Determination of Lignin-Carbohydrate Complexes Structure of Wheat Straw using Carbon-13 Isotope as a Tracer

To maximize the use of wheat straw as a feedstock for biofuels and other biorefinery products, the structure of lignin-carbohydrate complexes (LCCs) was characterized by injection of C-13 isotope-labeled xylose into living wheat straw. Afterwards, lignin-carbohydrate complexes were extracted from the harvested straw by the Bjorkman method. The extracted LCCs were chemically characterized by Fourier transform-infrared spectroscopy (FT-IR), sugar composition, molecular weight analysis, C-13-NMR, and HSQC. The results showed that LCCs in wheat straw were particularly enriched with xylan and exhibited narrow polydispersity (M-w/M-n < 1.5). NMR analysis showed that the lignin was linked with the carbohydrates through.-ester, phenyl glycoside, and benzyl ether bonds. In addition to S, G, and H lignin units, p-coumarate and ferulate were also in the LCCs. The substructures in lignin were beta-O-4', beta-beta', and beta-5'. Quantitative data analysis of C-13-NMR combined with HSQC showed that the lignin in the LCCs of wheat straw contained guaiacyl (G), syringyl (S), and p-hydroxyphenyl (H) units in the proportion of 5:4:1 (S:G:H). The main lignin substructure, beta-O-4', comprised 71.64% of the isolated lignin. The total LCC linkages (the sum of phenyl glycoside, gamma-ester and benzyl ether bonds) were 210.86/100C(9) in C-13-LCC, which was dominated by phenyl glycoside linkages, followed by gamma-ester bonds and minor amounts of benzyl ether bonds. Lignin and xylan in the LCCs of wheat straw were mainly linked by benzyl ether bonds and phenyl glycoside linkages.