Highly Acylated (Acetylated and/or p-Coumaroylated) Native Lignins from Diverse Herbaceous Plants

Abstract

The structure of lignins isolated from the herbaceous plants sisal ( Agave sisalana), kenaf ( Hibiscus cannabinus), abaca ( Musa textilis) and curaua ( Ananas erectifolius) has been studied upon spectroscopic (2D-NMR) and chemical degradative (derivatization followed by reductive cleavage) methods. The analyses demonstrate that the structure of the lignins from these plants is highly remarkable, being extensively acylated at the gamma-carbon of the lignin side chain (up to 80% acylation) with acetate and/or p-coumarate groups and preferentially over syringyl units. Whereas the lignins from sisal and kenaf are gamma-acylated exclusively with acetate groups, the lignins from abaca and curaua are esterified with acetate and p-coumarate groups. The structures of all these highly acylated lignins are characterized by a very high syringyl/guaiacyl ratio, a large predominance of beta- O-4' linkages (up to 94% of all linkages), and a strikingly low proportion of traditional beta-beta' linkages, which indeed are completely absent in the lignins from abaca and curaua. The occurrence of beta-beta' homocoupling and cross-coupling products of sinapyl acetate in the lignins from sisal and kenaf indicates that sinapyl alcohol is acetylated at the monomer stage and that, therefore, sinapyl acetate should be considered as a real monolignol involved in the lignification reactions.

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].…”

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

“…Nowadays, the best information on lignin structure related to phenylpropanoid units and inter-unit linkages may be derived from 2D NMR spectroscopy obtained by preparations according to the MWL method (Ralph et al 1999(Ralph et al , 2004Capanema et al 2004Capanema et al , 2005Zhang et al 2006;del Río et al 2008;Rencoret et al 2008a). To obtain more information about the unaltered native lignin, some other methods have recently been proposed for the ''in situ'' analysis of lignins by 2D NMR of whole plant materials.…”

“…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).…”