Effect of carboxyl content on the physicochemical properties and intramolecular water distribution of 6-carboxyl chitooligomer prepared by laccase-TEMPO

6-carboxyl chitooligomer was prepared by degradation of chitosan with H₂O₂ followed by catalytic oxidation in the laccase-2,2,6,6-Tetramethylpiperidine-1-oxyl (TEMPO) system. The TEMPO was used as a single variable to alter the content of introduced carboxyl groups. The resulting oxidation products were analyzed by ion chromatography, scanning electron microscopy, and ¹H low-field NMR. Moisture absorption-retention ability, antioxidation, and thermal stability also were characterized. The results showed that the carboxyl content in the molecules increased with increasing TEMPO. With the addition of 8% TEMPO (mass fraction relative to chitooligomers), the carboxyl content and the oxidation degree of the product reached 4.90 mmol/g and 96.53%, respectively. Under these conditions, it also had the highest ABTS radical cation scavenging ability of 85.47% and improved thermal stability. When the amount of TEMPO was reduced to 2%, the oxidation product had a degree of oxidation of ~50% and showed a unique molecular structure and supreme moisture absorption-retention ability. A higher associated water content was observed inside the molecules where more bound water was locked in the process of moisture absorption. Furthermore, the physicochemical properties of 6-carboxyl chitooligomer were much higher than chitooligomer and oligomeric sodium hyaluronate (HA-Oligo). © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47509.