Cofactors electrochemical regeneration

The incorporation of nanomaterials to modify the surface of electrodes is one of the most frequently used strategies to enhance the current density of bioelectrocatalytic processes. Nanostructured electrodes improve bioelectrocatalysis by means of providing high electrochemically active surface area, improving the electronic communication between biocatalysts and electrodes, increasing the conductivity or lowering the overpotential for cofactor electrochemical regeneration. ... 

An immobilized-enzyme continuous-flow reactor incorporating a continuous direct electrochemical regeneration of NAD + has been proposed. To retain the low molecular weight cofactor NADH/NAD+ within the reaction system, special hollow fibers (Dow ultrafilter UFb/HFU-1) with a molecular weight cut-off of 200 has been used [32],... 

Apart from enzymatic cofactor regeneration, both chemical and electrochemical regeneration methods have attracted attention due to their greater flexibility. The electrochemical regeneration of cofactors has a few advantages, especially the use... 

Though electrochemical regeneration of nicotinamide cofactors has some drawbacks, it is a beneficial technology for ecological reasons. Because of the use of electrodes as clean reagents, this method is very interesting for synthetic applications. 

Many different systems have heen suggested for the regeneration of (mostly) one of the coenzymes NAD, NADH, NADP or NADPH. Only a few publications which appeared after the review articles of the Whitesides group (87-89) shall be mentioned (90,91). For the photochemical and electrochemical regeneration of nicotinamide cofactors see (92,93). 

In addition to their in vivo function, these cofactors have important in vitro functions as co-factors in enzymatic synthesis. Because these co-factors are too expensive to be used in equimolar amounts, many methods have been developed for their regeneration. These include chemical (66), biological (67) and electrochemical (68) methods. En2ymatic regeneration has found particular utihty in this appHcation (69). 

Surface-modified electrodes were used for prevention of high overpotentials with direct oxidation or reduction of the cofactor, electrode fouling, and dimerization of the cofactor [7cj. Membrane electrochemical reactors were designed. The regeneration of the cofactor NADH was ensured electrochemically, using a rhodium complex as electrochemical mediator. A semipermeable membrane (dialysis or ultrafiltration) was integrated in the filter-press electrochemical reactor to confine... 

The principal strategies of cofactor regeneration - namely the enzymatic, chemical and electrochemical approach - are presented in Scheme 43.2 and have been reviewed recently [17, 21-23]. This chapter does not intend to be exhaustive rather, it focuses on the systems where a transition-metal complex and... 

Considering that these two transition-metal complexes are the only ones reported for the electrochemical cofactor reduction, the results are quite promising and show the need for further research in this field to identify new catalysts. In addition to the use of soluble redox mediators in electrochemical cofactor regeneration, modified electrodes have also been used. Details on these systems can also be found in the above-mentioned reviews [31, 32]. 

Besides the electrochemical application, the (Cp )Rh(bpy)-complex 9 can also be used to reduce cofactors with hydrogen. In a recent study it was compared with ruthenium complex 13 [RuC12(TPPTS)2]2 (TPPTS tris(w-sulfonatophenyl)-phosphine Scheme 43.5). Both complexes were used to regenerate the cofactors in the reduction of 2-heptanone to (S)-2-heptanol, catalyzed by an ADH from Thermoanaerobium brockii (TfrADH) [46, 47]. The TON for both catalysts was 18. 

Regeneration of the oxidized form of the cofactors, while not within the frame of this chapter, is needed for several biotransformations (e.g., oxidative kinetic resolution of diols). In these procedures, transition-metal complexes have also been applied. For this task, Ru(phend)3 complex and derivatives thereof can be used, either with oxygen or in an electrochemical procedure [49-51]. 

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