OXIDATION OF DIMETHOXYLATED AROMATIC-COMPOUNDS BY LIGNIN PEROXIDASE FROM PHANEROCHAETE-CHRYSOSPORIUM

The stabilities of the cation radicals of veratryl alcohol, 3,4-dimeth oxytoluene and 1,4-dimethoxy-benzene were compared by monitoring the f ormation 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 coupli ng of two cation radicals, or a cation radical with a neutral molecule , were obtained only in trace amounts or not at all. This suggests tha t the cation radical of veratryl alcohol rapidly loses a benzylic prot on to form a benzylic radical which undergoes further reactions to for m veratraldehyde. In contrast, the LiP oxidation of 3,4-dimethoxytolue ne generated the dimeric product imethoxy-6-methylphenyl)-4-methyl-1,2 -benzoquinone as the major product. Several other monomeric and dimeri c products were produced in lower yields. The generation of these dime ric products indicates that the cation radical of 3,4-dimethoxytoluene is considerably more stable than that of veratryl alcohol. This sugge sts that the electronegative benzylic oxygen of veratryl alcohol incre ases the acidity of the benzylic protons, destabilizing the veratryl a lcohol cation radical. LiP oxidized 1,4-dimethoxybenzene to generate 1 ,4-benzoquinone and 2-(2,5-dimethoxyphenyl)-1,4-benzoquinone as the ma jor products. The formation of these products indicates that the catio n radical of 1,4-dimethoxybenzene also is relatively stable, as previo usly demonstrated by ESR. All of these results indicate that the verat ryl alcohol cation radical generated by Lip oxidation is unstable, sug gesting that it would not act as a diffusible radical mediator in LiP- catalyzed reactions.