Synthesis of chitosan-gallic acid conjugate: structure characterization and in vitro anti-diabetic potential

In this study, chitosan grafted copolymer with gallic acid (GA) was synthesized by a novel and efficient free radical mediated method. The optimal grafting conditions, structural characterization, α-glucosidase and α-amylase inhibitory activities of chitosan grafted copolymers were investigated. Results showed that the maximum grafting ratio (128.3 mg GA equivalents/g) was obtained at 12 h with 5 g/L chitosan, 16 g/L GA, 2 g/L ascorbic acid and 0.2 M hydrogen peroxide. UV-vis, Fourier-transform infrared and nuclear magnetic resonance spectroscopy all confirmed the successful grafting of GA onto chitosan. The conjugation of GA onto chitosan probably occurred between amine (C-2), hydroxyl groups (C-3 and C-6) of chitosan and carboxyl groups of GA, forming amide and ester linkages, respectively. Differential scanning calorimetry and X-ray diffraction spectra indicated that GA grafted chitosan (GA-g-chitosan) had decreased thermal stability and crystallinity as compared to chitosan. Notably, GA-g-chitosan showed increased α-glucosidase and α-amylase inhibitory activity with the increase of grafting ratio. These results indicated the potential of GA-g-chitosan in the development of an effective anti-diabetic agent.