Lignins, the aromatic biopolymers present in all vascular plant cell walls, may embody as many as 10 different linkages between the constituent (p-hydroxyphenyl)propane residues. The final step in lignin biosynthesis has traditionally been thought to involve the random coupling of radical intermediates produced during the dehydrogenative polymerization of monolignol precursors. However, the enzyme-catalysed dehydropolymerization of coniferyl alcohol in homogeneous solution has now been found to be strongly affected by small quantities of macromolecular lignin components. Under these circumstances, when the radical concentrations remained low, the molecular weight distributions of the dehydropolymerisates formed with horseradish peroxidase and H2O2 exhibited dramatic increases in the populations of the largest species. The phenomenon was not suppressed by prior methylation of the aromatic hydroxyl groups on the causative lignin macromolecules which, therefore, were able to promote the formation of large covalent species from the monolignol without direct participation in radical coupling. If operative in lignifying tissues, such an effect could provide the organization necessary for replicating sequences of interunit linkages along macromolecular lignin chains. In this regard, the enzyme-catalysed dehydropolymerization of coniferyl alcohol would display the character of a typical template polymerization reaction.
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Lignins, among all biopolymers, are second only to cellulose in abundance and yet little has explicitly emerged about the actual configurations of their constituent macromolecular chains. These aromatic cell wall components are formed in all vascular plants and woody tissues through the dehydrogenative polymerization of no more than three monolignols, which differ only in the methoxy substituents around the aromatic ring [l]. The precursors are believed to be sequentially incorporated into polymeric lignin structures in an order, viz. p-hydroxycinnamyl (p-coumaryl) alcohol, 4-hydroxy-3-methoxycinnamyl (coniferyl) alcohol, and 4-hydroxy-3,5-dimethoxycinnamyl (sinapyl) alcohol, which follows their positions in the biosynthetic pathway . In the process they are oxidatively coupled to form as many as ten different inter- * Paper no. 22,419 of the Scientific Journal Series of the Minnesota Agricultural Experiment Station. funded through Minnesota Agricultural Experiment Station Project no. 43-68, supported by Hatch Funds. t Author to whom correspondence should be addressed.
Copyright 2004 Elsevier Science B.V., Amsterdam. All rights reserved.
- Coniferyl alcohol
- Dehydrogenative polymerization
- Lignin biosynthesis
- Template polymerization