Enzymic asymmetric hydrolysis of D,L-p-hydroxyphenylglycine methyl ester in aqueous ionic liquid co-solvent mixtures.

An aqueous solution of ionic liquid BMIM.BF(4) (1-butyl-3-methylimidazolium tetrafluoroborate) was used as the reaction medium for the asymmetric hydrolysis of D,L-p-hydroxyphenylglycine methyl ester to enantiopure L-HPG (L-p-hydroxyphenylglycine) for the first time. Papain was screened out from five proteases and two lipases tested because of its high activity and enantioselectivity for this reaction. A comparative study demonstrated that papain is more active, enantioselective and stable in the sodium phosphate buffer solution of BMIM.BF(4) than in aqueous buffer and in the buffer solutions of several typical organic solvents examined. L-HPG with an e.e. (enantiomeric excess) of 95.6% and the yield of 47.4% (on a mole basis) could be obtained when the reaction was performed in a buffer solution of 12.5% (v/v) BMIM.BF(4) (pH 7.0). The aqueous ionic liquid co-solvent increased papain's half-life by 3.1-, 3.8-, 4.7- and 16.3-fold when compared with aqueous buffer, aqueous propan-2-ol, aqueous acetonitrile and aqueous 1,4-dioxan co-solvent respectively at 40 degrees C. When hydrolysis was conducted under reduced pressure, the yield, the e.e. of the L-HPG and the e.e. of remaining D-hydroxyphenylglycine methyl ester achieved were as high as 49.8, 98.1 and >99.0% respectively.