Structural Variations of Lignin Macromolecules from Early Growth Stages of Poplar Cell Walls

The natural lignin macromolecule is structurally complex and heterogeneous polymers. Herein, to comprehend the dynamic structural characteristics of poplar (Populus tomentosa) lignin during the growth stages, the native lignins were isolated from poplar woods with different growth stages (3, 6, and 18 months). Meanwhile, the microscopic distribution of lignin and structural features of components in the cell wall of poplar woods were monitored through CRM and CP/MAS C-13 NMR, respectively. The isolated native lignin samples were synthetically characterized by NMR, GPC, and Py-GC/MS techniques for uncovering the structural variations of lignin during different growth stages. Solution-state 2D-HSQC NMR spectra of the poplar cell wall were also performed to investigate the structural characteristics of the cell wall components in poplar woods. All the results indicated that the lignin macromolecule was gradually polymerized with the increasing growth period of poplar. On the basis of the results obtained, possible structural models of the lignin macromolecule from different growth stages of poplar were proposed. The panoramic picture of chemical and structural characteristics of the lignin macromolecule is significant not only for the understanding of structural variations of the lignin macromolecule in the plant cell walls during different growth stages but also for biomass deconstruction and valorization.