Linkage-dependent terminal localization of p-hydroxyphenyl-containing substructures in Cryptomeria japonica compression wood lignin

Lignin is a major aromatic polymer in plant cell walls and contributes to mechanical strength, hydrophobicity, and resistance to microbial degradation. In softwoods, compression wood formed in response to gravitational stimuli contains lignin enriched in p -hydroxyphenyl (H) units, but how these units are incorporated into the lignin polymer remains incompletely understood. Previous studies using methylation followed by thioacidolysis suggested that H-units in compression wood lignin mainly occur as phenolic terminal units; this supports the so-called capping effect, whereby further chain propagation from the H-units is unlikely to occur. However, this analysis relies solely on the β-O-4 type substructure, and the substructure of condensed H units remains unanalyzed. This study demonstrated the linkage-dependent terminal localization of H-unit substructures in cellulolytic enzyme lignin (CEL) isolated from Cryptomeria japonica (Japanese cedar) compression wood. Experiments using β-O-4 type lignin model polymers showed that the substructures in which the phenolic hydroxyl groups of the H-units form β-O-4 bonds undergo incomplete cleavage during thioacidolysis. However, it was found that almost no incomplete cleavage occurred during the thioacidolysis of methylated CEL, confirming that H units with β-O-4 linkage are primarily located at phenolic termini. It was also found that H units within the β-5 and 5-5 type substructures primarily exist as phenolic terminal structures. Although the H units within the 4-O-5 substructure also primarily possessed terminal phenolic hydroxyl groups, it was shown that, compared to other H-unit substructures, the proportion of etherified internal structures was relatively high. These results refine the capping model for H units and suggest that the strength of the capping effect depends on the type of linkage.