activated at 298 K than at 77 K.

In Vitro Hydrolysis of Zinc Chlorophyllide a Homologues by a BciC Enzyme.

Chlorosomes in green photosynthetic bacteria are the largest and most efficient light-harvesting antenna systems of all phototrophs. The core part of chlorosomes consists of bacteriochlorophyll c, d, or e molecules. In their biosynthetic pathway, a BciC enzyme catalyzes the removal of the C132-methoxycarbonyl group of chlorophyllide a. In this study, the in vitro enzymatic reactions of chlorophyllide a analogues, C132-methylene- and ethylene-inserted zinc complexes, were examined using a BciC protein from Chlorobaculum tepidum. As the products, their hydrolyzed free carboxylic acids were observed without the corresponding demethoxycarbonylated compounds. The results showed that the in vivo demethoxycarbonylation of chlorophyllide a by an action of the BciC enzyme would occur via two steps: (1) an enzymatic hydrolysis of a methyl ester at the C132-position, followed by (2) a spontaneous (nonenzymatic) decarboxylation in the resulting carboxylic acid.

Chlorophyllide a oxidoreductase Preferentially Catalyzes 8-Vinyl Reduction over B-Ring Reduction of 8-Vinyl Chlorophyllide a in the Late Steps of Bacteriochlorophyll Biosynthesis.

Bacteriochlorophyll a (BChl) is an essential pigment for anoxygenic photosynthesis. In late steps of the BChl biosynthesis of Rhodobacter capsulatus, the C8 vinyl group and C7=C8 double bond of 8-vinyl chlorophyllide a (8 V-Chlide) are reduced by a C8 vinyl reductase (8VR), BciA, and a nitrogenase-like enzyme, chlorophyllide a oxidoreductase (COR), respectively, to produce 3-vinyl-bacteriochlorphyllide a. Recently, we discovered 8VR activity in COR. However, the kinetic parameters of the COR 8VR activity remain unknown, while those of the COR C7=C8 reductase activity and BciA have been reported. Here, we determined the kinetic parameters of COR 8VR activity by using 8 V-Chlide. The Km value for 8 V-Chlide was 1.4 µM, which is much lower than the 6.2 µM determined for the C7=C8 reduction of Chlide. The kinetic parameters of the dual activities of COR suggest that COR catalyzes the reduction of the C8 vinyl group of 8 V-Chlide preferentially over C7=C8 reduction when both substrates are supplied during BChl biosynthesis.

BciC-Catalyzed C132 -Demethoxycarbonylation of Metal Pheophorbide†a Alkyl Esters.

Bacteriochlorophyll c molecules self-aggregate to form large oligomers in the core part of chlorosomes, which are the main light-harvesting antenna systems of green photosynthetic bacteria. In the biosynthetic pathway of bacteriochlorophyll c, a BciC enzyme catalyzes the removal of the C132 -methoxycarbonyl group of chlorophyllide a, which possesses a free propionate residue at the C17-position and a magnesium ion as the central metal. The in vitro C132 -demethoxycarbonylations of chlorophyll a derivatives with various alkyl propionate residues and central metals were examined by using the BciC enzyme derived from one green sulfur bacteria species, Chlorobaculum tepidum. The BciC enzymatic reactions of zinc pheophorbide a alkyl esters were gradually suppressed with an increase of the alkyl chain length in the C17-propionate residue (from methyl to pentyl esters) and finally the hexyl ester became inactive for the BciC reaction. Although not only the zinc but also nickel and copper complexes were demethoxycarbonylated by the BciC enzyme, the reactions were largely dependent on the coordination ability of the central metals: Zn>Ni>Cu. The above substrate specificity indicates that the BciC enzyme would not bind directly to the carboxy group of chlorophyllide a, but would bind to its central magnesium to form the stereospecific complex of BciC with chlorophyllide a, giving pyrochlorophyllide a, which lacks the (132 R)-methoxycarbonyl group.

Growth model of chlorosome antenna by the environment-dependent stepwise assembly of a zinc chlorophyll derivative.

A zinc chlorophyll derivative possessing an oligoethylene glycol ester at the 17-propionate residue was prepared as a model of specific pigments in chlorosomes, such as bacteriochlorophylls-c, d, and e, by chemical modification of naturally occurring chlorophyll-a. The zinc chlorophyll derivative aggregated in aqueous or hexane solutions containing 1% (v/v) ethanol to give red-shifted and broadened Soret/Qy absorption bands with intense circular dichroism signals, indicating the formation of its chlorosome-like J-type self-aggregates. The atomic force microscope images of the self-aggregates prepared in aqueous or hexane solutions showed thin tube-like (ca. 3 nm diameter) or thick rod-like aggregates (> 20 nm diameter), respectively. After standing these solutions for several days, the nanotubes or nanorods further assembled to give ribbon- or bundle-like aggregates, respectively. The latter transformation (tube to ribbon) was triggered by hydrogen bonding between oligoethylene glycol esters located outside of the tubes via water or ethanol molecules. These dynamic supramolecular transformations may also be useful for revealing the growth process of bacteriochlorophyll self-aggregates in a chlorosome.

A Synthetic Chlorophyll Dimer Appending Fullerene: Effect of Chlorophyll Pairing on (Photo)redox Properties.

Accurately mimicking structure and function of natural chlorophyll (Chl) assemblies is very challenging. Herein, we report the synthesis of a fullerene-appended Chl dimer being capable of intramolecular photoinduced charge separation (CS) with a unique structure reminiscent of reaction centers (RCs) in phototrophs. Structural analyses revealed that the Chl dimer adopts a bird-like structure in which two Chl components overlapped partially with one of the four pyrrole rings in a Chl ring similar to in a Chl pair in the natural RC complexes. A comparative study including voltammetry and spectrometric analyses using the Chl dimer and its corresponding monomer with and without a fullerene moiety was performed to gain insight into the effect of Chl pairing on (photo)redox properties. Our results suggest that the present dimer motif that closely resemble the Chl pair in natural RCs lead to more facile oxidation and lower energy of the CS state of the Chl dimer than those of the Chl monomer, resulting in its photoredox behavior different from that of the monomer Chl.