Extraction of flavonoids from Glycyrrhiza residues using deep eutectic solvents and its molecular mechanism

• Fourteen deep eutectic solvents (DESs) were used to extract flavonoids from Glycyrrhiza residues. • ChCl-glycolic acid had the greatest capacity for extracting four flavonoids. • Optimal extraction rate was 83.03% higher than that using 60% ethanol. • Molecular mechanism associated with DES-flavonoid interactions were analyzed. • Hydrogen bond strength was significant for the DES-flavonoid extraction process. The re-extraction of bioactive compounds from Chinese herb residues is significant for the maximum utilization of biomass resources. However, conventional alcoholic and aqueous solvents are unsuitable for recovering those compounds. In this study, 14 deep eutectic solvents (DESs) were used to improve the efficiency of flavonoid extraction from Glycyrrhiza residues. The maximal total extraction rate (10.68 mg/g) for four flavonoids (liquiritin, isoliquiritin, liquiritigenin, and isoliquiritigenin) was achieved using choline chloride-glycolic acid as the DES under optimal conditions. The extraction rate was 83.03 % higher than that by 60 % ethanol, a traditional solvent. Analysis of the associated molecular mechanism based on density functional theory showed that interactions between the solvent and liquiritin were dominated by hydrogen bonds followed by Van der Waals forces, whereas the bonding between the solvent and liquiritigenin only involved Van der Waals forces, thereby verifying the significance of the strength of hydrogen bonding in the DES-flavonoid extraction process.