The activity and molecular interaction of lysozyme in adding four ionic liquids aqueous solutions

Abstract

In this work, four hydrophilic ionic liquids (ILs, 1-butyl-3-methylimidazolium tetrafluoroborate ([C4mim]BF4), 1-butyl-3-methylimidazolium chloride ([C4mim]Cl), 1-butyl-3-methylimidazolium bromide ([C4mim]Br) and 1,3-dimethylimidazolium iodide ([dmim]I)) were selected to probe their effect on lysozyme activity and the molecular interaction in aqueous solutions. Specifically, the effects of pH and concentration of ILs on lysozyme activity were investigated by using the spectrophotometric turbidity assay. The results demonstrated that the effect of ILs on lysozyme activity depended on the concentration of ILs. Fixing the pH at 6.85, lysozyme activity rose firstly and then reduced with the increase of concentration of four ILs, reaching the maximum at 75 mM. The UV–Vis spectra illustrated that the interaction between ILs and lysozyme was weak. The results of emission and synchronous fluorescence spectra indicated that ILs had a quenching effect on the fluorescence of lysozyme, and the quenching effect increased with the increasing concentration of ILs. Meanwhile, the fluorescent quenching mechanism of ILs on lysozyme was dynamic quenching, and ILs had significant effects on the tryptophan (Trp) residues Trp62 or Trp108 at the active site of lysozyme, which suggested that effects of ILs on lysozyme activity could be attributed to the conformational changes of lysozyme triggered by changes in the microenvironment surrounding Trp62 or Trp108.