Enantioselective Oxidation of Racemic Citronellol with an Interface Bioreactor

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Microbial oxidation of racemic citronellol was done on the interface between a nutrient agar plate and decane (interface bioreactor). While Rhodococcus accumulated (R)-Citronellal (21-80% e.e.), many yeasts accumulated (S)-citronellic acid (20-68% e.e.) as the oxidation product. For the production of (R)-citronellal and (S)-citronellic acid, Rhodococcus equi IFO 3730 and Hansenula anomala IFO 0147 were the best strains, respectively. While citronellol-oxidation in R. equi IFO 3730 was (S)-selective, citronellal-oxidation was (R)-selective. Although H anomala had also both (S)-selective alcohol and aldehyde dehydrogenases (constitutive, NAD⁺ -dependent), the equilibrium of the alcohol dehydrogenase Shifted to. reduction Of citronellal. Geotrichum candidum JCM 01747 accumulated very high concentrations of citronellic acid (conversion yield 90% in 5 days or accumulation 65 mg/ml in 13 days) in spite of the strong toxicity of the acid, although the optical purity was low. For the alleviation of the toxicity of citronellol and accumulation of toxic citronellic acid, the interface bioreactor is superior to the organic-aqueous two-liquid-phase system (dispersion system).