Efficient enantioselective reduction of 4′-methoxyacetophenone with immobilized Rhodotorula sp. AS2.2241 cells in a hydrophilic ionic liquid-containing co-solvent system

The biocatalytic enantioselective reduction of 4′-methoxyacetophenone to (S)-1-(4-methoxyphenyl)ethanol was successfully conducted in a hydrophilic IL-containing co-solvent system using immobilized Rhodotorula sp. AS2.2241 cells. Of all the tested ILs, the best results were observed with the novel IL 1-(2′-hydroxy)ethyl-3-methylimidazolium nitrate (C₂OHMIM·NO₃), which showed a good biocompatibility with the cells and increased the cell membrane permeability moderately, thus improving the efficiency of the bioreduction. To better understand the bioreduction, several crucial influential variables were also examined. The optimal C₂OHMIM·NO₃ content, buffer pH, reaction temperature and substrate concentration were 5.0% (v/v), 8.5, 25 °C and 12 mM, respectively. Under the optimized conditions, the initial reaction rate, the maximum yield and the product e.e. were 9.8 μmol/h g_cell, 98.3% and >99%, respectively, which are much better than the results previously reported. The established biocatalytic system has proven to be highly effective for the reduction of other aryl ketones. Also, the cells exhibited excellent operational stability in the presence of C₂OHMIM·NO₃. Moreover, the ILs can accumulate within the cells, suggesting that ILs are likely to interact with the related enzymes within the cells.