Metal-coordinating controlled oxidative degradation of chitosan and antioxidant activity of chitosan-metal complex

A new method of metal-coordinating controlled oxidative degradation of chitosan leading to low molecular weight chitosan with uniform molecular weight distribution is reported in this paper. Chitosan is converted into chitosan-metal complex in which chitosan chain can be easily broken at weak points caused by coordinating bond. IR, UV, XRD, DSC and elemental analysis proved coordination between chitosan and the metal ion and the presence of weak points existing on the chitosan chain of complexes. Coordinating conditions (such as metal amount, speed of stirring, speed of metal salt addition) are controlled to obtain the complex with uniform metal distribution and uniform weak points distribution. H2O2 is added to break chitosan chain at those weak points to get low molecular weight chitosan. Degradative speed with different metal ion is different as follow: Cu2+ > Co2+ > Mn2+ > Ni2+. With the increase of oxidant amount, temperature, and pH, and decrease of O-2 content, degradative velocity increase. HPLC/GPC spectra show that degradative velocity of complexes is faster than that of chitosan, and the molecular weight distribution is much more narrow than that of chitosan. Low molecular weight chitosan and its complexes with Cu(II) or Co(II) possess good O-2(.-) scavenging activity.