Spectroscopic properties of oxidation species generated in the lignin of wood fibers by a laccase catalyzed treatment: electronic hole state migration and stabilization in the lignin matrix

A laccase catalyzed oxidative treatment of wood pulp fibers has been found to induce unusual modifications of these fibers that are qualitatively diff erent from those encountered when more severely degraded fibers are subject ed to similar enzymatically catalyzed oxidative treatments. These results s uggest that the physical/conformational state of the lignin of wood fibers determines which oxidation pathways dominate in a given oxidative treatment , leading to different lignin modifications depending on both the chemical and the physical structure of the lignin polymer. Spectroscopic measurement s (ESR, IR, UV-Vis and fluorescence) show that the laccase treatment result s in the formation of two different species in the dried fibers: one is int erpreted as chemically transformed (via oxygen) lignin products, and the ot her as initial oxidation radicals which have gained stabilization against t ransformation into the first mentioned products via a migration mechanism. It is argued that these initial radicals may likely be cation radical (or h ole state) parts in lignin. The migration mechanism is identified with site -to-site transfer or 'hopping' via electron transfer and it is postulated t hat this mechanism 'carries' cation radical parts of the lignin, produced a t the surface of the fiber, into parts of the lignin where chemical transfo rmation pathways are suppressed due to the lignin conformational state. The possible existence of such a migration mechanism, the relative dominance o f which should depend sensitively on the polymer conformational state, may have implications for the biogeneration and biodegradation of lignin as wel l as for oxidative treatments of non-natural conjugated polymers. (C) 1999 Published by Elsevier Science B.V. All rights reserved.