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The utilization of the light-harvesting and electron-transferring function of chlorophylls (Chls) has received attention for visible-light driven hydrogen production. In this work, a series of Chl derivatives based on pyropheophorbide-a (Pyro-a) conjugated with a viologen moiety, including a Pyro-a methyl ester directly bonded with the viologen at the 3-position 1, its 31-methylene analog 2 and Pyro-a connected with the viologen in the 17-substituent 3, were synthesized from chemical modification of naturally occurring Chl-a and characterized in terms of their photochemical and photophysical properties. As the photoexcited singlet state of the Pyro-a moiety was strongly quenched by the viologen moiety in a molecule, the effective photoinduced intramolecular electron transfer from Pyro-a to the bonded viologen moiety occurred. Moreover, these molecules were applied as a photosensitizer in the system for visible-light driven hydrogen production with platinum nanoparticles via intramolecular reduction of the bonded viologen moiety. Efficient photoreduction of external methyl viologen and successive hydrogen production on platinum nanoparticles were achieved using the synthetic conjugate of Pyro-a with the viologen moiety as a photosensitizer. In particular, effective visible-light driven hydrogen production was accomplished using 3 and platinum nanoparticles via the reduction of external methyl viologen.

Effective light-driven conversion of CO2 to formic acid with a hybrid system consisting of photocatalyst TiO2 nanoparticle (P25), methylviologen (MV2+) as an electron mediating molecule and biocata...

From the view point of green chemistry, CO2 utilization technologies with solar energy including the photoredox system have been received a lot of attention. As one of them, photoredox system containing a photosensitizer and a catalyst catalyzing a reaction of a carbon–carbon bond formation from CO2 as a feed stock were constructed. In a recent study, we reported the visible light-induced malate (C4 compound) production from pyruvate (C3 compound) and CO2 due to carbon–carbon bond formation with the system consisting an electron donor, a photosensitizer, diphenylviologen (PV2+) derivative as an electron mediator in the presence of malic enzyme (ME). However, the interaction between a photosensitizer and PV2+ derivative has not been clarified yet. In this study, water-soluble PV2+ derivative, 1,1′-bis(p-sulfonatophenyl)-4,4′-bipyridinium salt (PSV2+) was synthesized, and its electro-, photochemical properties were evaluated. Moreover, the photoredox properties of PSV2+ with water-soluble Zn porphyrin were studied using fluorescence spectroscopy and steady state irradiation. The fluorescence of Zn porphyrin was quenched by PSV2+ and the two-electron reduced form of PSV2+ were produced with Zn porphyrin with steady state irradiation. In addition, reaction solution containing triethanolamine, tetraphenylporphyrin tetrasulfonate, pyruvate, ME, Mg2+ and PSV2+ in CO2 saturated bis-tris buffer (pH 7.4) was irradiated with visible light, the oxaloacetate and malate were produced. This result indicates that PSV2+ is an efficient electron mediator in the visible light-induced redox system for carbon–carbon bond formation with ME from CO2 as a feedstock.

Formate dehydrogenase (FDH) is an attractive biocatalyst for converting CO 2 to formic acid in ambient conditions. FDH is applied to the catalyst for the visible-light induced CO 2 – formic acid conversion system consisting of an electron donor, a photosensitizaer and an electron carrier. For improvement of the formic acid production efficiency with this system, the enhancement of electron relay processes among a photosensitizer, an electron carrier and FDH has been needed. We aimed to solve this problem by developing novel electron carrier based on the viologen derivative instead of methylviologen (MV), widely used as an electron carreir. In this study, viologen derivatives with carbamoyl group, 1,1′-dicarbamoylmethyl-4,4′-bipyridinium diiodide (CV) and 1-carbamoylmethyl-1′-methyl-4,4′-bipyridinium diiodide (CMV) were synthesized and applied to the electron carrier for photoinduced CO 2 -formic acid conversion system consisting of triethanolamine, water-soluble zinc porphyrin and FDH. By using CV and CMV, the effective formic acid production in the system of water-soluble zinc porphyrin and FDH was improved compared with that of MV.

