Using ionic liquid [EMIM][CH₃COO] as an enzyme-'friendly' co-solvent for resolution of amino acids

An ionic liquid (IL), 1-ethyl-3-methylimidazolium acetate [EMIM][CH₃COO], was used in 0-4.0 M (∼60% IL, v/v), as a nonvolatile organic medium for the enzymatic resolution of amino acids. When dl-phenylalanine methyl ester was studied as a model substrate, high enantiomeric excesses (ee) of l-amino acid were obtained in all ionic concentrations; however, lower yields were observed at high IL concentrations. This IL is more enzyme-'friendly' than the hydrophilic organic solvent acetonitrile and those ILs containing chaotropic anions (such as [EMIM][OTs]). Among three proteases and two lipases investigated, lyophilized Bacillus licheniformis protease exhibited the best enantioselectivity and activity. Highly enantioselective resolutions were also produced for several other amino acids in 2.0 M IL. Interestingly, high ee were also found in deuterium oxide (D₂O) rather than in ordinary water, and a further enhancement was achieved with the co-existence of [EMIM][CH₃COO]. The heavy water effect was explained in terms of protein stabilization by D₂O. The secondary structural changes of enzyme in various media were interpreted by the second derivatives of FT-IR spectra.