Asymmetric Reduction Of Acetophenone By Candida Valida Under Compressed CO₂

The asymmetric reduction catalyzed by whole cells is a promising way for the synthesis of chiral compounds, with the advantages of mild reaction conditions and high stereoselectivity. Besides, changing mediums or systems of asymmetric reduction is also a field received great interests. The asymmetric reduction of acetophenone(ACP) to optically active a-phenylethanol(α-PEA) by Candida valida under compressed CO₂ was investigated in this work.Firstly, Candida valida was treated by compressed CO2 before the asymmetric reduction under atmospheric pressure. In this reduction, (R)-α-PEA was excessive with 70-80% enantiomeric excess (e.e.). The effects of different treatment conditions were investigated. With the increasing of treatment time pressure and temperature,a-PEA yield was obviously decreased, while e.e. values showed a bit changed.Then, the asymmetric reduction of ACP by free Candida valida under compressed CO2 was investigated. The properties of reaction were studied by changing reaction time, ACP concentration, cells mass, temperature, pressure, adding NaHCO₃ , co-substrate, and agitation, etc. It was shown that this biocatalysis under compressed CO2 was both feasible and effective. The optimal reaction conditions was obtained: 24.4mmol/L of ACP, 0.1g/ml of glucose as co-substrate, 0.4g/ml of wet cells in 20ml PBS buffer(0.2mol/L, pH 7.0) under 6.0MPa, 30℃ reacting for 30 hours. α-PEA yield and e.e. of (R)-α-PEA could reach as high as 90% and 80-90% respectively, superior compared with that of reduction under atmospheric pressure.Finally, controlling of stereoselectivity were tried, like changing cell cultivation ways, heat pretreatment and using different yeasts. We found that cells in log phase of aerobic fermentation performed higher a-PEA yield and e.e. of (R)-a-PEA. However, heat pretreatment had no positive effects on the stereoselectivity. The asymmetric reduction catalyzed by baker's yeast behaved much lower a-PEA yield than that by Candida valida, and (S)-a-PEA was excessive. Reuse of Candida valida in the reduction under compressed CO2 showed quick decreasing of yield and e.e..