Highly Efficient Enzymatic Conversion of Rutin to Isoquercitrin and l-Rhamnose Using Deep Eutectic Solvents

Deep eutectic solvents (DESs), considered to be "green media", have recently been employed in multiple applications including biotransformations. Here, a DES was used in the production of isoquercitrin and l-rhamnose from rutin by enzymatic transformation with recombinant α-l-rhamnosidase from Aspergillus niger. A solution synthesized from choline chloride and urea (ChCl-U) performed best in the conversion of rutin among 24 kinds of DESs. When 130 g/L rutin was used as the substrate, the conversion rate was 100% and the productivity of l-rhamnose and isoquercitrin was 208.68 mM/h under optimized conditions in ChCl-U solution, which was 1.12 times higher than that in buffer (186.92 mM/h). The recycling of ChCl-U solution was also investigated by aqueous two-phase system extraction of l-rhamnose, and ChCl-U/K2HPO4 was screened as the suitable extraction solvent. When the ChCl-U content was 34%, the extraction efficiency of l-rhamnose was 79.06% with a purity and primary crystallization yield of 96.28 and 66.49%, respectively. After five cycles of ChCl-U solution, the average enzymatic conversion rate of recycling was 95.63% with an average purity of 94.22% of the isoquercitrin prepared. After 10 cycles of reuse, the residual conversion rate was 61.14%. The results indicate that the DES has obvious advantages in biocatalytic processes involving flavonoid conversion.