Tertiary/quaternary amine derivatives of sophorolipid for baicalin solubility and antioxidating performance improvement: solubilization or hydrotropy?

Baicalin is a flavonoid compound extracted from the Scutellaria baicalensis plant. It possesses strong free radical scavenging ability and has a wide range of medical applications in antibacterial, anti-inflammatory, and anti-tumor fields. However, its limited solubility in water restricts its application. In this study, we prepared SL-DMEA by the amidation of acidic sophorolipids and N, N-dimethylethanolamine, while SL-TMEAI was synthesized via the quaternization of SL-DMEA with iodomethane. The structures of SL-DMEA and SL-TMEAI were confirmed by 1 H NMR, FT-IR, and Mass spectra. Surface tension and micelle microenvironment indicated that SL-DMEA and SL-TMEAI exhibit superior surface activity. Furthermore, the solubility of baicalin in aqueous solutions containing SL-DMEA and SL-TMEAI was approximately 70 mg/L and 75 mg/L, while it was only 18 mg/L in SL-COOH solution, which were significantly higher than the solubility in pure water (4 mg/L). Similarly, SL-DMEA and SL-TMEAI can enhance the antioxidant activity of baicalin. Specifically, in the SL-TMEAI aqueous solution, the free radical scavenging rate of baicalin was increased by 10 %–20 %. Density functional theory calculations revealed that SL-DMEA and SL-TMEAI formed stable hydrogen bonds with baicalin molecules, which can increase the solubility of baicalin in water and expose the groups that react with free radicals in the solution, thereby enhancing the antioxidant activity. These results demonstrated that SL-DMEA and SL-TMEAI could significantly enhance the solubility and antioxidating of baicalin in water through a hydrotropy mechanism.