Molecular conversion reaction system containing a dye and a biocatalyst catalyzing a reaction of a carbon–carbon bond formation from CO2 as a feedstock were constructed. For example, NADP-malic enzyme (ME, EC1.1.1.40) catalyzes the reaction of introducing CO2 as a carboxy group to pyruvate (C3 compound) to form malate (C4 compound) via oxaloacetate in the presence of natural co-enzyme NADPH. In a recent study, visible light-induced oxaloacetate production from pyruvate and CO2 with the system consisting an electron donor, a photosensitizer, diphenylviologen (PV2+) derivative as an electron mediator and ME was reported. However, the visible light-induced reduction properties of PV2+ with water-soluble Zn porphyrin have not been clarified. In this study, chemical and photo-redox properties of PV2+ were studied. In addition, to produce malate, CO2 saturated reaction solution containing triethanolamine, tetraphenylporphine tetrasulfonate (H2TPPS), pyruvate, PV2+ and ME in the presence of Mg2+, co-factor for ME was irradiated with visible-light, the oxaloacetate and malate were produced.

One-electron reduced bipyridinium salt (BP˙) acts as a co-enzyme for formate dehydrogenase (FDH) from Candida boidinii in CO2 reduction to formic acid. By using BP˙ with a carbamoyl group (CMV˙), the CO2 reduction catalytic activity of FDH was improved compared with those of other BP˙s. Moreover, CMV˙ did not act as a co-enzyme for FDH in the presence of more than 20 times higher concentration of CMV˙ against FDH. This is the first report of BP˙ with the abnormal co-enzymatic behavior on the catalytic activity of CO2 reduction by FDH.

The effect of the functional ionic group of 4,4′-bipyridinium salt derivatives (4,4′-BPs) as the electron carrier on the visible-light driven conversion of CO2 to formic acid with the system consisting of water-soluble zinc tetraphenylporphyrin tetrasulfonate (ZnTPPS) and formate dehydrogenase (FDH) in the presence of triethanolamine (TEOA) as an electron donor was investigated. 1,1′-Diaminoethyl- (DAV), 1-aminoethyl-1′-methyl- (AMV), 1-carboxymethyl-1′-methyl- (CMV) and 1,1′-dicarboxymethyl-4,4′-bipyridinium salt (DCV) were prepared as the 4,4′-BPs with the functional ionic group. Irradiation of a CO2 saturated buffer solution containing TEOA, ZnTPPS, 4,4′-BP and FDH with visible light irradiation resulted in the production of formic acid. By using 4,4′-BPs with the cationic aminoethyl-group, DAV or AMV as an electron carrier, the effective visible-light driven formic acid production based on the CO2 reduction was observed compared to the 4,4′-BPs with the anionic carboxymethyl-group, CMV or DCV. The formic acid production rate with DAV was approximately 3.2 times higher than that of the system with DCV.

The kinetic properties of formate reduction to formaldehyde with formaldehyde dehydrogenase (FldDH) using single-electron reduced methylviologen (MV˙) as a co-enzyme were clarified. By using natural co-enzyme NADH, actually, no formaldehyde production due to formate reduction with FldDH was observed. In contrast, we discovered that MV˙ activates the FldDH activity for formate reduction to formaldehyde for the first time.

Acid phosphatase (APase) was immobilized in discrete mesospaces of an assembly of silica nanoparticles (SiO2NPA) to be used as a heterogeneous catalyst for the hydrolysis of organophosphates often found as pesticide residues in agricultural products. APase immobilized in SiO2NPA exhibited higher catalytic activity than APase supported on conventional porous supports for the hydrolysis of p-nitrophenyl phosphate (p-NPP) with quantitative yield of p-nitrophenol below pH 5.5 due to the efficient diffusion of substrates in the SiO2NPA. Especially, a heterogeneous catalyst prepared by the co-accumulation method, in which silica nanoparticles (SiO2NPs) dispersion containing APase was simply dried to assemble a composite catalyst (APase/SiO2NPA), exhibited four times faster rate for the hydrolysis of p-NPP than the catalyst prepared by the equilibrium adsorption of APase in pre-assembled SiO2NPA. The catalytic stability of immobilized APase above pH 6.0 was enhanced by surface modification of SiO2NPs with 3-aminopropyltriethoxysilane (APase/SiO2NPA-NH2) due to the strong electrostatic interaction between APase and the protonated amino groups at the pH condition. Such stability enhancement was hardly obtained by cross-linking treatment of SiO2NPA to improve the robustness. These results suggest that electrostatic interaction between APase and SiO2NPs is crucial to enhance catalytic stability in the wide range of pH as well as preparation methods for the stable encapsulation.

