Monolignol dehydrogenative polymerization in vitro in the presence of dioxane and a methylated β-β′ dimer model compound

Lignin formation is believed to occur by polymerization of resonance-stabilized monolignol radicals formed by enzymatic oxidation. Recently, different hypotheses suggested that lignin polymerization is influenced by surfaces in the cell wall which can be polysaccharides or proteins. The latter is called the proteinaceous dirigent sites/template polymerization hypothesis. According to another hypothesis, lignin itself is believed to act as a template and replicate its primary structure. In this work, dehydrogenative polymerization (DHP) of the lignin precursor coniferyl alcohol was performed in vitro in the presence and absence of pinoresinol dimethyl ether (a β-β′ dimer model). Another peculiarity of the experiments was the presence of dioxane which afforded a high solubility of the reactants. The question was whether the presence of β-β′ dimer model would change the structure of the DHP formed. The DHPs were analyzed by quantitative 13C NMR, GC-FID, and GC-MS. The dimer model as a template in the homogeneous polymerization state (in solution) did not influence the DHP structure.