Tyrosinase-mediated grafting and crosslinking of natural phenols confers functional properties to chitosan

• Tyrosinase oxidizes phenols to quinones that then graft to chitosan.
• Phenolics are an abundant and diverse group of natural products.
• Caffeic acid grafting confers mechanical properties to chitosan solutions or films.
• Caffeic acid grafting also confers redox properties to chitosan films.
• Biofabrication enlists biology's materials and mechanisms for materials science.

Physics and chemistry underpinned the remarkable advances in materials science that occurred during the 20th century, while biology is poised to provide the scientific underpinning for materials science advances of the 21st century. Biofabrication is the emerging approach to broadly apply biology's materials and mechanisms to create structure and function. Here, we describe one such biofabrication methodology, the use of tyrosinase to graft phenolics to the aminopolysaccharide chitosan. Phenolics are a broad class of abundant natural products and we provide results from a single phenolic reactant, caffeic acid, to illustrate the potential for enzymatically imparting functional properties to chitosan. We show that tyrosinase oxidation mediates the grafting of caffeic acid to chitosan and possibly even results in the covalent crosslinking of chitosan. When this enzymatic reaction is performed in a chitosan solution (pH < 6), it is observed to induce a sol–gel transition. At higher pHs, chitosan forms an insoluble film and this enzymatic reaction can alter the film's mechanical properties. Importantly, caffeic acid grafting also confers redox-activity to the chitosan film, enabling the film to accept, store and donate electrons. The broader efforts to enlist tyrosinase to build macromolecular structures and impart functions are discussed.

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