Studies of xylan interactions and cross-linking to synthetic lignins formed by bulk and end-wise polymerization: a model study of lignin carbohydrate complex formation

Abstract

The mechanism of lignin carbohydrate complex formation by addition of polysaccharides on quinone methide (QM) generated during lignin polymerisation was investigated using a model approach. Dehydrogenation polymers (DHPs, lignin model compounds) were synthesized from coniferyl alcohol in the presence of a glucuronoarabinoxylan (GAX) extracted from oat spelts, by Zutropfverfahren (ZT) and Zulaufverfahren (ZL) methods. The methods ZT and ZL differed in their distribution of QM over the reaction period but generated roughly the same QM amount. Steric exclusion chromatography of the ZT and ZL reaction products showed that only the ZT reaction produced high molar mass compounds. Covalent linkages in the ZT reaction involving ether bonds between GAX moiety and alpha carbon of the lignin monomer were confirmed by (13)C NMR and xylanase-based fractionation. The underlying phenomena were further investigated by examining the interactions between GAX and DHP in sorption experiments. GAX and DHPs were shown to interact to form hydrophobic aggregates. In the ZT process, slow addition permitted polymer reorganisation which led to dehydration around the lignin-like growing chains thereby limiting the addition of water on the quinone methide formed during polymerisation and thus favoured lignin-carbohydrate complex (LCC) formation.

Localization of Cell Wall Polysaccharides in Normal and Compression Wood of Radiata Pine: Relationships with Lignification and Microfibril Orientation

“…These polysaccharides are known to form chemical bonds with lignin to form lignin-carbohydrate complexes (Tenkanen et al, 1999;Lawoko et al, 2005;Barakat et al, 2007), and since hemicelluloses are deposited prior to or concurrently with lignin (Kim et al, 2010a(Kim et al, , 2010b(Kim et al, , 2010c(Kim et al, , 2011, the possibility that they influence lignin deposition seems reasonable. This study extends these earlier results by definitively colocalizing lignin and polysaccharides on the same field of view and measuring their association by image analysis.…”

“…In a comparison of different polysaccharides, Iwata et al (1998) found that beech (Fagus sylvatica) glucomannans have the highest affinity for cellulose, followed by xyloglucan, xylan, and arabinogalactan. Mannans may also associate with xylans and with lignin (Lawoko et al, 2005;Barakat et al, 2007).…”

Microwave-assisted extraction of lignin from triticale straw: Optimization and microwave effects

“…Wheat straw lignin is known to contain arabinoxylans as the main carbohydrate impurities with lignin being linked with arabinose side chains of xylan by ether bonds (Sun et al, 1997). The majority of these ether linkages occur at the a-position of the propanoid chains of lignin as initially suggested by their possible cleavage under acidic conditions (Mc Donough, 1993;Cornu et al, 1994;Lawther et al, 1996) and later confirmed by NMR and xylanase-based fractionation (Barakat et al, 2007). Applying MW instead of CH decreased significantly the xylose content of the lignin but had almost no effect on the arabinose content.…”

Supramolecular Self-Assembled Chaos: Polyphenolic Lignin’s Barrier to Cost-Effective Lignocellulosic Biofuels

“…Dehydration enhances the local polysaccharide concentration and also reduces water attack on the quinone methide intermediate (Figure 4), enabling hemicellulose reactions to generate covalent benzyl esters and ethers of the LCCs (Figure 7). Consequently, the concentrations of lignin and LCCs probably track in the same direction [115]. Nucleation sites have been proposed for promoting lignin polymerization [28].…”

“…However, when 'Zulauf' technique was adopted, only noncovalent bonds were observed between xylan and DHP. The resulting xylan-DHP supramolecular nanocomposites were resistant to endoxylanase digestion [106,[113][114][115]. Uraki et al [116] reported that the hemicelluloses xylan and mannan adsorbed the monolignols non-covalently through hydrophobic forces.…”