Water-soluble chitosan derivatives and pH-responsive hydrogels by selective C-6 oxidation mediated by TEMPO-laccase redox system.

Chitosan is a polysaccharide with recognized antioxidant, antimicrobial and wound healing activities. However, this polymer is soluble only in dilute acidic solutions, which restricts much of its applications. A usual strategy for improving the functionality of polysaccharides is the selective oxidation mediated by 2,2,6,6-tetra-methyl-1-piperidinidyloxy (TEMPO) using laccase as a co-oxidant. In this work, the TEMPO-laccase redox system was used for the first time to selectively oxidize chitosan in order to produce tailored derivatives. The reaction was performed at pH 4.5 under continuous air supply and the oxidized products were characterized structurally and functionally. The TEMPO-laccase oxidation successfully added aldehyde and carboxylate groups to chitosan structure resulting in derivatives with oxidation between 4 and 7%. These derivatives showed increased solubility and decreased viscosity in solution. If chitosan is dissolved in diluted hydrochloric acid prior to TEMPO-laccase oxidation, a crosslinked chitosan derivative was produced, which was able to form a pH-responsive hydrogel.

Oxidative polymerization of lignins by laccase in water-acetone mixture.

The enzymatic oxidative polymerization of five technical lignins with different molecular properties, i.e. Soda Grass/Wheat straw Lignin, Organosolv Hardwood Lignin, Soda Wheat straw Lignin, Alkali pretreated Wheat straw Lignin, and Kraft Softwood was studied. All lignins were previously fractionated by acetone/water 50:50 (v/v) and the laccase-catalyzed polymerization of the low molecular weight fractions (Mw < 4000 g/mol) was carried out in the same solvent system. Reactivity of lignin substrates in laccase-catalyzed reactions was determined by monitoring the oxygen consumption. The oxidation reactions in 50% acetone in water mixture proceed with high rate for all tested lignins. Polymerization products were analyzed by size exclusion chromatography, FT-IR, and (31)P-NMR and evidence of important lignin modifications after incubation with laccase. Lignin polymers with higher molecular weight (Mw up to 17500 g/mol) were obtained. The obtained polymers have potential for applications in bioplastics, adhesives and as polymeric dispersants.