Ultrasound-Assisted Deep Eutectic Solvent Extraction of Total Flavonoids from Hainan Li-Medicine Polygonum hydropiper: Process Optimization, Antioxidant Activity and Molecular Mechanism

Ultrasound-assisted deep eutectic solvent (DES) extraction was developed for the efficient recovery of total flavonoids from Polygonum hydropiper L., a traditional Li medicinal plant from Hainan, China. Among 30 DES formulations screened, betaine-glycerol (Bet-Gly, 1:2) afforded the highest total flavonoid yield and was selected for further optimization. The extraction conditions were systematically optimized through single-factor experiments, Plackett–Burman screening, and Box–Behnken response-surface methodology. The predicted optimal conditions (water content 28.08%, solid-to-liquid ratio 1:37.58, and extraction time 30.69 min) were adjusted to 28.10%, 1:38, and 31 min for practical operation, yielding 3.98 mg RE/g DW, in close agreement with the predicted value of 4.13 mg RE/g DW. Compared with 70% ethanol and water, Bet-Gly increased the total flavonoid yield by 39.6% and 131.4%, respectively (p<0.05), and the resulting extract showed the lowest IC50 values in both ABTS and DPPH radical-scavenging assays. UPLC-Q-TOF-MS/MS tentatively identified eight phenolic constituents, mainly flavonoids, in the DES extract. Molecular dynamics simulations further revealed more homogeneous distributions of five representative flavonoids and more favorable flavonoid-solvent interaction energies in Bet-Gly than in water or 70% ethanol, indicating that electrostatic and van der Waals interactions contributed to enhanced flavonoid solvation. Overall, this study integrates DES screening, process optimization, chemical characterization, antioxidant evaluation, and molecular simulation, linking the improved extraction performance of Bet-Gly with favorable flavonoid-solvent interactions at the molecular level.