Formate dehydrogenase (FDH) is an attractive catalyst for the reduction of CO2 because CO2 is converted to formic acid by FDH at room temperature under normal pressure in neutral aqueous solution. The reduced form of methylviologen acts as an artificial co-enzyme for FDH in the conversion of CO2 to formic acid. To improve the catalytic activity of FDH in reducing CO2, viologen derivatives with ionic groups were synthesized as effective artificial co-enzymes for FDH. We used enzyme kinetic analysis to assess the effect of the ionic amino or carboxyl functional groups in the reduced form of the viologen derivatives on the catalytic activity of FDH with respect to the reduction of CO2. By using 1,1′-diaminoethyl-4,4′-bipyridinium salt, which is the reduced form of a viologen derivative with two amino groups, we optimized the reduction of CO2 to formic acid with FDH. The catalytic efficiency value (kcat/Km) of the reduced form of 1,1′-diaminoethyl-4,4′-bipyridinium salt was estimated to be more than 560 times larger than that of the natural co-enzyme NADH. From the analysis result, the CO2 reduction was influenced by the ionic group of the viologen derivative.

To develop a visible-light driven conversion of CO2 to formic acid system consisting of water-soluble zinc porphyrin, an electron carrier and formate dehydrogenase (FDH) in the presence of triethanolamine (TEOA) as an electron donor, 1,1′-diaminoethyl-4,4′-bipyridinium salt (DAV) as a novel electron carrier with high affinity for FDH was applied.

Recently, CO2 utilization technology, including artificial photosynthesis, has received much attention. In this field, CO2 is used as a feedstock for fuels, polymers and in other chemical processes. Of note are malic enzymes (MEs) which catalyze the reaction of malic acid to pyruvic acid and CO2 with the co-enzyme NADP+, and catalyze the reverse reaction of pyruvic acid and CO2 to malic acid with the co-enzyme NADPH. Thus, MEs are also an attractive biocatalyst for carbon–carbon bond formation from CO2. Studies of the visible light-induced malic acid production from pyruvic acid and CO2 using an electron donor, a photosensitizer, an electron mediator, ferredoxin-NADP+ reductase, NADP+, and ME have been reported. However, modification of these systems is required, as they are very complicated. In this study, the visible light-induced carbon–carbon bond formation from pyruvic acid and CO2 with ME using the photoreduction of 1,1′-diphenyl-4,4′-bipyridinium salt derivatives as a novel electron mediator with water-soluble tetraphenylporphyrin tetrasulfonate (H2TPPS) in the presence of triethanolamine (TEOA) as an electron donor was developed. When a sample solution containing TEOA, H2TPPS, 1,1′-diphenyl-4,4′-bipyridinium salt derivative, pyruvic acid, and ME in CO2-saturated bis–tris buffer was irradiated, the major product was oxaloacetic acid. Thus, a visible light-induced photoredox system for carbon–carbon bond formation from CO2 with ME using 1,1′-diphenyl-4,4′-bipyridinium salt derivative as an electron mediator was developed.

Formate dehydrogenase (FDH) is an attractive biocatalyst for CO2 reduction. The reduced form of methylviologen acts as an artificial coenzyme for FDH. For further improvement in the CO2 reduction catalytic activity of FDH, a viologen derivative with two amino groups, 1,1′-diaminoethyl-4,4′-bipyridinium salt (DA2+), was synthesized as the effective artificial coenzyme for FDH. By enzyme kinetic analysis, the catalytic efficiency, i.e., kcat/Km value of the reduced form of DA2+ was estimated to be more than 560 times larger than that of NADH, the natural coenzyme for FDH.

The kinetic properties of the conversion of CO2 to formic acid with formate dehydrogenase (FDH) using some dithionite-reduced 2,2′-bipyridinium salts (1,1′-ethylene-2,2′-bipyridinium dibromide diquat and 1,1′-dimethyl-2,2′-bipyridinium dichloride) as an artificial coenzyme were studied. By using the reduced form of diquat, the effective CO2 reduction to formic acid with FDH is accomplished. The catalytic efficiency, the kcat/Km value of the reduced form of diquat was ca. 126 times larger than that of NADH.

The kinetic properties of the conversion of CO2 to formic acid with formate dehydrogenase (FDH) using dithionite-reduced methylviologen as an artificial coenzyme were studied. The catalytic product...