Structural Basis for the Formation and Regulation of Lignin-Xylan Bonds in Birch

The covalent connectivity between lignin and polysaccharides forming the so-called lignin carbohydrate complexes (LCCs) is important to obtain fundamental knowledge on wood formation and may shed light on molecular aspects of wood processing. Although widely studied, unequivocal proofs of their existence in native-state biomass are still lacking, mainly because of harsh preanalytical fractionation conditions that could cause artifacts. In the present study, we applied a mild protocol for quantitative fractionation of LCCs and performed detailed structural studies using 2D HSQC NMR spectroscopy, P-31 NMR spectroscopy, and thioacidolysis in combination with GC-MS and GC with flame ionization detection. The detailed structural analysis of LCCs, including both lignin and the carbohydrate skeleton, unveiled insights into the role of molecular structure of xylan on the type of lignin-carbohydrate (LC) bonds formed. More specifically, it is shown that xylan LCCs differ in the degree of substitution of hydroxyl functionality on the xylan skeleton by the presence of acetyl- or 4-O-methylglucuronic acid. The highly substituted xylan had a lower prevalence of phenyl glycosidic and benzyl ether LC bond types than the lowly substituted xylan. In addition, structural differences in the lignin part of the LCCs were observed. On the basis of the results, it is suggested that acetylation on xylan regulates the type and frequency of LC bonds.