Structural variations of lignin macromolecule from different growth years of Triploid of Populus tomentosa Carr.

•A ultrahigh yield of native lignin based on enzymatic hydrolysis was firstly proposed.•The proposed lignin is an ideal sample to delineate the structures of entire lignin.•The lignin molecules from different growth years of poplar are structurally similar.•The morphological distribution of lignin was monitored by Confocal Raman Microscopy.

To better understand the variations of structural characteristics of lignin macromolecules during different growth years of Triploid of Populus tomentosa Carr, a novel lignin isolation procedure based on double ball-milling and enzymatic hydrolysis (DEL) was proposed in this study. The morphological distributions of lignin in the plant cell wall of these poplar wood samples were monitored by Confocal Raman Microscopy (CRM). The ultrahigh yields (105.1%-111.2%) of DELs were significantly higher than those (24.4-31.8%) of corresponding cellulolytic enzyme lignins (CELs). DELs and CELs were elaborately characterized by HPAEC, GPC, 2D-HSQC NMR and 31P NMR techniques, and NMR results showed that DEL samples possess similar structural features as compared to CEL counterparts except for the decreased S/G ratio and p-hydroxybenzoate (PB) as well as increased p-hydroxyphenyl units (H). There are no obvious differences in the structural characteristics except for high contents of PB and H units in DEL-1, as well as high S/G ratio and β-O-4′ linkages in DEL-5. It is believed that the DEL proposed in the present study can be used for characterizing the entire structural features of lignin macromolecules in the plant cell wall of different kinds of lignocellulosic biomass.

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