<p>The visible-light driven redox system of an electron donor, a photosensitizer, an electron mediator and formate dehydrogenase from Candida boidinii (CbFDH; EC.1.2.1.2) as a catalyst has received much attention for CO2 reduction to formate. In this system, elucidation of the electron transfer process between the electron mediator and CbFDH is important for improving efficiency of CO2 reduction to formate. 2,2′-Bipyridinium salt (2,2′-BP2+) is a remarkable electron mediator for CbFDH-catalyzed CO2 reduction. Especially, the direct interaction between cation radicals of 2,2′-BPs (2,2′-BP+•s) and CbFDH is a significant factor for the CO2 reduction. By using 1,1′-ethylene-2,2′-bipyridinium salt (DB2+), 1,1′-trimethylene-2,2′-bipyridinium salt (TB2+), 1,1′-tetramethylene-2,2′-bipyridinium salt (QB2+) and 1,1′-dimethyl-2,2′-bipyridinium salt (DM2+), 2,2′-BP+• with a small dihedral angle between the two pyridine rings (DB or TB) accelerates the CbFDH-catalyzed CO2 reduction. This work illuminated the direct interaction of the cation radicals of DB, TB, QB and DM in the substrate-binding site of CbFDH on the basis of a docking-simulated prediction. Moreover, the electron transfer process from the cation radicals of DB, TB, QB and DM to CO2 in the CbFDH was investigated based on the energy of the molecular orbital calculated by density functional theory (DFT). From these results, it can be predicted that efficient CO2 reduction to formate can be achieved by suppressing the three-dimensional structural change between the 2,2′-BP dication and the cation radical in the substrate-binding pocket of CbFDH as much as possible.</p>
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