Enzymatic dehydrogenative polymerization of monolignol dimers

The structure and biosynthesis of lignin are not yet fully understood, especially the step following the initial dimerization of monolignol. Liquid chromatograph mass spectrometer (LC-MS) was used to analyze the consumption rates of monolignol dimers formed by beta-O-4, beta-5, and beta-beta couplings between coniferyl alcohols in efforts to understand the activity of monolignol dimers in enzymatic dehydrogenative polymerization. We investigated the reaction kinetics in single-component and mixed-component reaction systems containing one and two species of the dimers, respectively. A difference was observed between the consumption rates of the three dimers we tested, and the consumption rate of one dimer in the single-component reaction was different from that in a mixed-component reaction. In qualitative LC-MS analyses, coniferyl alcohol oligomers were detected in the reaction products. Some monolignol tetramers were formed by 5-5 and 5-O-4 coupling between the dimers. The results of this work suggested that monolignol dimers with beta-5 and beta-beta linkages could function as radical mediators in enzyme-catalyzed polymerization.