Exploration of the chemical linkages between lignin and cellulose in poplar wood with 13C and Deuterium dual isotope tracer

Stable isotopes were used to label lignin and cellulose to accurately explain the chemical association between them in poplar wood. Both the 13C-labeled lignin precursors (coniferin-[Cα-13C] or coniferin-[Cγ-13C]) and the Deuterium-labeled cellulose precursor (UDP-glucose-[6,6-D2]) were added to a growing poplar plant. After 20 d cultivation, hemicellulose and lignin-carbohydrate complexes (LCC) were extracted from the newly-formed xylem by Björkman method, then the abundance of 13C and D were detected. It revealed that both lignin precursors and cellulose precursor could be efficiently deposited in the newly-formed xylem, while cellulose precursor was detected mainly deposited on cellulose. Then the LCC was degraded using cellulase and hemicellulase to obtain enzymatically degraded lignin-carbohydrate complexes (EDLCC). The 13C NMR，1H NMR and 2D NMR analyses of EDLCC clearly showed that lignin subunits in poplar wood were mainly connected through β-O-4, β-5, β-β structures, while cellulose was mainly connected to lignin by ether bonds, acetal bonds and ester bonds. The ether bonds and acetal bonds were between cellulose C6 and lignin Cα, while the ester bonds were between cellulose C6 and lignin Cγ. The study provides new evidence for the structural information between cellulose and lignin in poplar.