The monomer composition controls the ∑β-O-4/∑O-4 end monomer ratio of the linear lignin fraction

Abstract

Lignins are cell wall phenolic heteropolymers that result from the oxidative coupling of three monolignols bearing p-coumaryl (H), coniferyl (G), and sinapyl (S) units, in a reaction mediated by peroxidases. Here, we report the existence of a relationship between the ∑β-O-4/∑O-4 end monomer ratio of the linear lignin fraction, released through the specific cleavage of the alkyl ether linkages by thioacidolysis, and the G/S ratio of lignins, when this was estimated in differentially evolved vascular land plants. Most importantly, in the case of angiosperms, Gnetales, and lycopods, the ∑β-O-4/∑O-4 end monomer ratio was apparently predictable from the proportions at which the G and S units were mixed. In the case of G lignins (present in basal gymnosperms and ferns), the ∑β-O-4/∑O-4 end monomer ratio decayed exponentially to increase the O-4-linked dihydroconiferyl alcohol (DHCA) content. The results obtained suggest that the ∑β-O-4/∑O-4 end monomer ratio of the linear lignin fraction depends intimately on the lignin monomer composition, and, therefore, on the chemical nature of the radicals derived from three monolignols (coniferyl, dihydroconiferyl, and sinapyl alcohols), whose gain have been finely tuned during land plant evolution. © 2007 The Japan Wood Research Society.