The molecular mechanism of the interaction between pepsin and two typical ionic liquids (ILs), 1-butyl-3-methylimidazolium chloride ([C<sub>4</sub>mim]Cl) and 1-octyl-3-methylimidazolium chloride ([C<sub>8</sub>mim]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 (?<i>H</i>), and entropy change (?<i>S</i>) during the interaction of pepsin and ILs were estimated. Positive values of ?<i>H</i> and ?<i>S</i> 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<sup>?1</sup> 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<sup>?1</sup> for [C<sub>8</sub>mim]Cl and >0.30 mol L<sup>?1</sup> for [C<sub>4</sub>mim]Cl) cause the remarkable reduction of enzyme activity. The presence of ILs also does not improve the thermal stability of pepsin.

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