Shusaku Ikeyama

Novel artificial co-enzyme based on viologen skeleton for highly efficient CO2 reduction to formic acid with formate dehydrogenase

Formate dehydrogenase (FDH) is an attractive catalyst for the reduction of CO2 because CO2 is converted to formic acid by FDH at room temperature under normal pressure in neutral aqueous solution. The reduced form of methylviologen acts as an artificial co‐enzyme for FDH in the conversion of CO2 to formic acid. To improve the catalytic activity of FDH in reducing CO2, viologen derivatives with ionic groups were synthesized as effective artificial co‐enzymes for FDH. We used enzyme kinetic analysis to assess the effect of the ionic amino or carboxyl functional groups in the reduced form of the viologen derivatives on the catalytic activity of FDH with respect to the reduction of CO2. By using 1,1′‐diaminoethyl‐4,4′‐bipyridinium salt, which is the reduced form of a viologen derivative with two amino groups, we optimized the reduction of CO2 to formic acid with FDH. The catalytic efficiency value (kcat/Km) of the reduced form of 1,1′‐diaminoethyl‐4,4′‐bipyridinium salt was estimated to be more than 560 times larger than that of the natural co‐enzyme NADH. From the analysis result, the CO2 reduction was influenced by the ionic group of the viologen derivative.

Visible light-induced reduction system of diphenylviologen derivative with water-soluble porphyrin for biocatalytic carbon–carbon bond formation from CO2

Abstract From the view point of green chemistry, CO2 utilization technologies with solar energy including the photoredox system have been received a lot of attention. As one of them, photoredox system containing a photosensitizer and a catalyst catalyzing a reaction of a carbon–carbon bond formation from CO2 as a feed stock were constructed. In a recent study, we reported the visible light-induced malate (C4 compound) production from pyruvate (C3 compound) and CO2 due to carbon–carbon bond formation with the system consisting an electron donor, a photosensitizer, diphenylviologen (PV2+) derivative as an electron mediator in the presence of malic enzyme (ME). However, the interaction between a photosensitizer and PV2+ derivative has not been clarified yet. In this study, water-soluble PV2+ derivative, 1,1′-bis(p-sulfonatophenyl)-4,4′-bipyridinium salt (PSV2+) was synthesized, and its electro-, photochemical properties were evaluated. Moreover, the photoredox properties of PSV2+ with water-soluble Zn porphyrin were studied using fluorescence spectroscopy and steady state irradiation. The fluorescence of Zn porphyrin was quenched by PSV2+ and the two-electron reduced form of PSV2+ were produced with Zn porphyrin with steady state irradiation. In addition, reaction solution containing triethanolamine, tetraphenylporphyrin tetrasulfonate, pyruvate, ME, Mg2+ and PSV2+ in CO2 saturated bis-tris buffer (pH 7.4) was irradiated with visible light, the oxaloacetate and malate were produced. This result indicates that PSV2+ is an efficient electron mediator in the visible light-induced redox system for carbon–carbon bond formation with ME from CO2 as a feedstock.

Activation of the catalytic function of formaldehyde dehydrogenase for formate reduction by single-electron reduced methylviologen

The kinetic properties of formate reduction to formaldehyde with formaldehyde dehydrogenase (FldDH) using single-electron reduced methylviologen (MV˙) as a co-enzyme were clarified. By using natural co-enzyme NADH, actually, no formaldehyde production due to formate reduction with FldDH was observed. In contrast, we discovered that MV˙ activates the FldDH activity for formate reduction to formaldehyde for the first time.

Abnormal co-enzymatic behavior of a one-electron reduced bipyridinium salt with a carbamoyl group on the catalytic activity of CO2 reduction by formate dehydrogenase

One-electron reduced bipyridinium salt (BP˙) acts as a co-enzyme for formate dehydrogenase (FDH) from Candida boidinii in CO2 reduction to formic acid. By using BP˙ with a carbamoyl group (CMV˙), the CO2 reduction catalytic activity of FDH was improved compared with those of other BP˙s. Moreover, CMV˙ did not act as a co-enzyme for FDH in the presence of more than 20 times higher concentration of CMV˙ against FDH. This is the first report of BP˙ with the abnormal co-enzymatic behavior on the catalytic activity of CO2 reduction by FDH.

Light-driven CO2 reduction to formic acid with the hybrid system of biocatalyst and semiconductor based photocatalyst

Effective light-driven conversion of CO2 to formic acid with a hybrid system consisting of photocatalyst TiO2 nanoparticle (P25), methylviologen (MV2+) as an electron mediating molecule and biocata...