Deciphering the linkage type and structural characteristics of the p-hydroxyphenyl unit in Pinus massoniana Lamb compressed wood lignin

To reveal the existence of p-hydroxyphenyl (H) units in compressed wood lignin, four different milled wood lignins were extracted using Pinus massoniana Lamb compressed wood, Pinus massoniana Lamb normal wood, and sugarcane bagasse as raw materials. Then, three dehydrogenation polymers (DHPs) were synthesized using coniferyl/p-coumaryl alcohol as raw materials to reveal the interunit linkages of H units. The lignin and DHP samples were systematically characterized by ¹H, ¹³C, 2D HSQC, and ³¹P NMR techniques. Compared with the opposite wood milled wood lignin (OW-MWL) and the normal wood milled wood lignin (NW-MWL), the compressed wood milled wood lignin (CW-MWL) contained a large amount of H units, and the H/G ratio and the p-hydroxyphenyl OH group contents were 0.15 and 1.09 mmol/g, respectively. Through the characterization of CW-MWL and DHPs, it was revealed that p-hydroxyphenyl units mainly coupled with other units by β-O-4, β-β, and β-5 linkages. Compared to the sugarcane bagasse milled wood lignin, it was clearly demonstrated that the H unit rather than p-coumarate ester occurred in CW-MWL. This study comprehensively explored the structural characteristics and linkages of H units in compress wood lignin, and provided useful information for revealing the participation of H units in the construction of lignin macromolecules.