Preparation of chitosan oligomers and their antioxidant activity

Chitosan oligomers with different molecular weights were prepared by oxidative degradation method involving hydrogen peroxide (H₂O₂) and the combined degradation method using hydrogen peroxide and microwave radiation. Viscosity determination and end group analysis were applied to measure molecular weights of chitosan oligomers. Effects of concentration of H₂O₂ and degradation time on molecular weights of chitosan oligomers were studied. Both methods were effective to prepare chitosan oligomers from the initial chitosan (8.5×10⁵ Da). The degradation process of chitosan will be accelerated with the aid of microwave and degradation time may be reduced. The antioxidant activity of chitosan oligomers was evaluated as radical scavengers against superoxide anion and hydroxyl radical by application of flow injection chemiluminescence technology. Chitosan oligomers A, B, C and D (2300, 3270, 6120, and 15,250 Da) had different antioxidant activity. Among the four chitosan oligomers, oligomer D (15,250 Da) had the lowest scavenging ability against superoxide anion and hydroxyl radicals. For superoxide anion scavenging, the 50% inhibition concentrations (IC₅₀s) of other three oligomers A, B, and C were 5.54, 8.11, and 12.15 mg/mL, respectively. And for hydroxyl radical scavenging the values were 0.4, 0.76, and 1.54 mg/mL, respectively. At the concentration range examined, the maximal inhibiting efficacy of A, B, C, and D were 89, 75, 74, and 41% for superoxide anion, and 71, 65, 51, and 7% for hydroxyl radical. These results indicated that chitosan oligomers with lower molecular weight had better antioxidant activity.