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 different from those encountered when more severely degraded fibers are subjected to similar enzymatically catalyzed oxidative treatments. These results suggest 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 measurements (ESR, IR, UV-Vis and fluorescence) show that the laccase treatment results in the formation of two different species in the dried fibers: one is interpreted as chemically transformed (via oxygen) lignin products, and the other as initial oxidation radicals which have gained stabilization against transformation into the first mentioned products via a migration mechanism. It is argued that these initial radicals may likely be cation radical (or hole state) parts in lignin. The migration mechanism is identified with site-to-site transfer or 'hopping' via electron transfer and it is postulated that this mechanism 'carries' cation radical parts of the lignin, produced at the surface of the fiber, into parts of the lignin where chemical transformation pathways are suppressed due to the lignin conformational state. The possible existence of such a migration mechanism, the relative dominance of which should depend sensitively on the polymer conformational state, may have implications for the biogeneration and biodegradation of lignin as well as for oxidative treatments of non-natural conjugated polymers. (C) 1999 Published by Elsevier Science B.V. All rights reserved.