ON THE MECHANISM OF OXIDATION OF NONPHENOLIC LIGNIN MODEL COMPOUNDS BY THE LACCASE-ABTS COUPLE

Oxidation of veratryl alcohol by laccase in the presence of ABTS (2,2'azino-bis(3-ethylbenzthiazoline-6-sulphonic acid) yields veratraldehyde, but differs to the reaction catalysed by lignin peroxidase (LiP) because ring-opened products and quinones associated with the reactivity of radical cation intermediates are not formed. The laccase-ABTS couple is also unable to oxidise 1,4-dimethoxybenzene to benzoquinone. Similar results were obtained using horseradish peroxidase (HRP) and ABTS with H2O2 as oxidant. These data favour the view that the mechanism of the ABTS-coupled reaction does not involve radical cation formation and that the reactivity of laccase-ABTS is not equivalent to that of LiP. Similarly. no evidence for radical cation formation could be obtained when syringaldehyde was substituted for ABTS. It is suggested that the reactivity observed toward non-phenolic lignin model compounds with these enzyme-coupled systems involves a hydrogen atom abstraction mechanism. The importance of oxidation via hydrogen atom abstraction in lignin degradation remains to be elucidated.