DEHYDROGENATIVE POLYMERIZATION OF CONIFERYL ALCOHOL ON MACROMOLECULARLIGNIN TEMPLATES

Lignins, the aromatic biopolymers present in all vascular plant cell w alls, may embody as many as 10 different linkages between the constitu ent (p-hydroxyphenyl)propane residues. The final step in lignin biosyn thesis has traditionally been thought to involve the random coupling o f radical intermediates produced during the dehydrogenative polymeriza tion of monolignol precursors. However, the enzyme-catalysed dehydropo lymerization of coniferyl alcohol in homogeneous solution has now been found to be strongly affected by small quantities of macromolecular l ignin components. Under these circumstances, when the radical concentr ations remained low, the molecular weight distributions of the dehydro polymerisates formed with horseradish peroxidase and H2O2 exhibited dr amatic increases in the populations of the largest species. The phenom enon was not suppressed by prior methylation of the aromatic hydroxyl groups on the causative lignin macromolecules-which, therefore, were a ble to promote the formation of large covalent species from the monoli gnol without direct participation in radical coupling. If operative in lignifying tissues, such an effect could provide the organization nec essary for replicating sequences of interunit linkages along macromole cular lignin chains. In this regard, the enzyme-catalysed dehydropolym erization of coniferyl alcohol would display the character of a typica l template polymerization reaction. (C) 1997 Elsevier Science Ltd. All rights reserved.