Structural characteristics and biological activities of a pectic-polysaccharide from okra affected by ultrasound assisted metal-free Fenton reaction

Abstract This study aimed to investigate the impact of ultrasound assisted metal-free Fenton (H2O2-ascorbic acid system) reaction on structural and biological properties of a pectic-polysaccharide extracted from okra (OPP). Three degraded fractions (DOPP-1, DOPP-2, and DOPP-3) were successfully prepared by ultrasound assisted Fenton treatment of OPP for 0.5, 1.0, and 3.0 h, respectively. Molecular weights of OPP, DOPP-1, DOPP-2, and DOPP-3 were determined to be 2.84 × 105, 1.90 × 105, 8.27 × 104, and 4.65 × 104 Da, respectively. Besides, the ultrasound assisted Fenton treatment could obviously reduce the apparent viscosity, radius of gyration, and branching degree of OPP. However, its degree of esterification notably increased, suggesting that free uronic acid was more susceptible to hydroxyl radicals generated by H2O2-ascorbic acid system than that of esterified uronic acid. Furthermore, the ultrasound assisted Fenton treatment made no significant changes on the backbone and chain conformation of OPP. Moreover, an appropriate ultrasound assisted Fenton treatment can remarkably promote the antioxidant, hypoglycemic, and immunomodulatory activities of OPP. Indeed, the antioxidant and hypoglycemic activities of OPP were associated with its molecular weight and degree of esterification, and its immunomodulatory activity was closely correlated to its degree of branching and molecular weight, as well as degree of esterification. Results suggest that the ultrasound assisted metal-free Fenton reaction has potential applications in degradation and modification of okra pectic-polysaccharides to improve their biological activities. Furthermore, the degraded products derived from okra pectic-polysaccharides have potential application for the development of functional foods with antioxidant, hypoglycemic, and immunomodulatory activities.