We demonstrate for the first time the feasibility of using bacterial laccase as a biocatalyst for oxidizing ferulic acid (FA) to enable nucleophilic crosslinking of chitosan under neutral conditions, targeting future applications in medicine and the food industry. We have established optimal conditions achieving a high degree of chitosan crosslinking (up to 85%). Employing quantum-chemical modeling and spectral analysis techniques, it was determined that the predominant mechanism for the nucleophilic attack of chitosan amino groups on the phenoxyl radical – the oxidation product of FA – involves Michael addition, followed by radical recombination. This process results in the formation of crosslinks consisting of β-β’ coupled FA dimers while retaining the phenoxyl functionalities.

For the first time, for oxidative cross-linking of chitosan with a natural polyphenol, ferulic acid, the so-called small two-domain bacterial laccase, was used. Recombinant Streptomyces carpinensis laccase (ScaSL) has been shown to oxidize ferulic acid in neutral and alkaline environments, which is important for subsequent nucleophilic reactions of chitosan with oxidation products. The selected conditions (pH 7.0, molar ratio FA/NH₂ = 1:10) provide a cross-linking degree of more than 80%, which significantly exceeds the indicators previously achieved using other laccases. Based on the results of quantum chemical modeling and experimental studies using FTIR spectroscopy and XPS, as well as thermogravimetric and differential scanning calorimetry, a new mechanism for laccase-catalyzed cross-linking of chitosan with ferulic acid without the formation of Schiff bases is proposed. The resulting hydrogels have a uniform smooth microstructure, high oxygen permeability, a significant degree of swelling (270%), and stability over a wide pH range. The material neutralizes up to 95% of ABTS^+• cation radicals. Quantum chemical analysis within the framework of the Marcus theory has demonstrated that the chitosan–ferulic acid conjugate is superior in antioxidant capacity to ferulic acid dimers, despite the decrease in the matrix element of the bond. Application of two-domain bacterial laccase for modifying chitosan hydrogels with ferulic acid opens the way to the creation of environmentally friendly biomaterials with antioxidant protection.