Enzymatic esterification in aqueous miniemulsions.

Monoesters of various linear carboxylic acids (C7-C12) with omega-phenyl-labeled primary alcohols (C1-C5) were synthesized in aqueous miniemulsions with various lipases as biocatalysts. The reactants were dispersed in an aqueous solution of a nonionic surfactant to form long-term stable miniemulsions. The esterification of all of the systems could be catalyzed by the applied enzyme and yielded significant conversions of about 90%. The hydrophilicity of the reactants and the specificity of the enzyme toward the substrates determine the reaction velocity and the final conversion. As a model system the reaction of nonanoic acid and 3-phenylpropanol was extensively studied. Among various lipases, Lipase PS was determined to be the most effective, and for this reaction the parameters were optimized. A maximum conversion of 80% could be obtained in less than one hour of reaction time. In comparison with an acid-catalyzed esterification performed in miniemulsion with the same reaction parameters, the enzyme-catalyzed reaction showed a significantly faster conversion. The reactions proved that the application of the miniemulsion technique enables efficient direct enzyme-catalyzed esterification reactions from carboxylic acids and alcohols in the presence of large amounts of water.