Revealing the Topochemistry and Structural Features of Lignin during the Growth of Eucalyptus grandis × Eucalyptus urophylla

The heterogeneity of the topochemistry and molecular structure of lignin have impeded the potential application of lignocellulosic biomass. In this study, confocal Raman microscopy (CRM), 2D heteronuclear single-quantum coherence (2D HSQC), and phosphorus magnetic resonance spectroscopy (31P NMR) were used to elucidate the topochemistry and structural variations of Eucalyptus lignin during the growth. CRM results confirmed that the lignin concentration was in the following order: cell corner middle lamella (CCML) > compound middle lamella (CML) > S2 regions in cell walls. The growth rate of lignin concentration in CCML decreased from 1 to 4 years old, which was contrary to the tendency in S2 regions. Swollen residue enzyme lignin (SREL) was isolated as a representative sample to be used in the characterization of the structural features of lignin. High yield (83.9–94.2%) and low carbohydrate content (5.11–6.25%) were observed in SRELs. An alteration in predominant monosaccharide content in the lignin samples between galactose and glucose was found during the wood maturation. Moreover, 2D NMR showed that β–O–4′ (60.75–64.55%) and β–β′ (8.56–11.16%) were the main linkages in lignin. The S/G ratio, from 3.12 to 3.67, exhibited an increasing trend with growth in SRELs.