Cholinium-based deep eutectic solvents and ionic liquids for lipase-catalyzed synthesis of butyl acetate

Abstract

The aim of this study was to analyze the advantages and limitations of cholinium-based ionic liquids (ILs) and deep eutectic solvents (DESs) used as green solvents for immobilized Candida antarctica lipase B-catalyzed synthesis. The reaction of acetic anhydride with 1-butanol to give short chain ester butyl acetate was chosen as a model reaction. Results showed that selected ILs (choline glycinate, choline alaninate, choline asparaginate, choline malate) and DESs (choline chloride mixtures with glycerol Gly, ethylene glycol EG, and urea U as hydrogen bond donors in molar ratio 1:2) are poor media for tested reaction if applied as pure solvents (yield <5%). This happens probably due to the substrate ⿿entrapment⿿ within DES structure through H-bonding. The addition of water (as a protic solvent) to the DESs strongly enhances both enzyme activity and reaction yield. Among different DES:water mixtures tested, the one with EG as hydrogen bond donor at water content of 5% (w/w) proved to be the most effective, resulting in esterification yield 80% (higher than yield obtained in a reference solvent n-heptane). Kinetic parameters of Ping-Pong Bi⿿Bi reaction mechanism were estimated for reactions in ChCl:EG:water mixtures and the chosen model seems to fit reaction mechanism description. Furthermore, in all ChCl:EG:water tertiary mixtures about 50⿿70% lipase activity decrease was observed after 3 days incubation. The enzyme activity stayed unchanged even after extending the incubation time to 15 days. Finally, the used DESs were assayed for cytotoxicity in fish (CCO) and human (MCF-7) cell lines, where they exhibited low cytotoxicity (EC50>5mM) for both cell lines. Obtained results suggest that DESs are promising candidates for green biocatalysis.