Oxidation of dimethoxylated aromatic compounds by lignin peroxidase from Phanerochaete chrysosporium

The stabilities of the cation radicals of veratryl alcohol, 3,4-dimethoxytoluene and 1,4-dimethoxybenzene were compared by monitoring the formation of dimeric products during the oxidation of these substrates by lignin peroxidase (LiP). LiP oxidized veratryl alcohol to generate veratraldehyde as the major product. Several other monomeric products were obtained in low yield. Dimeric products resulting from the coupling of two cation radicals, or a cation radical with a neutral molecule, were obtained only in trace amounts or not at all. This suggests that the cation radical of veratryl alcohol rapidly loses a benzylic proton to form a benzylic radical which undergoes further reactions to form veratraldehyde. In contrast, the LiP oxidation of 3,4-dimethoxytoluene generated the dimeric product 3-(2,3-dimethoxy-6-methylphenyl)-4-methyl-1,2-benzoquinone as the major product. Several other monomeric and dimeric products were produced in lower yields. The generation of these dimeric products indicates that the cation radical of 3,4-dimethoxytoluene is considerably more stable than that of veratryl alcohol. This suggests that the electronegative benzylic oxygen of veratryl alcohol increases the acidity of the benzylic protons, destabilizing the veratryl alcohol cation radical. LiP oxidized 1,4-dimethoxybenzene to generate 1,4-benzoquinone and 2-(2,5-dimethoxyphenyl)-1,4-benzoquinone as the major products. The formation of these products indicates that the cation radical of 1,4-dimethoxybenzene also is relatively stable, as previously demonstrated by ESR. All of these results indicate that the veratryl alcohol cation radical generated by LiP oxidation is unstable, suggesting that it would not act as a diffusable radical mediator in LiP-catalyzed reactions.