Protocatechuic acid grafted onto chitosan: Characterization and antioxidant activity

In this study, protocatechuic acid (PA) was grafted onto chitosan (CS) by a carbodiimide mediated cross-linking reaction. The structural characterization, physical property and antioxidant activity of PA grafted CS (PA-g-CS) was investigated. As results, three copolymers with different grafting ratios (61.64, 190.11 and 279.69mg PAE/g) were obtained by varying the molar ratios of reaction substrates. PA-g-CS showed the same UV absorption peaks as PA at 258 and 292nm. As compared to CS, PA-g-CS exhibited a decreased band at 1596cm(-1) and a new band at 1716cm(-1), suggesting the formation of amide and ester linkages between PA and CS. New proton signals at δ6.77-7⋅33ppm were observed on (1)H NMR spectrum of PA-g-CS, assigning to the methine protons of PA. Signals at δ 150.8-116.6 ppm on (13)C NMR spectrum of PA-g-CS was assigned to the aromatic ring carbon of PA moieties. All the structural information confirmed the successful grafting of PA onto CS. SEM observation showed CS had a smooth surface, while PA-g-CS had a rough surface. TGA revealed the thermal stability of PA-g-CS was lower than CS. Antioxidant activity assays further verified the reducing power and DDPH radical scavenging activity of PA-g-CS was much higher than CS.