Occurrence and structural characteristics of highly acylated (acetylated and/or p-coumaroylated) native lignins from diverse herbaceous plants

Abstract

We have studied the occurrence of native acylated (with acetates and/or p-coumarates) lignin in a large set of vascular plants, including both angiosperms and gymnosperms by DFRC and HSQC-NMR. Acylated lignin units were found in all angiosperms, including mono- and eudicotyledons, but were absent in the gymnosperms analyzed. Acylation occurred exclusively at the γ-carbon of the lignin-side chain and predominantly on syringyl units. In some plants (e.g. sisal, kenaf, abaca, curaua or hornbeam), lignin acylation occurred at a very high extent, up to 80 %. The structure of these highly-acylated lignins was characterized by a very high syringyl/guaiacyl ratio, a predominance of β-O-4´ linkages (up to 94% of all linkages) and a very low proportion of β-β´ linkages, which indeed are completely absent in the lignins from abaca and curaua. In all cases, acylation appears to occur at the monomer stage and sinapyl and coniferyl acetates have been demonstrated to behave as real lignin monomers participating in lignification.