Occurrence of Naturally Acetylated Lignin Units

Abstract

This work examines the occurrence of native acetylated lignin in a large set of vascular plants, including both angiosperms and gymnosperms, by a modification of the so-called Derivatization Followed by Reductive Cleavage (DFRC) method. Acetylated lignin units were found in the milled wood lignins of all angiosperms selected for this study, including mono-and eudicotyledons, but were absent in the gymnosperms analyzed. In some plants (e.g., abaca, sisal, kenaf, or hornbeam), lignin acetylation occurred at a very high extent, exceeding 45% of the uncondensed (alkyl-aryl ether linked) syringyl lignin units. Acetylation was observed exclusively at the γ-carbon of the lignin side chain and predominantly on syringyl units, although a predominance of acetylated guaiacyl over syringyl units was observed in some plants. In all cases, acetylation appears to occur at the monomer stage, and sinapyl and coniferyl acetates seem to behave as real lignin monomers participating in lignification.

HSQC-NMR analysis of lignin in woody (Eucalyptus globulus and Picea abies) and non-woody (Agave sisalana) ball-milled plant materials at the gel state 10th EWLP, Stockholm, Sweden, August 25–28, 2008

“…The HSQC spectrum of sisal fibers, therefore, undoubtedly demonstrates that this lignin is extensively acylated and that acylation takes place only at the gposition of the lignin side-chain. Previous studies have indicated that acylation in sisal occurs exclusively with acetate groups and predominantly over syringyl units (del Río et al 2004(del Río et al , 2007. The presence of acetylated lignin units in the HSQC of sisal fibers excludes the possibility of acetylation as an artifact produced during the MWL isolation protocol.…”

“…The mechanically disintegrated and acetylated cell wall is then fully soluble in chloroform allowing the whole lignin to be analyzed directly (in the environment of dissolved polysaccharides) by NMR. Acetylation has the advantage to make readily visible the various OH groups; however, it can result in loss of important structural information since different lignins, mainly from grasses and other herbaceous plants, are known to be naturally acetylated to a high extent (Ralph 1996;Lu and Ralph 2002;del Río et al 2004del Río et al , 2007del Río et al , 2008, most probably as a mechanism for regulating lignin structure at the monolignol level (Martínez et al 2008).…”

Structural Modifications of Residual Lignins from Sisal and Flax Pulps during Soda-AQ Pulping and TCF/ECF Bleaching

“…The major lignin compounds released from this lignin were 4-methylphenol (4), guaiacol (5), 4-methylguaiacol (9), 4-ethylguaiacol (10), 4-vinylguaiacol (12), syringol (13), 4-methylsyringol (19), transisoeugenol (20), guaiacylacetone (24), 4-vinylsyringol (25), and trans-4-propenylsyringol (31). The Py-GC/MS data (Table 1) indicate a 11:74:15 (H:G:S) relative composition and a low S/G ratio of 0.2 for flax MWL.…”

Naturally p-Hydroxybenzoylated Lignins in Palms

“…Acylated lignins of four types have been encountered in various plants to date. Angiosperm trees have long been thought to have low-level lignin acetylation [15], whereas kenaf bast fiber lignins and lignins in various other plants are extensively acetylated [16][17][18][19][20][21]. Grasses produce the enzyme p-coumaroyl-CoA:monolignol transferase (PMT) [22,23], which produces γ-p-coumaroylated monolignols that are incorporated into the growing lignin polymer, particularly at advanced maturity [24][25][26].…”