Lipase-catalyzed Esterification Of L-ascorbic Acid In Non-aqueous Solvent And The Optimization Of Reaction Conditions

L-ascorbic acid，as a necessary vitamin, keeps the normal physiological functions ofhuman, and it also has the effect of resistance to oxidation. While it is oxidized easily and haspoor solubility in lipids, to overcome such disadvantages, several kinds of L-ascorbic acidderivatives are synthesized. This essay studied the synthesis of L-ascorbyl acetate between ofL-ascorbic acid and vinyl acetate with immobilized lipase as catalyst.The first section describe the synthesis of L-ascorbyl acetate with TLIM lipase, which isfrom thermomyces lanuginosus and immobilized in silica gel. To raise convert ratio, reactionmedia、water activity、substance mole ratio、temperature、lipase loading and “pH” memoryare invested in the experiment. Only a single factor is changed while other factors keptconstant in the selection of reaction conditions. The optimal reaction condition is as follow:the reaction is proceeded in anhydrous acetone, substrate mole ratio of vinyl acetate andL-ascorbic acid is3:1, lipase loading is30mg/mL, and the temperature is40℃. Water activityand reaction media are critical factors in the lipase-catalyzed reaction. The exist of water isnot favored for the TLIM lipase, the relation defined by former scholar about logP of organicsolvent and the catalytic activity is not fitted for this reaction. Molecular sieve is soaked inacetone for24h to remove the water in acetone, and saturated salt solutions are used toprepare reaction systems with different water activity. Lipase keeps “pH” memory effect innonaqueous solvent and TLIM lipase show high catalytic activity in neuter condition.Novozym435is used to catalyze the synthesis of L-ascorbyl acetate in the second section,which is similar to the process of catalytic reaction with TLIM lipase as catalyst.Novozym435is obtained when candida antarctica is immobilized in macroporous acrylic acidresin. The optimal condition is below: reaction media is anhydrous acetone, mole ratio ofvinyl acetate and L-ascorbic acid is5:1, the concentration of lipase is3mg/mL, reactiontemperature is40℃. Water activity should be strictly controlled to obtain high convert ratio.Compared with TLIM lipase, Novozym435is more easy to be affected by water activity andtemperature, but Novozym435can show relatively high catalytic activity in wide range of pH.The conversion of L-ascorbic acid with both kinds of lipases have exceeded90%in their respective optimal conditions, while the lipase loading of TLIM is10times ofNovozym435.