Purpose: To investigate the effect of salt form on the antioxidant activities of chitosan aqueous solution.

Methods: The antioxidant activities of chitosan acetate (CS-acetate), chitosan hydroxybenzotriazole (CS-HOBt), chitosan thiamine pyrophosphate (CS-TPP) and chitosan ethylenediaminetetraacetic acid (CS-EDTA) solution were determined employing various established in vitro system such as superoxide and hydroxyl radicals scavenging, metal ion chelating and reducing power. Their chemical structures were characterized by nuclear magnetic resonance (NMR) and Fourier transform infrared spectrophotometry (FT-IR).

Results: NMR and FT-IR show confirmed formation of chitosan salts. The 50 % inhibition concentration (IC₅₀) of superoxide and hydroxyl radicals was 0.349 – 1.34 and 0.34 – 1.54 mg/mL, respectively. Among the salt forms, CS-acetate (IC₅₀ = 0.349 mg/mL) showed the highest superoxide radical scavenging effect while CS-HOBt (IC₅₀ = 0.34 mg/mL) showed the highest hydroxyl radical scavenging effect. With regard to metal ion chelating activity, CS-EDTA showed the highest chelating activity (approx 100 % at 1 mg/mL) while the others showed 20 % activity at a concentration of 1 mg/mL. The results for reducing power indicate that CS-TPP had the highest reducing power. 

Conclusion: The results indicate that antioxidant activity varied with the salt form.  Thus, CS salts may be used as a source of antioxidants for pharmaceutical applications.