Theoretical study on CO2 reduction catalyzed by formate dehydrogenase using the cation radical of a bipyridinium salt with an ionic substituent as a co-enzyme

Abstract

Formate dehydrogenase from Candida boidinii (CbFDH; EC.1.2.1.2) is a useful enzyme for CO₂ reduction to formate in the photoredox system of a visible-light sensitizer and an electron mediator in the presence of an electron donor. The electron mediator, cation radicals of 4,4′-bipyridinium salts (4,4′-BPs) act as the co-enzyme for CbFDH in the CO₂ reduction to formate. We found that the CbFDH-catalyzed CO₂ reduction to formate could be controlled by the ionic substituents introduced into the cation radical of 4,4′-BPs [Y. Amao, Sustainable Energy Fuels, 2018, 2, 1928–1950]. By using 1,1′-diaminoethyl-4,4′-bipyridinium salt (DABP), 1-aminoethyl-1′-methyl-4,4′-bipyridinium salt (AMBP), 1,1′-carboxymethyl-4,4′-bipyridinium salt (DCBP), and 1-carboxymethyl-1′-methyl-4,4′-bipyridinium salt (CMBP), the introduction of an amino-group into 4,4′-BP accelerates the CbFDH-catalyzed CO₂ reduction to formate, while the introduction of a carboxy-group into 4,4′-BP slows the CO₂ reduction to formate. This work clarified the direct interaction of the cation radicals of DABP, DCBP, AMBP, CMBP, and MV in the substrate-binding site of CbFDH by the docking simulation. In addition, a mechanistic investigation for the CbFDH-catalyzed CO₂ reduction to formate with cation radicals of DABP, DCBP, AMBP, CMBP, and MV was carried out based on the energy of molecular orbitals calculated by density functional theory (DFT).