Quantification of Wheat Straw Lignin Structure by Comprehensive NMR Analysis

A further understanding of the structure of lignin from herbaceous crops is needed for advancing technologies of lignocellulosic biomass processing and utilization. A method was established in this study for analyzing structural motifs found in milled straw lignin (MSL) and cellulase-digested lignin (CEL) isolated from wheat straw by combining quantitative DC and HSQC NMR spectral analyses. The results showed that guaiacyl (G) was the predominant unit in wheat straw cell wall lignin over syringyl (S) and hydroxyphenyl (H) units. Up to 8.0 units of tricin were also detected in wheat straw lignin per 100 aromatic rings. Various interunit linkages, including beta-O-4, beta-5, beta-beta', beta-1, alpha, beta-diaryl ether, and 5-5',4-O-beta' as well as potential lignin-carbohydrate complex (LCC) bonds, were identified and quantified. These findings provide useful information for the development of biofuels and lignin-based materials.