Synthesis, characterization, and antioxidant activity of carboxymethyl chitosan derivatives containing sulfonium salt

To improve the solubility and bioactivity of chitosan, a new class of carboxymethyl chitosan derivatives possessing sulfonium salts was successfully designed and synthesized, including Methyl sulfide carboxymethyl chitosan (MCMCS), Ethyl sulfide carboxymethyl chitosan (ECMCS), Propyl sulfide carboxymethyl chitosan (PCMCS), and Butyl sulfide carboxymethyl chitosan (BCMCS). To determine the structure of the new class of the derivatives, methods of the Fourier transform infrared spectroscopy (FT-IR), 1H nuclear magnetic resonance spectrometer (1H NMR), and 13C nuclear magnetic resonance spectrometer (13C NMR) were used. Moreover, the antioxidant activity of the derivatives for three types of free radicals, i.e., hydroxyl radical, superoxide radical, and 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical was evaluated in vitro. In addition, the L929 cells were adopted to test the cytotoxicity of chitosan and its derivatives by CCK-8 assay. The class of the carboxymethyl chitosan derivatives showed a strong scavenging ability against the three free radicals at 1.6 mg/mL, with scavenging rate of over 70% and some up to 100%. At this high rate, the overall cell viability in the toxicity test reached more than 80%, indicating that the synthetic derivative had a little cytotoxicity. The results show that the introduction of carboxymethyl group to chitosan increased the water-solubility of chitosan, and the combination of sulfonate ions with different chain lengths further enhanced the antioxidant activity of chitosan. Therefore, the sulfonium-containing carboxymethyl chitosan derivatives had excellent bioactivity with good application prospects in food, biomedicine, and medical fields.