Protocatechuic acid grafted onto chitosan: Characterization and antioxidant activity

• PA was grafted onto CS by a carbodiimide mediated cross-linking reaction.
• PA-g-CS was characterized by several instrumental methods.
• Amide and ester linkages were formed between PA and CS.
• PA-g-CS showed lower crystallinity and thermal stability than CS.
• PA-g-CS exhibited higher antioxidant activity in vitro than CS.

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 279.69 mg 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 292 nm. As compared to CS, PA-g-CS exhibited a decreased band at 1596 cm−1 and a new band at 1716 cm−1, suggesting the formation of amide and ester linkages between PA and CS. New proton signals at δ6.77–7⋅33 ppm were observed on 1H NMR spectrum of PA-g-CS, assigning to the methine protons of PA. Signals at δ 150.8–116.6 ppm on 13C 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.