Thermally controlled biotransformation of glycyrrhizic acid via an asymmetric temperature-responsive polyurethane membrane.

Separating a target product from a relatively complex bioreaction system is often difficult. In this work, a "smart" bioreaction system was developed by using the special characteristic of temperature-responsive polyurethane (TRPU). By combining solvent evaporation with a wet phase inversion technique, an asymmetric membrane consisting of an integral and dense skin layer supported by a porous sublayer was prepared from a thermally responsive polyurethane that experiences a sudden free volume increase upon heating through a phase transition temperature of 56 °C. Subsequently, the asymmetric TRPU membrane served as the carrier of an immobilized enzyme, wherein β-glucuronidase was multipoint-conjugated by using biotin and streptavidin on the porous sublayer. Then, the material-asymmetric TRPU membrane served jointly as the antennae as well as the actuator, which reversibly responds to temperature to switch (on-off) the access of the reactant glycyrrhizic acid (GL). Under the optimal temperature (40 °C) and pH (7.0) conditions, the immobilized β-glucuronidase contributed to almost 33% yield of glycyrrhetinic acid 3- O -mono-β-d-glucuronide (GAMG) of the isolated counterpart for the same concentration of substrate (250 mg L -1 ) reaction for 24 h, while costing 1% of that of the isolated β-glucuronidase. Kinetic results showed that V max and K m values were 8.89 × 10 3 mg L -1 and 2.30 × 10 3 mg L -1 h -1 , respectively. The specific functional polymer-immobilized β-glucuronidase design serves as a bioreactor of GL into GAMG, as well as a separator deliberately irritated and controlled by temperature. This "smart" support material presents a potential facilitator for the separation of complex biotransformation reactions.