Degradation mechanisms of a nonphenolic β-O-4 lignin model dimer by Trametes versicolor laccase in the presence of 1-hydroxybenzotriazole

Abstract

Past studies have shown that Trametes versicolor laccase catalyzed Cα-Cβ cleavage, Cα-oxidation, β-ether cleavage and aromatic ring cleavage of a nonphenolic β-O-4 lignin model dimer, 1,3-dihydroxy-2-(2,6-dimethoxyphenoxy)-1-(4-ethoxy-3-methoxyphenyl)propane (I), in the presence of 1-hydroxybenzotriazole. In this study, we investigated the detailed degradation mechanism for these pathways of substrate I by laccase/1-hydroxybenzotriazole couple, based on incorporation experiments from H218O and 18O2. Oxidation of substrate I in H218O resulted in the incorporation of 18O into three aromatic ring cleavage products and a β-ether cleavage product. These results were identical with those for lignin peroxidase. We inferred that these cleavages of substrate I proceed via the β-aryl cation radical intermediate of I. However, the Cα-Cβ cleavage degradation product of I by the laccase/1-hydroxybenzotriazole couple was 4-ethoxy-3-methoxybenzoic acid, whereas it was 4-ethoxy-3-methoxybenzaldehyde for lignin peroxidase. It is clear that one atom of 18O from 18O2 was incorporated into 4-ethoxy-3-methoxybenzoic acid during the oxidation of substrate I. To confirm the reaction intermediate, we carried out the chemical reaction of 2-(2,6-dimethoxyphenoxy)-1-(4-ethoxy-3-methoxyphenyl)- 3-hydroxypropanone with alkaline hydrogen peroxide, since the expected hydroperoxide intermediate is quite similar to that formed from the reaction between the benzylic radical of I and O2. The result showed that the main degradation product was 4-ethoxy-3-methoxybenzoic acid. Thus, we propose a novel Cα-Cβ cleavage via a kind of Baeyer-Villiger reaction for nonphenolic β-O-4 lignin model dimer by laccase/1-hydroxybenzotriazole couple.