Ether-functionalized ionic liquids for nonaqueous biocatalysis: Effect of different cation cores

Ether-functionalized ionic liquids (ILs) usually have low viscosities, and can be designed to be compatible with enzymes. However, there is a lack of understanding of the effect of different ether-functionalized structures on the enzyme activity. We systematically evaluated new ether-functionalized ILs carrying different cation cores (pairing with Tf₂N⁻ anions) in two Novozym 435-catalyzed reactions: (1) the transesterification of ethyl sorbate with 1-propanol at 50 °C; (2) the ring-opening polymerization (ROP) of ε-caprolactone at 70 °C. The lipase showed different activities: in the first reaction, [CH₃OCH₂CH₂-Et₃N][Tf₂N] and [CH₃OCH₂CH₂-Py][Tf₂N] gave the highest reaction rates; in the second reaction, [CH₃OCH₂CH₂-PBu₃][Tf₂N] produced the highest molecular mass (M_w up to 25,400 Da). The lipase's thermal stability in [CH₃OCH₂CH₂-Et₃N][Tf₂N] was found much higher than that in t-butanol. The fluorescence spectra of free lipase (excited at 280 nm) in these ILs reveal that the wavelength of the maximum emission peak occurred at 314 nm for both [CH₃OCH₂CH₂PBu₃][Tf₂N] and [CH₃OCH₂CH₂PEt₃][Tf₂N], which matched closely with that (313 nm) in aqueous phosphate buffer (pH 7.5, 20 mM), while other ether-functionalized ILs led to various degrees of red shifts. In summary, the lipase activity is not only dependent on the IL structure, but also on the substrate and other reaction conditions.