Effect of cations and anions of ionic liquids on the stability and activity of lysozyme: Concentration and temperature effect

Abstract

Herein, we have shown the effect of cations and anions of three different ionic liquids (ILs); 1-Decyl-3-methylimidazolium chloride [DMIM][Cl], 1-Butyl-1-methylpyrrolidinium tetrafluoroborate [BMP][BF4] and 1-Butyl-1-methylpyrrolidinium bromide [BMP][Br] on the stability and activity of hen egg white lysozyme (HEWL). The spectroscopic techniques; fluorescence, time resolved fluorescence, UV–visible, CD spectroscopy and molecular docking method were applied to investigate the differential behavior of HEWL in the aqueous solution of these ILs. Fluorescence results showed that imidazolium-based IL binds more strongly than pyrrolidinium based ILs with HEWL. The docking results suggested that these ILs bind at the active site of HEWL mainly through electrostatic, hydrophobic and van der Waals interactions. Results revealed that depending upon cations and anions, these ILs exert concentration dependent effect on HEWL. The conformational stability of HEWL is maintained in [BMP][BF4] and [BMP][Br] ILs while it decreases with increasing concentration of [DMIM][Cl]. A slight increase of 1 °C in melting temperature (Tm) of HEWL is obtained in presence of [BMP][BF4] and [BMP][Br] ILs while a much decrease of about 16.9 °C in Tm is obtained in presence of [DMIM][Cl]. Surprisingly, different effects of these ILs were observed on the activity of HEWL against Gram-positive bacteria; Micrococcus lysodeikticus. The activity of HEWL increases with increasing concentration of [BMP][BF4] and [DMIM][Cl] up to certain concentration range while it decreases with increasing concentration of [BMP][Br]. Comparatively, higher activity is obtained in presence of [DMIM][Cl] than [BMP][BF4].