Shigetoshi Aono

Photoreduction of viologens and hydrogen evolution with hydrogenase

Abstract Ethyl-, propyl-, and n-butyl-viologens are found to be electron carries for photoinduced hydrogen production and also can be cofactors of hydrogenase. As these viologens are photoexcited by visible light irradiation or sunlight, simpler systems without another photosensitizer can be developed for the photochemical utilization of solar energy.

Regeneration of NADH and ketone hydrogenation by hydrogen with the combination of hydrogenase and alcohol dehydrogenase

The regeneration of nicotinamide-adenine dinucleotide (reduced form, NADH) by the reaction of NAD with hydrogen gas was carried out in the presence of the hydrogenase from Alcaligenes eutrophus. And the formations of alcohol, CO2, and 6-phospho-gluconate were observed by a combination of the above system and corresponding dehydrogenases. NADH was regenerated by hydrogen gas with the hydrogenase and recycled in these reactions.

Kinetics and mechanism of photo-induced methyl viologen reduction with an organic dye and hydrogen evolution from water by hydrogenase

The formation of the cation radical of methyl viologen was established when an aqueous solution containing an organic dye, methyl viologen, and mercaptoethanol was irradiated by visible light. The reduction of methyl viologen has been studied kinetically and the reduction rate was expressed as follows. where [S], [MV2+], [MV+], and [RSH] are the respective concentrations of the dye photosensitizer, oxidized and reduced forms of methyl viologen, and mercaptoethanol. On the basis of the rate expression, the reaction mechanism is discussed. On the addition of hydrogenase to the above photoirradiation system, hydrogen evolution was observed by the irradiation of visible light.

PHOTOREDUCTION OF VIOLOGENS WITH NADPH AS REVERSIBLE ELECTRON DONOR

Abstract— The photoreduction of viologens with NADPH as an electron donor was investigated. As the photosensitizer in that reaction, ZnTPP33‐ and HmP were applied. HmP dissolved by CTAB or Triton X‐100 was a more active photosensitizer than ZnTPPS33‐. The reduction rate of viologen under steady state irradiation depends remarkably on the type of surfactant used for dissolving HmP. The effect of the surface charge of micelle was examined by laser flash photolysis.

Photoinduced hydrogen evolution in micellar system

Photoreduction of viologens by the irradiation of the system containing NADPH, zinc mesotetraphenylporphyrintrisulfonate (Zn-TPPS33−), viologen and colloidal platinum has been investigated in the presence of surfactant micelles. In the presence of either cationic micelles or anionic micelles, a remarkable increase in the accumulation of the reduced form of viologen was observed. The existence of the micelles depressed both the quenching rate of the photoexcited Zn-TPPS33− by viologen and the back reaction rate, recombination rate of the oxidized Zn-TPPS33− and reduced viologen. Compared with both reactions, it was clarified that the recombination rate strongly influenced the viologen reduction rate. The effect of the micelles was explained by the electrostatic effect among the charges of the micellar surface, Zn-TPPS33− and viologen. By the addition of colloidal platinum to the system, photoinduced hydrogen evolution was also studied.

Effect of flavins on hydrogenase activity

Abstract The effect of flavin mononucleotide (FMN) and acriflavin on hydrogen evolution and D2-H2O exchange with hydrogenase are studied. When FMN is used for the hydrogen evolution reaction, an abnormal kinetic behavior is observed, where the activity increases and then decreases after passing through a maximum point. In the case of the D2-H2O exchange reaction, no effect is observed. When acriflavin is used, the activity decreases monotonously in both reactions. On the basis of these effects, the reaction mechanism and the active site of hydrogenase are discussed.

Thermostability and electron transfer activity of the ferredoxin from a thermophilic hydrogen oxidizing bacterium, Bacillus schlegelii

Abstract The Azotobacter -type 7Fe ferredoxin from Bacillus schlegelii was active as an electron carrier in the reduction system of cyt. c consisting of NADPH, FNR and cytochrome c (cyt. c ). The only [3Fe4S] cluster in the Fd molecule acts as the active site for the reduction of cyt. c . The activity of the ferredoxin once increased by 10, 20 and 20% by heat treatment at 60, 80 and 90°C, respectively. The increasing of the activity is caused by the interconversion of the [4Fe4S] cluster to the [3Fe4S] cluster with increasing of the [3Fe4S] cluster in the concentration.

Photoreduction of cytochrome c with zinc protoporphyrin reconstituted myoglobin (ZnPPMb)

Abstract Photoreduction of cytchrome c (cyt. c ) proceeded by irradiation of the visible light in the reaction system consisting of EDTA, ZnPPMb and cyt. c . About 90% of cyt. c was reduced in 20 min of irradiation of the light. Photoinduced electron transfer occurs between ZnPPMb and cyt. c in the above system. Laser flash photolysis showed that the oxidative quenching of 3 ZnPPMb∗ by the oxidized cyt. c took place and the quenching rate constant was 8.5 × 10 5 M −1 s −1 .

Substrate Specificity and Stereoselectivity of Epoxidation of Alkenes with Methylosinus trichosporium (OB3b)

The enantiomeric composition of the epoxide will depend on the type of reaction intermediate, the reaction face of the alkene, and the carbon atom at which the initial oxygen attack takes place. In this communication, the epoxidation of alkenes with whole-cell M. trichosporium was carried out to obtain information on the active site and the reaction mechanism

Photoinduced Hydrogen Evolution with Viologen-Linked Porphyrins

Abstract Viologen-linked water-soluble porphyrins with different methylene chain lengths between porphyrin and viologen were synthesized. These compounds were applied to photoinduced hydrogen evolution in a system containing NADPH, viologen-linked porphyrin, and hydrogenase under steady-state irradiation.