The use of non-edible plant parts from agricultural or forestry residues (lignocellulosic biomass) for producing advanced biofuels or chemicals is highly desirable to avoid competition with food and water resources. Lignocellulosic biomass, which forms the structure of plant cell walls, consists of three main polymers (Fig. 1): cellulose (35-45%), hemicellulose (25-30%), and lignin (15-30%), which are intricately bound together into a solid, recalcitrant structure.1 Among these components, polysaccharides such as cellulose and hemicellulose can be converted into monomeric sugars (e.g., glucose and xylose), which are used for bioethanol production via fermentation. A crucial step in the valorization of lignocellulosic components into bioethanol or chemicals is reducing the size of the polymers, particularly cellulose, the most abundant component.

Oxidative depolymerization of polysaccharides is highly desirable for efficient valorization of recalcitrant biomass. Until 2010, no enzymes were known to break down polysaccharides through oxidation. However, the discovery of Lytic Polysaccharide Monooxygenases (LPMOs), coppercontaining enzymes, changed this by enabling the breakdown of C-H bonds in glycosidic linkages, aiding polysaccharide degradation. LPMOs can use either O2 with an electron source or H2O2 to carry out this process and can act on crystalline polysaccharides, making them more accessible to other enzymes for further breakdown. This discovery has spurred increased research into C-H bond activation and has opened new possibilities for biofuels and biobased chemicals. Inspired by the recently discovered LPMOs, mononuclear copper complexes have been developed and studied in the literature.2,3 However, their activities have been evaluated on different substrates and under various conditions. In this study, we aimed to develop more robust and sustainable molecular catalysts for reproducible activity assays in aqueous solutions at near-neutral pH and mild conditions.4 We tested several complexes on substrates of increasing complexity: the model substrate para-nitrophenyl-β-D-glucopyranoside (p-NPG), cellobiose (glucose dimer), and more complex substrates such as chitin, cellulose, and agave bagasse. After comparing the assays, proof-of-concept was achieved, demonstrating that bioinspired copper complexes can effectively promote oxidative polysaccharide depolymerization. Overall, it is likely that different mechanistic pathways are involved when various systems are used, depending on the nuclearity and/or redox properties of the complexes. Ongoing research aims to further understand these mechanisms and optimize the catalyst’s stability.

1. Wakerley, D. W.; Kuehnel, M. F.; Orchard, K. L.; Ly, K. H.; Rosser, T. E.; Reisner, E, Nat Energy 2017, 2, 17021.
2. G. Vaaje-Kolstad, B. Westereng, S. J. Horn, Z. Liu, H. Zhai, M. Sørlie,V. G. H. Eijsink, Science 2010, 330, 219–222.
3. Z. Forsberg, M. Sørlie, D. Petrović, G. Courtade, F. L. Aachmann, G. Vaaje-Kolstad, B. Bissaro, Å. K. Røhr, V. G. Eijsink, Curr. Opin. Struct. Biol. 2019, 59, 54-64.
4. Rébecca Leblay, Rafael Delgadillo-Ruíz, Christophe Decroos, Christelle Hureau, Marius Réglier, Ivan Castillo,* Bruno Faure, and A. Jalila Simaan*, ChemCatChem 2023, 15, e202300933