The unmasking of lignin structures in wheat straw by alkali

This study reports on the structural modifications of wheat straw cell wall promoted by potassium carbonate and sodium hydroxide that lead to the unmasking of some lignin structures. The first impact of the treatments was the extraction of a particular fraction of lignin enriched in C-C linked structures compared to the mean composition in reference wheat straw. Concomitantly, an apparent increase in the amount of lignin monomers released by the cleavage of alkyl-aryl ether bonds was observed in alkali-extracted samples. By summing the amount of ether linked monomers analyzed by thioacidolysis in the solubilized lignin to that found in the extracted wheat straw, an excess of up to 37% is apparent, relative to the corresponding amount in the reference wheat straw. Other modifications of the cell wall were also found. Indeed, a fraction of uronic acids was lost during the treatments and a new fractionation pattern of the lignin-carbohydrate complexes was evidenced. It can thus be concluded that a significant proportion of lignin within the cell wall was unmasked after (i) the selective removal of a particular lignin fraction, (ii) a partial saponification of the esterified fraction of lignin with uronic acids and (iii) a modification of the interactions between the cell wall constituents.