Electrochemical Screening of Redox Mediators for Electrochemical Regeneration of NADH

Abstract

We report the electrochemical cofactor regeneration for electroenzymatic reduction as a promising alternative to the enzymatic recycling due to the avoidance of couple products. In the mediated electron transfer reaction appropriate redox mediators for the cofactor reduction are necessary. We have synthesized different redox mediators-based on rhodium complexes-and have screened their electrochemical performance at different pH values and concentrations. Suitable mediators have been tested in the presence of the cofactor NAD + and then additionally in the presence of D-sorbitol dehydrogenase (DSDH) from Rhodobacter sphaeroides. The electroenzymatically generated product was analyzed by HPLC. As result of the screening some mediator species were identified which enable an efficient electrochemical cofactor reduction.Biofunctionalization of metal surfaces is widely used in the field of analytical applications especially in clinical diagnostics or for biofuel cells. 1-3 But electrochemical cofactor regeneration can also be important for preparative applications like synthesis of enantiomerically pure fine chemicals as synthons in the pharmaceutical industry. Especially for the preparative conversion with dehydrogenases, which implies a mediated electron transfer from the enzyme to the electrode, a cofactor and a redox mediator are necessary. The redox mediator decreases the potential for the NAD + reduction which amounts to about −1.0 V vs. NHE at bare electrodes without mediator. 4 For preparative syntheses only catalytic amounts of the cofactor NAD + /NADH are necessary which can be recycled in an enzymatic way. 5 A promising alternative represents the electrochemical cofactor regeneration which is already extensively known for the electrochemical re-oxidation of NADH by the functionalization of metal surfaces with biomolecules in the research of biosensors and biofuel cells 6-9 but only few mediators for reduction of NAD + are known. Numerous mediators likely to oxidize NADH (i.e., NAD + regeneration) have been reported. 10 For example, the regeneration of NAD + cofactor by a nitro fluorenone redox mediator was intensively investigated by Mano and co-workers. [11][12][13] In this case, the redox mediator was covalently bound to a gold surface and the cofactor was attached to the mediator by electrostatic forces via calcium bridges. This has been notably applied in the field of bioelectrocatalysis using a modified variant of a dehydrogenase for electroenzymatic oxidation reactions 14 on the basis of a cysteine modified galactitol dehydrogenase from Rhodobacter spaeroides, which was directly immobilized onto a gold surface, leading to successful electrochemical regeneration of the cofactor.On the other hand, elaborating redox mediators enabling the onestep transfer of two electrons (or one hydride) to NAD + species to regenerate directly the active form of the reduced cofactor (i.e., 1,4-NADH) is not an easy task as such mediator should react with NAD + and not transferring directly the electrons (or hydride ion) t...

The coordination chemistry of organo-hydride donors: new prospects for efficient multi-electron reduction

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Characterization of the sorbitol dehydrogenase SmoS from Sinorhizobium meliloti 1021

“…Some advantages of biocatalysts include high product selectivity and low environmental or physiological toxicity (Chapman et al, 2018). As an example, galactitol dehydrogenase has been immobilized on gold electrodes for use in electrochemical reactors with the goal of generating precursor molecules for pharmaceuticals via reactions that regenerate reduced cofactors (Gajdzik et al, 2010(Gajdzik et al, , 2011Kornberger et al, 2009).…”

Electrocatalytic Biosynthesis using a Bucky Paper Functionalized by [Cp*Rh(bpy)Cl]+ and a Renewable Enzymatic Layer

“…The redox mediator [Cp*Rh(bpy)Cl] + has been proved to be the most efficient non‐enzymatic catalyst for the reduction of NAD(P) + to NAD(P)H ,. The presence of [Cp*Rh(bpy)Cl] + not only reduced the overpotential needed for this reaction, but also avoided the unfavorable formation of inactive NAD(P) 2 dimer …”