The demand for efficient utilization of biomass induces a detailed analysis of the fundamental chemical structures of biomass, especially the complex structures of lignin polymers, which have long been recognized for their negative impact on biorefinery. Traditionally, it has been attempted to reveal the complicated and heterogeneous structure of lignin by a series of chemical analyses, such as thioacidolysis (TA), nitrobenzene oxidation (NBO), and derivatization followed by reductive cleavage (DFRC). Recent advances in nuclear magnetic resonance (NMR) technology undoubtedly have made solution-state NMR become the most widely used technique in structural characterization of lignin due to its versatility in illustrating structural features and structural transformations of lignin polymers. As one of the most promising diagnostic tools, NMR provides unambiguous evidence for specific structures as well as quantitative structural information. The recent advances in two-dimensional solution-state NMR techniques for structural analysis of lignin in isolated and whole cell wall states (in situ), as well as their applications are reviewed.

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The strong association of condensed phenolic moieties in isolated lignins with their inhibition of enzymatic hydrolysis.

The use of ionic liquid (IL) in biomass pretreatment has received considerable attention recently because of its effectiveness in decreasing biomass recalcitrance to subsequent enzymatic hydrolysis. To understand the structural changes of lignin after pretreatment and enzymatic hydrolysis process, ionic liquid lignin (ILL) and subsequent residual lignin (RL) were sequentially isolated from ball-milled birch wood. The quantitative structural features of ILL and RL were compared with the corresponding cellulolytic enzyme lignin (CEL) by nondestructive techniques (e.g., FTIR, GPC, quantitative (13)C, 2D and (31)P NMR). The IL pretreatment caused structural modifications of lignin (cleavage of β-O-4 ether linkages and formation of condensed structures). In addition, lignin fragments with lower S/G ratios were initially extracted, whereas the subsequently extracted lignin is rich in syringyl unit. Moreover, the maximum decomposition temperature (T(M)) was increased in the order ILL < RL < CEL, which was related to the corresponding β-O-4 ether linkage content and molecular weight (M(w)). On the basis of the results observed, a possible separation mechanism of IL lignin was proposed.