Abstract
BackgroundBiocatalytic asymmetric reductions with whole cells can offer high enantioselectivity, environmentally benign processes and energy-effective operations and thus are of great interest. The application of whole cell-mediated bioreduction is often restricted if substrate and product have low water solubility and/or high toxicity to the biocatalyst. Many studies have shown that a biphasic system is often useful in this instance. Hence, we developed efficient biphasic reaction systems with biocompatible water-immiscible ionic liquids (ILs), to improve the biocatalytic anti-Prelog enantioselective reduction of acetyltrimethylsilane (ATMS) to (R)-1-trimethylsilylethanol {(R)-1-TMSE}, which is key synthon for a large number of silicon-containing drugs, using immobilized Candida parapsilosis CCTCC M203011 cells as the biocatalyst.ResultsIt was found that the substrate ATMS and the product 1-TMSE exerted pronounced toxicity to immobilized Candida parapsilosis CCTCC M203011 cells. The biocompatible water-immiscible ILs can be applied as a substrate reservoir and in situ extractant for the product, thus greatly enhancing the efficiency of the biocatalytic process and the operational stability of the cells as compared to the IL-free aqueous system. Various ILs exerted significant but different effects on the bioreduction and the performances of biocatalysts were closely related to the kinds and combination of cation and anion of ILs. Among all the water-immiscible ILs investigated, the best results were observed in 1-butyl-3-methylimidazolium hexafluorophosphate (C4mim·PF6)/buffer biphasic system. Furthermore, it was shown that the optimum substrate concentration, volume ratio of buffer to IL, buffer pH, reaction temperature and shaking rate for the bioreduction were 120 mM, 8/1 (v/v), 6.0, 30°C and 180 r/min, respectively. Under these optimized conditions, the initial reaction rate, the maximum yield and the product e.e. were 8.1 μmol/min gcwm, 98.6% and >99%, respectively. The efficient whole-cell biocatalytic process was shown to be feasible on a 450-mL scale. Moreover, the immobilized cells remained around 87% of their initial activity even after being used repeatedly for 8 batches in the C4mim·PF6/buffer biphasic system, exhibiting excellent operational stability.ConclusionsFor the first time, we have successfully utilized immobilized Candida parapsilosis CCTCC M203011 cells, for efficiently catalyzing anti-Prelog enantioselective reduction of ATMS to enantiopure (R)-1-TMSE in the C4mim·PF6/buffer biphasic system. The substantially improved biocatalytic process appears to be effective and competitive on a preparative scale.
Highlights
Biocatalytic asymmetric reductions with whole cells can offer high enantioselectivity, environmentally benign processes and energy-effective operations and are of great interest
Effect of various water-immiscible ionic liquids (ILs) on the anti-Prelog enantioselective reduction of ATMS to (R)-1-TMSE with immobilized Candida parapsilosis CCTCC M203011 Many studies have shown that a biphasic system is often useful in whole-cell biocatalysis if substrate and product have low water solubility or high toxicity to the biocatalyst [10,25,26]
The cell viability of immobilized Candida parapsilosis CCTCC M203011 with and without the addition of substrate ATMS were studied in the aqueous monophasic system as well as the IL-based biphasic systems
Summary
Biocatalytic asymmetric reductions with whole cells can offer high enantioselectivity, environmentally benign processes and energy-effective operations and are of great interest. Enantiopure silicon-containing chiral alcohols are becoming increasingly attractive due to the unique physical and chemical characteristics of the silicon atom, such as its larger atomic radius and smaller electronegativity than the carbon atom [3] These silicon-containing compounds play an important role in asymmetric synthesis and functional materials, and in the preparation of silicon-containing drugs, such as Zifrosilone, Cisobitan and TAC-101{4-[3,5-bis(trimethylsilyl)benzamido] benzoic acid}, which possess greater pharmaceutical activity, higher selectivity and lower toxicity than their carbon counterparts [4,5,6]. When compared to conventional chemical methods, biocatalytic asymmetric reductions using isolated enzymes or whole cells as biocatalyst, can offer high enantioselectivity, environmentally benign processes and energy-effective operations and of great interest [10]. The major advantages of using whole cells rather than isolated enzymes as biocatalysts are that cells provide a natural environment for the enzymes, preventing conformational changes in the protein structure that would lead to loss of activity in non-conventional medium, and are able to efficiently regenerate the expensive cofactors [11]
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