Mild pretreatment with Brønsted acidic deep eutectic solvents for fractionating β-O-4 linkage-rich lignin with high sunscreen performance and evaluation of enzymatic saccharification synergism.

Herein, a mild fractionation method by employing polyol-based Brønsted acidic DESs (BDESs) was proposed to extract lignin with well-preserved β-O-4 substructures and to enhance fermentable sugar yields simultaneously. For ethylene glycol-oxalic acid (EG-OA), more than 53 % of lignin was obtained and superb carbohydrate digestibility (i.e., glucose and xylose yields were reached to 94.6 % and 87.7 %, respectively) was achieved after pretreatment. Remarkably, detailed structural studies revealed that the polyol was incorporated into lignin, which stabilized reactive carbocation intermediates formed during BDESs treatment and prevented undesired recondensation reactions. This lignin protection mechanism was shown to play a key role in enzymatic hydrolysis enhancement and lignin valorization. The resultant β-O-4 linkage-rich lignin fractions were attractive for natural sunscreen applications due to their lighter color and excellent UV-blocking performance. Overall, this work proposed a sustainable and economically practical lignin-first biorefinery approach that is beneficial for achieving comprehensive valorization of lignocellulose.