Revealing the Topochemistry and Structural Features of Lignin during the Growth of Eucalyptus grandis x 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 (P-31 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 beta-O-4' (60.75-64.55%) and beta-beta' (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.