Monolignol Acylation and Lignin Structure in Nonwoody Plants

2D NMR revealed a correspondence between the molecular structure of lignins and their degree of acylation in several nonwoody angiosperms characterized by different lignin acylation types (acetate and/or p-coumarate esters) and extents (up to more that 90% of units). Some of the lignin substructures identified showed that acylation is produced at the monolignol level. Direct evidence was also provided by GC-MS identification of γacetylated sinapyl alcohol among plant extractives. Taking the above findings together, it is proposed that some angiosperms could use monolignol acylation as a mechanism to regulate the structure of lignin. In this way, acylation would result in lower presence of resinols and other β-β' interunit linkages in S-rich lignins, promoting formation of a highly-etherified (β-O-4' linked) and more labile lignin polymer. In contrast, low (or null) acylation would result in lignins with higher content of C-C interunit linkages, that will be more recalcitrant towards chemical and/or biological degradation.