Monolignol acylation and lignin structure in some nonwoody plants: A 2D-NMR study

Abstract

Three non-wood lignins differing in their molecular structure were isolated and characterized by 2D-NMR. The Musa textilis milled-wood lignin (MWL), with a syringyl-to-guaiacyl (S/G) ratio of 9, is strongly acylated (near 85% of side-chains) at the γ-carbon by both acetates and p-coumarates, as estimated from 13C-1H correlations in Cγ-esterified and Cγ-OH units. The p-coumarate C3,5-H3,5 signal was completely displaced by acetylation, and disappeared after alkali treatment, indicating that p-coumaric acid was esterified maintaining its free phenolic group. By contrast, the Cannabis sativa MWL (S/G ~0.8) was free of acylating groups, and the Agave sisalana MWL (S/G ~4) showed high acylation degree (near 80%) but exclusively with acetates. Extensive Cγ-acylation results in the absence (in M. textilis lignin) or low abundance (4% in A. sisalana lignin) of β-β' resinol linkages, which require free Cγ-OH to form the double tetrahydrofuran ring. However, minor signals revealed unusual acylated β-β' structures confirming that acylation is produced at the monolignol level, in agreement with chromatographic identification of γ-acetylated sinapyl alcohol among the plant extractives. In contrast, resinol substructures involved 22% side-chains in the C. sativa lignin. The ratio between β-β' and β-O-4' side-chains varied from 0.32 in the latter lignin to 0.02-0.07 in the two other MWL, and was inversely correlated with the degree of acylation. The opposite was observed for the S/G ratio that was directly correlated with the acylation degree. Monolignol acylation in these and other angiosperms is discussed as a mechanism contributing to regulate the structure of lignin.