Hofmeister series of ionic liquids: kosmotropic effect of ionic liquids on the enzymatic hydrolysis of enantiomeric phenylalanine methyl ester

The kinetic hydrolysis of enantiomeric phenylalanine methyl ester catalyzed by Bacillus licheniformis   protease was performed in aqueous solutions of several hydrophilic ionic liquids (ILs). The protease enantioselectivity was found related to the kosmotropicity of individual cations and anions of ILs. The ion effectiveness in enhancing the enzyme enantioselectivity follows the Hofmeister series: kosmotropic anions and chaotropic cations stabilize the enzyme. In this application, the Hofmeister series of ILs was established in an order of decreasing effectiveness for anions: PO43- > citrate3−, CH3COO−, EtSO4-, CF3COO− > Br− > OTs−, BF4- and for cations: [EMIM]+ > [BMIM]+ > [HMIM]+. The overall IL kosmotropicity was quantified by the δ value (difference in the Jones–Dole viscosity B-coefficients of anion and cation). In general, a high enzyme enantioselectivity was observed in a solution of IL with a high δ value.

Graphical abstractDuring hydrolysis of chiral phenylalanine, high enantiomeric ratios (E) of protease were observed in [EMIM][EtSO4] solutions. l-Phenylalanine (98% ee) was obtained in 0.5 M [EMIM][EtSO4] at 20 min reaction time.Figure optionsDownload full-size imageDownload as PowerPoint slide