Optimization of an Ultrasound-Assisted Aqueous Two-Phase Extraction Technique for Verbascoside From Leaves of Syringa pubescens Turcz. and Activity Evaluation

An efficient ultrasound-assisted aqueous two-phase extraction method was developed for the extraction of verbascoside from the leaves of Syringa pubescens Turcz. The ethanol/ammonium sulfate system was used as the extraction solvent. Based on the results of single-factor experiments, the key parameters, including ethanol concentration, ammonium sulfate concentration, and sample mass, were further optimized using response surface methodology. The optimal extraction conditions were obtained as follows: ethanol concentration of 30% (w/w), ammonium sulfate concentration of 19% (w/w), sample mass of 0.63 g, ultrasonic power of 150 W, extraction time of 30 min, and temperature of 40°C. Under these conditions, the experimental verbascoside yield (5.23 ± 0.29 mg/g) was in good agreement with the model-predicted value (5.33 mg/g), confirming the reliability of the established model. Furthermore, the verbascoside extract exhibited potent radical scavenging activities with half‑maximal inhibitory concentration values of 0.039 mg/mL against α, α-diphenyl-β-picrylhydrazyl and 0.058 mg/mL against 2,2'-azino-bis (3-ethylbenzthiazoline-6-sulphonic acid), as well as a ferric-reducing antioxidant power of 2.78 mg Fe 2 + /g at 2.5 mg/mL. This study demonstrates that the ultrasound-assisted aqueous two-phase extraction method is an efficient, green, and reliable approach for the extraction of verbascoside from S. pubescens leaves, and the obtained verbascoside extract shows promise as a natural antioxidant agent for potential applications in the food and pharmaceutical industries.