Study On Lipase-Catalyzed Resolution Of DL-Menthol

Menthol is the main component of peppermint oil and is one of the three major spices in the world.It exists in eight stereoisomeric forms but only L-menthol has great commercial application value,which is widely used in production of food,medicine,cosmetics,tea,cigar and so on.In this paper,lipase-catalysed resolution of DL-menthol and the optimization of reaction conditions in organic solvent and solvent-free system were studied.GC method was established to analyze the conversion of substrates,optical purity of products and concentration of by-products using the chromatographic column CP-Chirasil-Dex CB and AT-FFAP.Through the screening of several lipases with transesterification activity,we could get Lipozyme TL IM(Thermomyces lanuginosus lipase)which exhibited good enantioselectivity and high efficiency for resolution of DL-menthol.The most suitable way to preserve enzyme was mixed with molecular sieve e(4A)uniformly,then storing at 4?refrigerator.Effect of various reaction parameters on the conversion and enantioselectivity were studied,including type of acyl donor,solvent,molar ratio,temperature,enzyme loading and substrate concentration.The optimum reaction conditions were found to be transesterification with vinyl acetate at 30?in methyl tert-butyl ether(MTBE)with a mole ratio(vinyl acetate:DL-menthol)of 5:1,molecular sieve of 1.8 g and an enzyme concentration of 200 g/L.At optimum reaction conditions,L-menthyl acetate with 99.3%enantiomeric excess(ee_p)was obtained when the conversion was 34.7%with the reaction time of 12 h at the substrate concentration of 0.5 M.Moreover,the enzyme displayed a strong tolerance for high substrate concentration of up to 1.5 M without loss of the enantioselectivity.The enzyme activity was reserved 85.2%with a good enantioselectivity after 20 cycles of reaction.Vinyl acetate was employed as acyl donor and solvent in solvent-free system.The optimum conditions were achieved at a mole ratio(vinyl acetate:DL-menthol)of2:1,molecular sieve of 1.8 g and an enzyme concentration of 0.5 g at 30?.Under the above conditions,ee_p was 99.1%when the conversion was 22.7%after 6 h.The recyclability of the enzyme in solvent-free system was inferior to that in organic solvent,and the enzyme activity was reserved 85.2%after 20 cycles of reaction.The results indicated that Lipozyme TL IM was a promising biocatalyst in the resolution of DL-menthol in organic solvent and solvent-free system.