Effect of reaction media concentration on the solubility and the chemical structure of lignin model compounds

In plant cell walls, lignin polymerization occurs in concentrated polysaccharide gel. The effect of high polysaccharide concentrations on the structure of lignin-like polymers (DHPs=dehydrogenation polymers), were investigated by running lignification-like polymerization under three reaction conditions in which the concentrations of all reactants (xylan/coniferyl alcohol (CA)/oxidising system) were gradually increased. Control experiments were also run in similar conditions but without polysaccharides. DHPs showed increased solubility with increased concentrations of reactants in the presence of xylans but were mostly insoluble in buffer control experiments. The structures of DHPs were characterized by thioacidolysis and size exclusion chromatography (SEC). Results indicated that the frequency of beta-alkyl aryl ether bonds and DHP molecular weight increased with increasing concentration of the reaction mixture in the presence of xylans whereas those of control DHPs decreased slightly under the same conditions. This emphasizes the role of the pre-existing polysaccharide gel and high concentrations existing in the cell wall during construction of the lignin polymer.