(378e) Kinetics of Electron Mediators in Bio-Electro-Chemical Reactors

Processes involving a bioelectrochemical reactor have been around for over thirty years and the ability to create bioproducts from these processes was confirmed at least twenty years ago. The application of this technology is varied, but most recent speculation about their uses has centered around waste treatment and fuel cells. Most studies center around conversion of wastes or inexpensive compounds (e.g. glycerol and glucose) into more useful materials such as hydrogen, ethanol, and electric current via utilization of the metabolic functions of microbial systems. Some studies have also focused on metabolic changes that affect product distribution of these systems.

More recently, electroenzymatic reactors have been developed which utilize an electron mediator that can be reduced by a three-electrode system and subsequently act as the substrate for enzymatic redox reactions to produce valuable products. Some examples of products that can be formed this way include statins, keto alcohols, chiral alcohols from alpha-halo ketones, and xylitol, among many others. Most of the enzymes involved in these reactions as well as the whole cell systems mentioned above utilize NAD(P)H as a cofactor to reduce the substrates into products. While direct electrochemical reduction of NAD(P)+ to NAD(P)H is possible, high overpotentials are required and yields of enzymatically active NAD(P)H decrease with each iteration such that after a few regeneration cycles, no enzymatically active cofactor remains. Indirect reduction of NAD(P)+ is achieved by direct reduction of an additional electron mediator which can subsequently reduce NAD(P)+.

In the present study, we analyzed the reduction kinetics of various electron mediators that can be used in these systems. Kinetics of Methyl Viologen, Benzyl Viologen, Neutral Red, and Anthraquinone-2,6-disulfonate reduction on graphite electrodes in buffered solution were measured. The analyzed data were fit to kinetic models to determine which of these mediators could be used as a suitable substrate for indirect NAD(P)H formation. These studies will prove critical to determining a viable solution for NAD(P)H regeneration in both whole cell and enzymatic electrochemical systems.