Boronate affinity-mediated magnetic solid phase extraction and bioactivities of polysaccharides from beverage plants

Polysaccharides are of great significance in food production, but their isolation highly relies on multi-staged liquid-liquid extraction. In this study, a boronate affinity-mediated magnetic solid phase extraction (BA-MSPE) method was initiated for the effortless and efficient extraction of polysaccharides using boronic acid-grafted magnetic nanospheres (MNPs@B(OH)₂) as extractants. MNPs@B(OH)₂ showed fine class selectivity toward cis-diol containing compounds at weak alkaline condition (pH 7.5~8.5) and higher binding capacity than that of MNPs without boronic acid functionalization. Fast binding dynamics with a binding equilibrium within 10 min, stronger affinity toward polysaccharides (K_d as low as 10⁻³~10⁻⁶ M level) than that of small molecular cis-diol compounds (K_d in the range of 10⁻¹~10⁻⁴ M level), and good recyclability (the binding capacity decreased less than 13% after ten times consecutive extraction) could also be observed for MNPs@B(OH)₂. Finally, the BA-MSPE of polysaccharides was performed with three beverage plants as real samples, including tea leaves, soybeans, and Lycium barbarum. Antioxidant activity of polysaccharide extractives was verified by DPPH radical scavenging assays, giving a radical scavenging rate of 31.4% and 18.8% for crude extractives of TPS (tea polysaccharide) and LBPS (Lycium barbarum polysaccharide), respectively. Microscopic imaging combining with MTT and trypan blue staining trials uncovered that the extractives were of dosage-dependent antitumor bioactivities, giving the cell mortality rates over 91.8% and 77.2% for MCF-7 and A549 cells in the presence of 5.0 mg/mL TPS, and 56.6% and 40.0% with the equal dosage of LBPS, respectively. As the BA-MSPE strategy is simple and eco-friendly, there will be more potential for the application of cis-diol compound purification.