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

Abstract

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.