Structural Variation of Lignin and Lignin−Carbohydrate Complex in Eucalyptus grandis × E. urophylla during Its Growth Process

Milled wood lignin (MWL), lignin−carbohydrate complex-rich fraction (LCC-AcOH), celluloytic enzyme lignin (CEL), and enzymatic hydrolysis residue (EHR) were sequentially isolated from Eucalyptus grandis × E. urophylla
under mild conditions, and the variations of lignin−carbohydrate complex (LCC) linkages and lignin structures during the eucalyptus growth were investigated. The 2D HSQC NMR analysis showed that β-O-4′ and β−β′ were the main
linkages in lignin, while other substructures were present in much lower amounts. The amounts of β-O-4′ in the MWL and LCC-AcOH fractions showed an increased tendency and those in the CEL and EHR fractions had no obvious variation with the eucalyptus growth. The S/G ratios of the MWL, LCC-AcOH, and CEL fractions increased first and then decreased, whereas those of the EHR fractions decreased with the tree age. The amount of phenyl glycoside (PhGlc) in the LCC-AcOH fractions varied consistent with the S/G ratio. The variation of the amount of benzyl ether (BE) in the MWL fractions was parallel to the
S/G ratio, while that in the CEL fractions was contrary to it. These findings will provide some evidence for the structural variation of lignin and LCC in Eucalypt during its growth process.