Reaction Mechanisms and Structural and Physicochemical Properties of Caffeic Acid Grafted Chitosan Synthesized in Ascorbic Acid and Hydroxyl Peroxide Redox System.

The ascorbic acid (AA) and hydroxyl peroxide (H2O2) redox pair induced free radical grafting reaction is a promising approach to conjugate phenolic groups with chitosan (CS). In order to reveal the exact mechanisms of the AA/H2O2 redox pair induced grafting reaction, free radicals generated in the AA/H2O2 redox system were compared with hydroxyl radical (•OH) produced in the Fe2+/H2O2 redox system. Moreover, the structural and physicochemical properties of caffeic acid grafted CS (CA-g-CS) synthesized in these two redox systems were compared. Results showed that only ascorbate radical (Asc•-) was produced in the AA/H2O2 system. The reaction between Asc•- and CS produced novel carbon-centered radicals, whereas no new free radicals were detected when •OH reacted with CS. Thin layer chromatography, UV-vis, Fourier transform infrared, and nuclear magnetic resonance spectroscopic analyses all confirmed that CA was successfully grafted onto CS through Asc•-. However, CA could be hardly grafted onto CS via •OH. CA-g-CS synthesized through Asc•- exhibited lower thermal stability and crystallinity than the reaction product obtained through •OH. For the first time, our results demonstrated that the synthesis of CA-g-CS in the AA/H2O2 redox system was mediated by Asc•- rather than •OH.