Role of molecular oxygen in lignin peroxidase reactions

Homogeneous lignin peroxidase (diarylpropane oxygenase) oxidized veratryl alcohol to veratryl aldehyde under anaerobic conditions in the presence of either H2O2, m-chloroperoxybenzoic acid (mCPBA), or p-nitroperoxybenzoic acid (pNPBA). Lignin peroxidase also oxidized the 1-(3',4'-diethoxyphenyl)-1,2-dihydroxy-(4"-methoxyphenyl)-propane I under anaerobic conditions in the presence of mCPBA to yield 3,4-diethoxybenzaldehyde III and 1-(4'-methoxyphenyl)-1,2-dihydroxyethane IV. In contrast to what occurs under aerobic conditions, under anaerobic conditions no 2-hydroxy-1-(4'-methoxyphenyl)-1-oxoethane V was obtained. During the diarylpropane I cleavage under anaerobic conditions, 18O from H2(18)O was incorporated into the alpha-position of the phenylglycol IV. Lignin peroxidase also hydroxylated 1-(4'-ethoxy-3'-methoxyphenyl)propane II at the alpha-position to yield 1-(4'-ethoxy-3'-methoxyphenyl)-1-hydroxypropane VI under anaerobic conditions in the presence of mCPBA. During the phenylpropane II hydroxylation under anaerobic conditions, 18O from H2(18)O was incorporated into the alpha-position of VI. These results are rationalized according to a mechanism involving an initial one-electron oxidation of the diarylpropane I by the lignin peroxidase compound I to form a benzene pi cation radical which undergoes alpha, beta cleavage to produce a benzaldehyde and a C6C2 benzylic radical. The latter is then attacked by O2 to form a hydroperoxy radical which may decompose through a tetroxide to form the phenylglycol IV and phenylketol V. Under anaerobic conditions the C6C2 benzylic radical is probably oxidized to a carbonium ion which would be subsequently attacked by H2O to yield the phenylglycol V.