Activity and thermal stability of lysozyme in alkylammonium formate ionic liquids—influence of cation modification

Abstract

The stability and activity of hen's egg white lysozyme in the presence of four protic room temperature ionic liquids (ethylammonium formate (EAF), propylammonium formate (PAF), 2-methoxyethylammonium formate (MOEAF) and ethanolammonium formate (EtAF)) have been investigated. Near UV CD experiments have been used to determine protein structure in aqueous solutions of 25 wt%, 50 wt% and 75 wt% ionic liquid, and to assess the proteins ability to refold after heating to 90 °C. It was determined that EAF and MOEAF are similarly effective refolding additives, while PAF is more effective at promoting refolding at concentrations up to ∼62.5 wt%, but at higher PAF concentrations the protein spontaneously denatures. Both of these effects are attributed to the increased hydrophobicity of the cation. EtAF is shown to stabilise lysozyme against unfolding at high temperature, and renaturing appears to be near complete upon cooling. Studies of enzyme kinetics reveal increased protein activity in the presence of all ionic liquids examined, but the most significant increase is noted for EtAF, where rates are six times higher than those determined for lysozyme in buffered water.