LPMO-Catalyzed Oxidation of Cellulosic Fibers with Controlled Addition of a Reductant and H₂O₂

Cellulose-derived biomaterials offer a sustainable and versatile platform for various applications. Enzymatic engineering of these fibers, particularly using lytic polysaccharide monooxygenases (LPMOs), shows promise due to the ability to introduce functional groups onto cellulose surfaces, potentially enabling further functionalization. However, harnessing LPMOs for fiber engineering remains challenging, partly because controlling the enzymatic reaction is difficult and partly because limited information is available about how LPMOs modify the fibers. In this study, we explored controlling LPMO-mediated fiber oxidation by sequentially adding a reductant (gallic acid, GA) and H₂O₂, using three different carbohydrate-binding module (CBM)-containing LPMOs. An in-depth analysis of the soluble products and the M_n, M_w, and carbonyl content in the fiber fraction indicates that fiber oxidation can indeed be controlled by adjusting the amount of GA and H₂O₂ added to the reaction. In particular, at lower overall dosages of GA and H₂O₂, corresponding to low oxidation levels, fiber oxidation occurs rapidly with almost no release of soluble oxidized products. Conversely, at higher dosages, fiber oxidation levels off, while oxidized oligosaccharides continue to be released and the fibers are eroded. Importantly, next to demonstrating controlled fiber oxidation, this study shows that different cellulose-active LPMOs modify the fibers in different manners.