Laccase oxidation of phenolic azo dyes was examined with a commercially available laccase from Pyricularia oryzae as the model. Methyl-, methoxy-, chloro-, and nitro-substituted derivatives of 4-(4-sulfophenylazo)phenol were examined as substrates for this laccase. Only the substituents on the phenolic ring were changed. Among the dyes examined, only 2-methyl-, 2-methoxy-, 2,3-dimethyl-, 2,6-dimethyl-, 2,3-dimethoxy-, and 2,6dimethoxy-substituted 4-(4-sulfophenylazo)-phenol served as substrates. Preliminary kinetic studies suggest that 2,6-dimethoxy-substituted 4-(4-sulfophenylazo)-phenol is the best substrate. Laccase oxidized the 2,6dimethyl derivative of 4-(4-sulfophenylazo)-phenol to 4-sulfophenylhydroperoxide (SPH) and 2,6-dimethyl-1,4-benzoquinone. The 2-methyl-and 2-methoxy-substituted dyes were oxidized to SPH and either 2-methyl-or 2-methoxy-benzoquinone. Six products were formed from laccase oxidation of the 2,6-dimethoxy-substituted dye. Three of them were identified as SPH, 4-hydroxybenzenesulfonic acid, and 2,6-dimethoxybenzoquinone. A mechanism for the formation of benzoquinone and SPH from laccase oxidation of phenolic azo dyes is proposed. This study suggests that laccase oxidation can result in the detoxification of azo dyes.

Lignin peroxidase (LiP) is an extracellular enzyme produced by the lignin-degrading fungus Phanerochaete chrysosporium and is involved in azo dye degradation by this organism. In this study, LiP oxidation of the sulfonated azo dyes 4-(4'-sulfophenylazo)-2,6- dimethylphenol (I), Orange II [1-(4'-sulfophenylazo)-2-naphthol] (II), a dimethyl analog of Orange II [1-(2',6'-dimethyl-4'-sulfophenylazo)-2-naphthol] (III), and 4-(4'-sulfonamidophenylazo)-2,6-dimehtylphenol (IV) was examined. Azo dye I was oxidized to 2,6-dimethyl-1,4-benzoquinone and 4-sulfophenyl hydroperoxide. Orange II (II) was oxidized to 1,2-naphthoquinone and 4-sulfophenyl hydroperoxide. The dimethyl analog of Orange II (III) was oxidized to 1,2-naphthoquinone and 2,6-dimethyl-4-sulfophenyl hydroperoxide. Azo dye IV was oxidized predominantly to 2,6-dimethyl-1,4-benzoquinone and another product, tentatively characterized as 4-sulfonamidophenyl hydroperoxide. In the 18O-labeling studies with 18O2, oxygen incorporation into the phenyl hydroperoxides from the oxidation of I and III was observed. A mechanism for azo dye degradation consistent with product identification and the 18O-labeling studies is proposed. Two successive one-electron oxidations of the phenolic ring of an azo dye by the H2O2-oxidized forms of LiP produces a carbonium ion. Then water attacks the phenolic carbon bearing the azo linkage, producing an unstable hydroxy intermediate which breaks down to yield a quinone and a sulfo- or sulfonamidophenyldiazene. The phenyldiazene is oxidized by O2 to generate the corresponding phenyldiazene radical, which eliminates N2 to yield a sulfo- or sulfonamidophenyl radical. O2 scavenges the latter to yield the corresponding hydroperoxide.(ABSTRACT TRUNCATED AT 250 WORDS)

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