Structural Characterization of Wheat Straw Lignin as Revealed by Analytical Pyrolysis, 2D-NMR, and Reductive Cleavage Methods

TLDR

Wheat‑straw lignin is a p‑hydroxyphenyl‑guaiacyl‑syringyl polymer with an H:G:S ratio of 6:64:30 and linked to p‑coumarates and ferulates. The lignin structure was examined using analytical pyrolysis, 2D‑NMR, and DFRC derivatization. 2D‑NMR and DFRC revealed that wheat‑straw lignin is dominated by β‑O‑4′ ethers (~75%) and phenylcoumarans (~11%), contains incorporated tricin, is partially γ‑acylated (~10%) mainly with acetates on guaiacyl units, and has negligible p‑coumarate esters on monomer conjugates, implying their preferential role in condensed or terminal structures.

Abstract

The structure of the lignin in wheat straw has been investigated by a combination of analytical pyrolysis, 2D-NMR, and derivatization followed by reductive cleavage (DFRC). It is a p-hydroxyphenyl-guaiacyl-syringyl lignin (with an H:G:S ratio of 6:64:30) associated with p-coumarates and ferulates. 2D-NMR indicated that the main substructures present are β-O-4'-ethers (∼~75%), followed by phenylcoumarans (~11%), with lower amounts of other typical units. A major new finding is that the flavone tricin is apparently incorporated into the lignins. NMR and DFRC indicated that the lignin is partially acylated (~10%) at the γ-carbon, predominantly with acetates that preferentially acylate guaiacyl (12%) rather than syringyl (1%) units; in dicots, acetylation is predominantly on syringyl units. p-Coumarate esters were barely detectable (<1%) on monomer conjugates released by selectively cleaving β-ethers in DFRC, indicating that they might be preferentially involved in condensed or terminal structures.