Fractionation of wheat straw Dioxane lignin reveals molar mass dependent structural differences

⿢Wheat straw lignin was fractionated into six distinct molar-mass-fractions.⿢High-mass fractions contained more H units and were linked to linear arabinoxylan.⿢Low-mass fractions contained additional glucan and exclusively tricin units.⿢One fraction was rich in lignin nucleation sites on arabinoxylan ferulates.⿢Lignin nucleation sites were suggested to involve dibenzodioxocin structures.

Dioxane lignin was isolated from ball-milled wheat straw by neutral dioxane-water extraction and subsequently molar-mass-fractionated by flash chromatography in dimethylformamide. The eluted lignin containing material was pooled into six distinct molar mass fractions that were in turn structurally characterized in detail by wet chemistry and NMR spectroscopy methods. Fractions of higher molar mass were found to be enriched in p-hydroxyphenyl units and contained more p-hydroxycinnamic acid units. They were found mainly associated with linear arabinoxylan, while in low-molar-mass fractions additional glucan contributions were found. Fractions of lower molar mass consisted of relatively more guaiacyl units and showed exclusive association with tricin units. One distinct lignin fraction with lower lignin content supposedly contained high ratios of arabinoxylan chains esterified by ferulic acid and cross-linked via ferulic acid dimerization, structures which were considered to represent lignin nucleation sites. High abundance of dibenzodioxocin type structures in this fraction indicated that they could be involved in cross-linking hemicelluloses networks with lignin moieties.