A key requirement in the effort to replace petroleum products with renewable-based products is an efficient and affordable method to depolymerize lignin from biomass for downstream valorization. Reductive processes maintain the energy content of the phenolic units of lignin but usually involve high pressures and temperatures. One mechanism for biological lignin depolymerization entails reduction of keto-aryl ether bonds through a series of reactions that include an SN2 mechanism with the thiol redox mediator glutathione. This enzymatic β-aryl ether cleavage pathway is the main catabolic pathway to cleave the most prevalent linkage in lignin, the β-O-4 bond. First, oxidation of the α-hydroxy by a NAD+-dependent dehydrogenase is required. Next, nucleophilic substitution of the aryl ether is achieved with an etherase glutathione cofactor. The final step is reductive cleavage of the thioether intermediate with a lyase glutathione cofactor, liberating the aromatic fragment and formation of a glutathione disulfide. Mimicking this chemistry in a simple protein-free process, we surveyed small organic thiols for their ability to cleave aryl ether models found in lignin. High yields were achieved for both synthetic dimers and polymers. We next extended our study to lignin obtained from biomass and found molecular weight reductions of up to 60%. In both models and biomass-derived lignin, oxidation was required, as seen in the enzymatic pathway. Our work provides mechanistic insight, substrate scope, and utility in polymeric systems. Furthermore, this thiol mediator has the potential to be electrochemically reactivated providing a mild, green approach to depolymerize lignin for bio-derived fuels and chemicals.
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