Enzyme-mediated free radical polymerization of acrylamide in deep eutectic solvents

Abstract

Deep eutectic solvents (DES) have attracted considerable attention as reaction media in biocatalysis due to their particular solvent properties. It has been demonstrated that some enzymes do not denature in these solvents and that under certain conditions, enzyme-catalyzed reactions in DES can result in higher conversions and reaction rates than those obtained in conventional media. In this work we report the enzyme-mediated free radical polymerization of acrylamide in nearly non-aqueous choline chloride–urea (CCl–U) and choline chloride–glycerol (CCl–Gly) DES. The catalytic activity of the enzyme, horseradish peroxidase (HRP), was observed to be lower at high DES concentrations than in phosphate buffer solution, whereas the thermal stability was enhanced. According to fluorescence and UV-vis spectroscopy studies, the drop in the enzyme activity could be a consequence of the partial denaturation of HRP in the “hydrated” DES. Despite this significant loss in activity, HRP was still able to initiate the free radical polymerization of acrylamide with full monomer conversions. For the CCl–U system, polymers with similar average molecular weight and slightly narrower polydispersities compared to those synthesized in a totally aqueous environment were obtained. Furthermore, taking advantage of the low freezing point of CCl–Gly, it was possible to synthesize polyacrylamide at 4 °C, while no polymer was obtained using water at the same temperature. These results illustrate the potential of DES to enable biotransformations in a wide range of temperatures, leading for instance to new strategies in materials synthesis, such as enzymatic cryo-polymerization.