A B S T R A C TEffect of chemical structures of some 2,2 0 -bipyridinium salts (BP 2+ ) as the electron carrier molecules on the visible-light induced conversion of CO 2 to formic acid with the system consisting of water-soluble zinc tetraphneylporphyrin tetrasulfonate (ZnTPPS) and formate dehydrogenase (FDH) in the presence of triethanolamine (TEOA) as an electron donor molecule was investigated. Irradiation of a CO 2 saturated solution containing TEOA, ZnTPPS, BP 2+ and FDH with visible light resulted in production of formic acid. By using 1,1 0 -ethylene-2,2 0 -bipyridinium dibromide (DB 2+ ) as an electron carrier molecule, the effective formic acid production was observed compared with the other 2,2 0 -bipyridinium salt derivatives.

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Formate dehydrogenase (FDH) is a useful biocatalyst for CO2 reduction to formic acid in a photoredox system consisting of a photosensitizer and an electron carrier. The electron carrier, single-electron reduced 2,2′-bipyridinium salts (2,2′-BP2+s) act as the co-enzyme for FDH in the reaction of CO2 to formic acid. An advantage of 2,2′-BP2+s is the easy change of structural geometry and the various single-electron reduction potentials. For further improvement of CO2 reduction catalytic activity of FDH, various 2,2′-BP2+s were synthesized as effective artificial co-enzymes for FDH. The effect of the structural geometry and the single-electron reduction potential in the single-electron reduced form of 2,2′-BP2+s on the CO2 reduction catalytic activity of FDH was studied by enzymatic kinetic analysis in detail for the first time. Especially, the catalytic efficiency, kcat/Km value of the single-electron reduced 1,1′-ethylene-2,2′-bipyridinium salt was c.a. 126 times larger than that of native co-enzyme, NADH. These results showed that catalytic activity of FDH can be manipulated with complete control by using 2,2′-BP2+ without changing the structure of FDH and has opened a new avenue for the approach of NAD+/NADH redox free system with FDH using an inexpensive small electron carrier molecule.