Free radical-mediated conjugation of chitosan with tannic acid: Characterization and antioxidant capacity

Chitosan (CS) is a natural biopolymer with versatile applications. In this work, CS was functionalized with tannic acid (TA) by a free radical grafting procedure to improve its antioxidant capacity. The resulting TA-conjugated CS (TA-CS) showed a high TA equivalent content of 267.1 ± 5.4 mg TAE/g. The formation of TA-CS was verified by UV–vis, FTIR, and 1H NMR analyses. Moreover, TA-CS showed a lower thermal stability and crystallinity than CS and exhibited a good water-solubility of 15.0 ± 0.3 mg/mL. The DPPH radical scavenging activity of TA-CS reached a maximum of 91.6%, which was 6.4-fold higher than that of CS. Similar results were also observed in ABTS and hydroxyl radical scavenging assays. Moreover, TA-CS showed a higher superoxide radical scavenging activity and ferrous ion chelating activity than both TA and CS. These results suggest that conjugating with TA is an available way to enhance the antioxidant capacity of CS.