Characterization of Lignin Structures and Lignin–Carbohydrate Complex (LCC) Linkages by Quantitative ¹³C and 2D HSQC NMR Spectroscopy

TLDR

To characterize lignin structures and lignin‑carbohydrate complex linkages, the authors isolated milled wood lignin and mild acidolysis lignin from ball‑milled poplar wood. Quantitative (13)C and 2D HSQC NMR spectroscopy were employed to determine the structural features of these lignins. The analyses revealed that acetylated 4‑O‑methylgluconoxylan is the dominant carbohydrate associated with lignin, with acetyl groups mainly on C2 and C3; MWL and MAL share similar structures dominated by β‑O‑4′ aryl ether, resinol, and phenylcoumaran linkages (abundances 41.5–43.3, 14.6–12.7, 3.7–4.0 per 100 aryl units), S/G ratios of 1.57–1.62, and quantified phenyl glycoside and benzyl ether LCC linkages, while γ‑ester linkages remained ambiguous.

Abstract

To characterize the lignin structures and lignin-carbohydrate complex (LCC) linkages, milled wood lignin (MWL) and mild acidolysis lignin (MAL) with a high content of associated carbohydrates were sequentially isolated from ball-milled poplar wood. Quantification of their structural features has been achieved by using a combination of quantitative (13)C and 2D HSQC NMR techniques. The results showed that acetylated 4-O-methylgluconoxylan is the main carbohydrate associated with lignins, and acetyl groups frequently acylate the C2 and C3 positions. MWL and MAL exhibited similar structural features. The main substructures were β-O-4' aryl ether, resinol, and phenylcoumaran, and their abundances per 100 Ar units changed from 41.5 to 43.3, from 14.6 to 12.7, and from 3.7 to 4.0, respectively. The S/G ratios were estimated to be 1.57 and 1.62 for MWL and MAL, respectively. Phenyl glycoside and benzyl ether LCC linkages were clearly quantified, whereas the amount of γ-ester LCC linkages was ambiguous for quantification.