Chitosan Modification By Laccase/Phenolic Compounds

As an abundant renewable resource, the chitosan from the deacelation reaction of chitin exhibits some unique properties such as biodegradability, biocompatibility, broad-spectrum antimicrobial properties, film-forming and fiber-forming property. It shows great market potential and application prospect. However, the expansion of chitosan’s application fields is limitted by the weak antioxidant ability, and the improvement of chitosan’s antioxidant ability attracts the extensive attention. The physical degradation, chemical modification and biological modification are effective methods to improve the antioxidant ability of chitosan, while the biological modification has much greater develope potential because of its good specificity, small environmental pollution and mild reaction conditions.In this paper, the chitosan were treated by laccase with different phenolic compounds. The results showed that the phenolic compounds could be grafted on the chitosan by laccase catalysis. The antioxidant activitives of all modified chitosans were improved at different degree. The lauryl gallate (LG) is the best compound that can enhance the antioxidiant activities of chitosan by laccase. The optimal process conditions of improving the antioxidant activity of chitiosan were:the chitosan/LG ratio at 1:1(w/w), the chitosan concentration of 0.15%(w/v), the temperature of 30℃, pH 4.5, the laccase dosage of 200 U/g chitosan, the chitosan solution/methanol ratio of 20:3(v/v), and the reaction time of 24 h.The chitosan was modified by laccase combined with LG (laccase/LG). The results showed that the ABTS·+ and DPPH-scavenging effects were enhanced significantly by laccase with lauryl gallate under the optimal conditions, and increased with the increase of chitosan concentration. The ABTS·+ scavenging effect of chitosan (1 mg/ml) reached a maximum after 30 min and then tend to stabilization. Compares to the chitosan alone, the ABTS·+ scavenging effect of the modified chitosan by laccase/LG increased 20.54% to 85.42%. The DPPH·scavenging effect modified chitosan also increased with the increase of reaction time, and increased from 1.26%(control) to 9.44% after 60 min. Compared to the untreated chitosan, the reducing power of the modified chitosan also increased significantly. In addition, the contact angle of the modified chitosan film increased from 81.0° to 95.5° and its water vapor transmission rate decreased from 45.96 g/m2·h to 27.49 g/m2·h. The swelling ratio significantly decreased significantly and the thermal stability decreased slightly compared with the untreated chitosan film. The tensile strength of chitosan film decreased from 76.67 Mpa to 60.48 Mpa, and the elongation at break decreased 51.8% to 38.3%.The synergistic results from the FT1R, UV, Elemental analysis and grafting ratio determination showed that the LG was grafted on the chitosan by laccase, and the amino groups of chitosan were the main reaction sites.The alkali lignin (AL) is the byproduct from the papermaking industry. In this paper, the chitosan was treated by laccase with the alkali lignin. The properties of the resulted chitosan film were studied. It showed that the reducing power, the scavenging effects of ABTS·+ and DPPH·of modified chitosan increased significantly compared to untreated chitosan. These antioxidition activities increased with the increase of chitosan concentration, and the rate of increase was much higher than that of untreated chitosan. Compared to the untreated chitosan, the ABTS·+ scavenging effect of modified chitosan film (1mg/ml) increased from 24.69% to 98.40%, and the DPPH· scavenging effect increased from 2.28% to 70.57%. The reducing power of modified chitosan (10 mg/ml) increased from 0.079 to 0.423. Obviously, the laccase/alkali lignin treatment could significantly enhance the antioxidant activities of chitosan. Compared to untreated chitosan, the thermal stability of modified chitosan slightly increased while its viscosity decreased. The crystalline region of modified chitosan was damaged. The laccase-catalyzed oxidation product of AL was grafted on the chitosan. The results from elemental analysis and grafting ratio showed that the amino groups of chitosan were the main reaction sites.