Isolation, Structural Characterization, and In Vitro Antioxidant Activity of Polysaccharides from Cynanchum auriculatum Royle ex Wight

A novel polysaccharide from Cynanchum auriculatum Royle ex Wight was isolated, structurally characterized, and its antioxidant activity was evaluated. The crude extract was purified by ion exchange and size exclusion chromatography to obtain a homogeneous fraction, CAP2-1. CAP2-1 displayed a weight-average molecular mass of 184.17 kDa and is mainly composed of galactose, arabinose, and galacturonic acid. Structural analysis revealed that CAP2-1 is a highly branched acidic arabinogalactan-type polysaccharide with a backbone of →6)-β-D-Galp-(1→, →3,6)-β-D-Galp-(1→, and →4)-α-D-GalpA-(1→ units, and side chains enriched in α-L-arabino furanose residues. Ultrasonic degradation produced a lower-molecular-weight derivative, UCAP2-1, which exhibited significantly stronger free radical scavenging ability compared with CAP2-1 (p < 0.01). These findings suggest that molecular weight reduction enhances antioxidant properties by improving electron-donating capacity and accessibility to reactive sites. This study reveals the structure–antioxidant relationship of CAP2-1 and UCAP2-1 and highlights UCAP2-1 as a promising natural antioxidant.