Locating the binding domains of lysozyme with ionic liquids in aqueous solution via spectroscopic studies

The binding domains of lysozyme with ionic liquids (ILs, [C₄mim]BF₄, [C₄mim]Cl, [C₄mim]Br and [dmim]I) in aqueous solution was investigated by studying molecular interactions using spectroscopic techniques. Ultraviolet spectroscopy (UV) showed that the addition of ILs increased the absorption peak intensity of lysozyme at 210 nm by enhancing peptide bond valence electron transition. It is also found that a weak interaction between ILs and lysozyme chromophore groups was generated by analyzing the changes of absorption peak intensity near 280 nm. Fluorescence and Synchronous Fluorescence spectra results showed that four ILs had quenching effect on the fluorescent substances of lysozyme, and the quenching effect rose with increasing ILs concentration. Meanwhile, the interaction between lysozyme and ILs molecules is mainly based on Van der Waals force and two Tryptophan (Trp) residues (Trp62 or Trp108) at the active site of lysozyme molecules play a critical role in binding ILs to their own molecules.