Modification of Condensed Lignin by Deep Eutectic Solvents: Insight into the Mechanism of Improving the Enzymatic Saccharification

Liquid hot water pretreatment (LHWP) can convert hemicellulose in biomass into valuable xylooligosaccharides, but the utilization of glucan in the solid residue after hemicellulose extraction is usually constrained due to the significant lignin condensation during the LHWP. To alleviate lignin’s inhibition on glucan saccharification, this study established a variety of functional deep eutectic solvents (DESs) for the pretreatment of the solid residues after LHWP. The applied DES pretreatments resulted in different glucan saccharification yields. Under the premise of similar lignin removal, the ChCl/ethylene glycol (EG) system obtained near complete glucan digestibility, while the acid or alkali DESs had much lower enzymatic saccharification yields (35.7% and 20.6%). The mechanism of boosting the glucan saccharification by ChCl/EG was comprehensively analyzed by 2D-HSQC NMR, hydrophobicity, GPC and Langmuir adsorption isotherm of enzymes onto the cellulolytic enzyme lignin. Results indicated that lignin remaining in the ChCl/EG pretreated substrates had a lower hydrophobicity (2.8 L/g) and enzyme adsorption (4.6 mg/g), resulting from the EG grafting onto the α-position of the lignin side chain (as high as 5.0/100 Ar). The LHWP-DES pretreatment in this study unveiled the mechanism for cellulose digestibility enhancement using three common DESs and maximized the enzymatic hydrolysis yield, which provided a new scheme for the high-value utilization of biomass.