Antioxidant activity of chitosan and effect of molecular weight and pH

The antioxidant activity of chitosan with different initial molecular weights were evaluated by the radical scavenging activity method using 1,1-diphenyl-2-picrylhydrazyl (DPPH) at different pHs. Fig.1 shows the antioxidant activity of chitosan with different initial molecular weights at different pHs. Scavenging activity of DPPH radical decreased with the increasing of molecular weights of chitosans and antioxidant activity is directly affected by the amount of molecular weights. The results showed that the antioxidant activity of chitosan with different molecular weights depend on the pH values and CS5 (M=98 kD) has the highest antioxidant activity at pH=3.5 and CS1 (M=876 kD) has the lowest antioxidant activity at pH=6.5. These results are possibly due to the low-molecular weight chitosan has lower H bonds than medium and high-molecular-weight chitosan, and with decreasing of intermolecular interaction (Van der Waals) and hydrogen bonds in low Mw, the lone pair of electrons or hydrogen compounds from C–O and N–H may be more available[1]. The effect of pH on the antioxidant activity is probably because of the scavenging mechanism of chitosan that free radical can react with the residual free amino (-NH2) groups to form stable macromolecule radicals, and the -NH2 groups can form ammonium groups by absorbing a hydrogen ion from the acidic solution [2]. At low pH these amino (-NH2) groups are protonated, making chitosan a cationic polyelectrolyte but, at neutral and high pH, deprotonation of amino groups enables the formation of intermolecular hydrogen bonds and concomitant dehydration of the internal region of the associated chains leading to gelation. Intramolecular hydrogen bonding between the hydroxyl O3 and ring O5 atoms confers rigidity to a single chitosan chain, while intermolecular hydrogen bonding between the amine N and hydroxyl O3 as well as O6 atoms is responsible for the selfassembly and the anti-parallel packing of the chains in the crystalline domains [3].