Pickering Emulsion as an Efficient Platform for Enzymatic Reactions without Stirring

To address the current limitations of enzymatic reactions, we develop a novel strategy to conduct stirring-free biphasic enzymatic reactions. This strategy involves translation of a conventional biphasic enzymatic reaction to a water-in-oil (W/O) Pickering emulsion system by adding a small amount of solid particle emulsifier. In such a system, enzymes, for example, a Candida Antarctica lipase B (CALB), are compartmentalized within millions of micron-sized water droplets, while organic substrates are dissolved in the oil phase (outside the droplets). It was demonstrated that CALB-catalyzed hydrolysis kinetic resolution of racemic esters in the stirring-free Pickering emulsion system gave favorable reaction efficiency and enantioselectivity as compared to those for the conventional biphasic system under stirring conditions, which was due to the large reaction interfacial area and the short molecule distances created by the Pickering emulsion droplets. The specific activity was found to depend on the water droplet size, highlighting the importance of the presence of droplets in the reaction system. Moreover, the convenient and effective recycling of CALB could be achieved through simple demulsification by centrifugation. After 27 reaction cycles, the ee values of ester and alcohol were still as high as 87.5% and 99%, respectively, which significantly exceed those of the conventional biphasic reaction. The high recyclability may be attributed to avoiding stirring that often causes damage to the three-dimensional structure of enzymes. This study compellingly demonstrates that a Pickering emulsion is an innovative platform to efficiently process enzymatic reactions without need for stirring and immobilization.