Electromicrobial reductions

Fig. 18. Electromicrobial reduction of a-keto acids with Proteus vulgaris in the presence of methyl viologen as mediator...

Enoate reductase, 2-hydroxy carboxylate viologen oxidoreductase (HVOR) and AMAPORs (Section 5) are enzymes able to accept reversibly single electrons fi-om artificial mediators such as viologens and others. These mediators transfer electrons fi om or to electrodes. Therefore by the presence of the aforementioned enzyme activities biocata-lytic redox reactions can be carried out in electrochemical cells. As already mentioned in Section 1.2 AMAPORs catalysing Reactions [8] and/or [8a] are rather ubiquitous. Electromicrobial reductions can also be carried out with yeasts (Section 1.2). Since the potential of the working electrode can be chosen at will, reductions as well as... 

Two typical examples for an electroenzymic and an electromicrobial reduction of 6 mmol ( )-2-methylcinnamate and 24.6 mmol 2-oxo-5-methylpentanoate, respectively, are given in Fig. 5 and 6(111). 

Fig. 6 demonstrates the preparative electromicrobial reduction of of 2-oxo-4-methyl-pentanoate to (i )-2-hydroxy-4-methylpentanoate. 

Fig. 6. Scan of the current during the electromicrobial reduction of phenylpyruvate. The cathode compartment contained 200 ml 0.1 Mpotassium phosphate buffer, pH 7.0, 20 mmol phenylpyruvate and 3 mM methylviologen. The working potential was set to -760 mV versus SCE. After adding 0.075 g wet packed cells of Proteus vulgaris and starting to stir at a rate of 400 rpm a current of 35 mA was obtained. The amount of P. vulgaris was too much since the electrochemical reaction Reaction [1]) was limiting (48) (catholyte not blue). Therefore at A the starting concentration of 3 mM methylviologen was doubled to 6 mM. According to HPLC, 96 % of the expected quantity of (R)-phenyllactate was formed after 12 h. The productivity number was approximately 125 000.

A few organic mediators are used such as ArsN/ArsN" for oxidations and viologens for reductions. One special example is the electromicrobial reduction of an a-keto acid to an a-hydroxy acid using the enzyme system of Proteus vulgaris (Ep) and methyl viologen (MV) as the electron shuttle (Scheme 5, Fig. 8) [124]. 

Electromicrobial and microbial reduction of 6-chloropyridine-3-carboxylate. The productivity numbers (PN) are given after 7 and 24 h for the uptake of 4 electrons. The organic phase was 30 ml ethylene glycol diethyl ether. 

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