Specific ion effects of ionic liquids on enzyme activity and stability

Catalytic performance of two model enzymes, Penicillium expansum lipase (PEL) and mushroom tyrosinase, was examined in aqueous solution with addition of 14 different ionic liquids (ILs) and has been found to correlate well with the IL's kosmotropic/chaotropic properties, which are assessed by the viscosity B coefficients (B+ for cations and B− for anions). The activity and stability of PEL were similarly correlated with B+ values but not with B− of the ILs tested. PEL can be activated and stabilized by addition of ILs (e.g., 5.2-fold activation and 1.4-fold stabilization in the presence of 0.63 M [choline][Ac] and 0.27 M [NHMe3][MeSO3], respectively). Choline ILs activated PEL but imidazolium ones deactivated it. The results indicate that the IL cations play a crucial role in affecting the enzyme performance and that ammonium ILs composed of chaotropic cations (favorably with H-bonding capability) and kosmotropic anions are favored for enzyme catalysis. The Hofmeister effect of ILs on PEL was confirmed by the kinetic and thermostability studies and structural analysis on tyrosinase. Our investigations on both enzymes have thus demonstrated that ILs can affect the enzyme functioning through the Hofmeister effect, and the mechanisms have been discussed in terms of the influence of the IL cations and anions on the surface pH, active site conformation, and catalytic mechanism of each specific enzyme, following the Hofmeister series.