Enantioselective oxidation of secondary alcohols at a quinohaemoprotein alcohol dehydrogenase electrode

Quinohaemoprotein alcohol dehydrogenase fromComamonas testosteroni was co-immobilized with a redox polymer (a poly(vinylpyridine) complex functionalized with osmium bis(bipyridine) chloride) on an electrode. The enzyme electrode readily oxidizes primary alcohols and secondary alcohols with maximum current densities varying between 0.43 and 0.98 A m^-2 depending on the substrate and the operation temperature. The affinity of the enzyme for aliphatic alcohols increases with the chain length of the substrate (i.e., 1-pentano1 [K_m = 0.006 mM] is a much better substrate than ethanol [K_m= 2.2 mM]). The same property is observed for secondary alcohols in the series 2-propanol (K_m = 22 mM) to 2-octano1 (K_m = 0.05 mM). The enzyme electrode is enantioselective in the oxidation of secondary alcohols. A strong preference is observed for the S-2-alcohols; the enantioselectivity increases with increasing chain length. The enantiomeric ratio (E) increases from 13 for (R,S)-2-butanol to approximately 80 for (R,S)-2-heptanol and (R,S)-2-octanol. This makes the enzyme electrode, potentially, a powerful tool for the preparation of a large range of alkanones and/or for the (kinetic) resolution of racemic alcohols.