Peroxidase activity can dictate the in vitro lignin dehydrogenative polymer structure

Abstract

The objective of this study was to assess the influence of the peroxidase/coniferyl alcohol (CA) ratio on the dehydrogenation polymer (DHP) synthesis. The soluble and unsoluble fractions of horseradish peroxidase (HRP)-catalyzed CA dehydrogenation mixtures were recovered in various proportions, depending on the polymerization mode (Zutropf ZT/Zulauf ZL) and HRP/CA ratio (1.6–1100 purpurogallin U mmol −1). The ZL mode yielded 0–57%/initial CA of unsoluble condensed DHPs (thioacidolysis yields <200 μmol g −1) with a proportion of uncondensed CA end groups increasing with the HRP/CA ratio (7.2–55.5%/total uncondensed CA). Systematically lower polymer yields (0–49%/initial CA) were obtained for the ZT mode. In that mode, a negative correlation was established between the β-O-4 content (thioacidolysis yields: 222–660 μmol g −1) and the HRP/CA ratio. In both modes, decreasing the HRP/CA ratio below 18 U mmol −1 favoured an end-wise polymerization process evidenced by the occurrence of tri-, tetra- and pentamers involving at least one β-O-4 bond. At low ratio, the unsoluble ZT DHP was found to better approximate natural lignins than DHPs previously synthesized with traditional methods. Besides its possible implication in lignin biosynthesis, peroxidase activity is a crucial parameter accounting for the structural variations of in vitro DHPs.