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

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-coumnarate 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.