Effect of microwave extraction and chemical modification on physicochemical characteristic and bioactivity of Prunella vulgaris L. polysaccharide-dominated-conjugates.

This study aimed to probe the influence of extraction and modification techniques on polysaccharide characteristic and activity. In this work, Prunella vulgaris L. (PVs) polysaccharides (PVPMs) were extracted by using an optimized microwave extraction technique, isolated by gel chromatography to obtain a polysaccharides-dominated-conjugates (PVPM-1), and modified by zincation, selenylation, sulfation and phosphorylation to obtain corresponding derivatives Zn-PVPM-1, Se-PVPM-1, S-PVPM-1 and P-PVPM-1. Chemical ingredient, monosaccharide composition, structural properties, antioxidant, hypoglycemic and antibacterial activities of PVPM-1 and derivatives were systematically compared. Results demonstrated that compared with the natural PVPM-1, the contents of uronic acid (UAC), total phenols (TPC), total flavones (TFC), sulfate group (SGC) and galacturonic acid (GalA) of the derivatives, increased by 11.45%-71.89%, 32.28%-110.12%, 3.02%-91.41%, 20.76%-81.86% and 33.33%-113.99%, but the molecular weight (Mw) and protein content (PC) decreased by 10.39%-34.61% and 12.22%-74.81%, respectively. Moreover, the modifications did not alter the primary monosaccharide types and main functional groups of PVPM-1, but induced mild degradation of polysaccharide chains, manifested as a relative decrease in neutral sugar content and an increase in UAC. Bioactivity assays showed that P-PVPM-1 exhibited the highest antioxidant capacity (compared with PVPM-1, the scavenging activities of DPPH·, ·OH, ·O2- and ABTS+· increased by 43.7%, 92.01%, 75.62% and 41.7%, respectively) and enzyme inhibition abilities (α-amylase and α-glucosidase activity enhanced by 107.63% and 126.67%), while Se-PVPM-1 possessed the best antibacterial activity (inhibitory capacities of E. coli and S. aureus enhanced by 5.21% and 44.63%). This study demonstrates that four chemical modifications synergistically regulate the chemical composition, molecular structure, electrical properties, and network conformation of conjugates, thereby significantly enhancing their antioxidant, hypoglycemic, and antibacterial activities and promoting their application in the food and pharmaceutical fields.