Effective Artificial Co-enzyme Based on Single-Electron Reduced Form of 2,2′-Bipyridinium Salt Derivatives for Formate Dehydrogenase in the Catalytic Conversion of CO2 to Formic Acid

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'-BP(2+)s) act as the co-enzyme for FDH in the reaction of CO2 to formic acid. An advantage of 2,2'-BP(2+)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'-BP(2+)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'-BP(2+)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, k(cat)/K-m, 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.