A deep eutectic solvent binary-phase system for alkaloid extraction from Chinese herb Evodia lepta residue and its mechanism

Bioactive compound extraction using deep eutectic solvent has the challenges of high solvent viscosity and difficult product separation. In present study, we designed a biphasic deep eutectic solvent (DES) system (levulinic acid–glycerol–K2HPO4 aqueous solution with ethyl acetate phase) to extract alkaloids from the residue of the Chinese herb Evodia lepta. The aqueous phase of DES with low viscosity enabled the mass extraction of alkaloids from plant cells, and the extracted compounds were transferred in situ and concentrated in the hydrophobic ethyl acetate phase. The biphasic extraction reduced the alkaloid accumulation in the aqueous phase and improved the extraction efficiency. The optimal extraction process was the binary system consisting of 20 mL of ethyl acetate and 50 mL of DES of levulinic acid–glycerol with molar ratio of 1.5:1 containing 30% K2HPO4 solution (0.4 g/mL), with a solid-DES ratio of 1:20 g/mL, under the extraction conditions of 40 min, 75 °C. The total alkaloids with highest yield of 757.71 μg/g, 587.45% higher than that achieved using traditional methanol extraction, was transferred to the ethyl acetate phase with selectivity of 72.03, 39.16, and 80.67% for skimmianine, dictamnine, and evodiamine, respectively. The molecular mechanism based on the Density Functional Theory indicated that the present solvent possesses higher capacity of forming hydrogen bonds with alkaloids than other tested solvents. Moreover, evodiamine showed lower binding energy with ethyl acetate, which explained why evodiamine extracted into the aqueous phase transferred quickly to the hydrophobic phase.