Syringyl α-carbonyl-type tetrahydrofuran β–β dimer and its hemiacetal were identified in the dehydrogenation products of sinapyl alcohol (SA), a hardwood lignin monomer, by biomimetic oxidation using laccase from Trametes versicolor (LacT). To investigate the presence of these structures in native lignin, two-dimensional heteronuclear single-quantum coherence nuclear magnetic resonance (2D HSQC NMR) analysis was performed on milled wood lignins (MWLs) prepared from eucalyptus and acacia woods and dehydrogenation polymer (S-DHP) prepared by the biomimetic oxidation of SA using horseradish peroxidase (HRP). Diagnostic NMR signals corresponding to the α-carbonyl-type β–β (IIIox1) and its hemiacetal (IIIox2) structures were observed in the HSQC NMR spectra of MWLs and S-DHP, albeit with low signal intensities, demonstrating their existence in hardwood lignins. The formation of IIIox1 and IIIox2 from SA in the biomimetic laccase/O2 and peroxidase/H2O2 systems supported that these α-oxidized β–β structures originated from the lignin polymerization process in plant cell walls.

Dehydrogenative polymerization of coniferyl alcohol (CA) and sinapyl alcohol (SA) was conducted using commercial laccases, fungal laccase from Trametes versicolor ( LacT ) and plant laccase from Rhus vernicifera ( LacR ), at pH 4–7 to investigate how the enzymatic polymerization of monolignols differs between these two laccase systems. The enzyme activity of LacT was the highest at pH 4, whereas that of LacR was the highest at pH 7. A dehydrogenation polymer (DHP) was obtained only from CA in both laccase systems, although the consumption rate of SA was higher than that of CA. 1 H– 13 C HSQC NMR analysis showed that DHPs obtained using LacT and LacR contained lignin substructures, including β- O -4, β- O -4/α- O -4, β-β, and β-5 structures. At pH 4.5, the β- O -4 structure was preferentially formed over the β- O -4/α- O -4 structure, whereas at pH 6.5, the β- O -4/α- O -4 structure was preferred. The pH of the reaction solution was more vital to affect the chemical structure of DHP than the origin of laccases.

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