Fig. 3

2D ¹H–¹³C HSQC spectra of ¹³C-labeled β-O-4 lignin oligomer model compounds. Superimposed 2D ¹H–¹³C HSQC spectra of the long- and short-chain models in 50 mM acetic acid-d₄ buffer prepared using D₂O (pD 5.0) and 10% (v/v) DMSO-d₆ are shown in blue and red, respectively. 2D ¹H–¹³C HSQC spectra of the lignin oligomer models with a ¹³C-labeled aromatic rings and β positions (4^(Arβ)); b ¹³C-labeled α positions (4^(α)); and c ¹³C-labeled methoxy groups (4^(m)). d Magnified view of the aromatic region in (a). The noise observed in ¹H-chemical shift of 4.75 corresponded to the solvent residual peak of water