Lignin Structure and Interunit Linkages

The intact linkage structures and subunits within the lignin polymer can now be resolved by high resolution 2D HSQC NMR based on ¹H-¹³C correlations.^8,51-60 Approximately 85% of the lignin interunit linkages have been identified (Figure 3).^55,58 Absolute quantitation of composition and linkages can be obtained from a 1D ¹³C and a 2D HSQC spectrum.^55,58-60 β-ethers (β-O-4) account for 40 to 65% of the total linkages in wood lignins.^{7,8,37,53,55,59} Other linkages include α-ethers (α-O-4) and C-C types, such as β-5, β-β, β-1 and 5-5. Atypical monomers in lignin have also been detected by NMR. Most if not all linkage types have been corroborated as PO-catalyzed oxidative coupling products.^7,8,47,61-67