Oxidation of primary hydroxyl groups in chitooligomer by a laccase–TEMPO system and physico-chemical characterisation of oxidation products

• A laccase–TEMPO system can selectively oxidise the C6 hydroxyl group of chitooligomer into carboxyl group.
• The oxidised product shows superior ability of moisture-absorption, moisture-retention and anti-oxidant.
• The oxidised product has the potention application value in pharmaceutical and cosmetics industries.
• Na+ content can characterise the carboxylate ions content of the oxidised product.

The aim of this study was to investigate the oxidation of chitooligomer by a laccase–TEMPO system which had not previously been examined. Chitooligomer was treated with laccase and TEMPO in order to evaluate the potential of a laccase–TEMPO system to improve the moisture absorption, moisture retention, and antioxidant abilities of chitooligomer. Chitooligomer was prepared by degradation of high molecular weight chitosan with hydrogen peroxide followed by oxidation using a laccase–TEMPO system. 13C NMR and carboxylate ion content detection results indicated that the laccase–TEMPO system could selectively oxidise the C6 hydroxyl group of the chitooligomer into carboxyl group; molecular weight distribution changes suggest that the structure of the oxidised product had changed and the molecular size and molecular weight decreased with the molecules in aqueous solution having a compact structure. Oxidation of chitooligomer by a laccase–TEMPO system resulted in a significant improvement in the moisture absorption, moisture retention and antioxidant abilities. The oxidised product has potential application values in the pharmaceutical and cosmetics industries.