Isolation, Characterization, and Depolymerization of L-Cysteine Substituted Eucalyptus Lignin

作者：Shi, Lanlan; Zhang, Tanhao; Zhou, Xin; Yao, Lu; Yang, Linjie; Yue, Fengxia^*; Lan, Wu^*; Lu, Fachuang^*

来源：GLOBAL CHALLENGES, 2022, 6(4): 2100130.

DOI：10.1002/gch2.202100130

• 2D NMR
• condensation
• l-cysteine lignin
• model compounds

Lignin condensation reactions are hard to avoid or control during separation, which is a deterrent to lignin isolation and post-conversation, especially for the full utilization of lignocelluloses. Selective protection of beta-aryl ether linkages in the isolation process is crucial to lignin valorization. Herein, a two-step acid/alkali separation method assisted with L-cysteine for eucalyptus lignin separation is developed, and the isolated L-cysteine lignins (LCLs) are comprehensively characterized by 2D NMR, P-31 NMR, thioacidolysis, etc. Compared to the two-step control treatment, a much higher beta-O-4 content is preserved without reducing the separation efficiency assisted by L-cysteine, which is also significantly higher than alkali lignin and kraft lignin. The results of hydrogenolysis show that LCLs generate a much higher monomer yield than that of control sample. Structural analysis of LCLs suggests that lignin condensation reaction, to some extent, is suppressed by adding L-cysteine during the two-step acid/alkali separation. Further, mechanistic studies using dimeric model compound reveals that L-cysteine may be the alpha-carbon protective agent in the two-step separation. The role of L-cysteine in the two-step lignin isolation method provides novel insights to the selective fractionation of lignin from biomass, especially for the full valorization of lignocellulosic biomass.