Investigation of the interaction of pepsin with ionic liquids by using fluorescence spectroscopy.

The molecular mechanism of the interaction between pepsin and two typical ionic liquids (ILs), 1-butyl-3-methylimidazolium chloride ([C4mim]Cl) and 1-octyl-3-methylimidazolium chloride ([C8mim]Cl), was investigated with fluorescence spectroscopy, ultraviolet absorption, and circular dichroism spectroscopy at a pH value of 1.6. The results suggest that ILs could quench the intrinsic fluorescence of pepsin, probably via a dynamic quenching mechanism. The fluorescence quenching constants were determined by employing the classic Stern-Volmer equation. The constant values are very small, indicating that only a very weak interaction between ILs and pepsin exists. The Gibbs free-energy change, enthalpy change (?H), and entropy change (?S) during the interaction of pepsin and ILs were estimated. Positive values of ?H and ?S indicate that the interaction between ILs and pepsin is mainly driven by hydrophobic interaction. Synchronous and three-dimensional fluorescence spectra demonstrate that the addition of ILs (0-0.20 mol L?1 for each IL) does not bring apparent changes to the microenvironments of tyrosine and tryptophan residues. Activity experiments show that the activity of pepsin is concentration dependent; higher concentrations of ILs (>0.22 mol L?1 for [C8mim]Cl and >0.30 mol L?1 for [C4mim]Cl) cause the remarkable reduction of enzyme activity. The presence of ILs also does not improve the thermal stability of pepsin.