Reactivity of p-Hydroxyphenyl Units in Enzymatic Co-Oxidative Coupling of p-Coumaryl and Coniferyl Alcohols

The p-hydroxyphenyl (H) unit is an aromatic structure found in lignin, particularly abundant in compression wood and grass, and is derived from the incorporation of p-coumaryl alcohol (p-CMA). Although the structural and biosynthetic aspects of lignin have been extensively studied, the polymerization reactivity of H-unit during lignification remains poorly understood. In this study, horseradish peroxidase (HRP)-catalyzed homo- and co-oxidative coupling reactions (initial stage of enzymatic dehydrogenative polymerization) with p-CMA and/or coniferyl alcohol (CA) were performed to investigate monolignol consumption, dilignol formation, and their potential involvement in subsequent polymerization. The results suggest that p-CMA acts as a redox shuttle mediator, promoting the dimerization of CA. Furthermore, dilignols containing a phenolic hydroxy group at the H unit showed significantly lower polymerization reactivity, suggesting that H units may exert a “capping effect” that could limit further lignin growth.