Unexpected polymerization mechanism of dilignol in the lignin growing.

Lignin is an essential component of higher plants, which is built by the enzymatic dehydrogenative polymerization of monolignols. First, monolignol is enzymatically oxidized to produce the phenoxy radical, which can form resonance hybrids. Two radical resonant hybrids are coupled with each other to yield dilignol with various linkage types, of which the main structures are β-O-4' (I), β-5' (II) and β-β' (III). However, the reaction mechanism behind the addition lignol radicals to dilignol is not yet fully understood. Here, we show an unexpected reaction with structure II during enzymatic dehydrogenative polymerization, which involves cleavage of a covalent linkage and creation of a new radical coupling site. This implied that the β-5 dilignol diversifies the growing pattern of lignin. This discovery elucidates a novel mechanism in lignin polymerization.