Manganese peroxidase (MnP) oxidized l-(3, 5-dimethoxy-4-hydroxyphenyl)-2-(4-(hydroxymethyl)-2-methoxyphenoxy)-l, 3-dihydroxypropane (I) in the presence of MnIIand H2O2 to yield l-(3, 5-dimethoxy-4-hydroxyphenyl)-2-(4-(hydroxymethyl)-2-methoxyphenoxy)-l-oxo-3-hydroxypropane (II), 2, 6-dimethoxy-l, 4-benzoquinone (III), 2, 6-dimethoxy-1, 4-dihydroxybenzene (IV), 2-(4-(hydroxymethyl)-2-methoxyphenoxy)-3-hydroxypropanal (V), syringaldehyde (VI), vanillyl alcohol (VII), and vanillin (VIII). MnP oxidized II to yield 2, 6-dimethoxy-l, 4-benzoquinone (III), 2, 6-dimethoxy-l, 4-dihydroxybenzene (IV), vanillyl alcohol (VII), vanillin (VIII), syringic acid (IX), and 2-(4-(hydroxymethyl)-2-methoxyphenoxy)-3-hydroxypropanoic acid (X). A chemically prepared MnIII-malonate complex catalyzed the same reactions. Oxidation of I and II in H218O under argon resulted in incorporation of one atom of 18O into the quinone III and into the hydroquinone IV. Incorporation of one atom of oxygen from H2180 into syringic acid (IX) and the phenoxypropanoic acid X was also observed in the oxidation of II. These results are explained by mechanisms involving the initial one-electron oxidation of I or II by enzyme-generated MnIIIto produce a phenoxy radical. This intermediate is further oxidized by MnIII to a cyclohexadienyl cation. Loss of a proton, followed by rearrangement of the quinone methide intermediate, yields the Cα-oxo dimer II as the major product from substrate I. Alternatively, cyclohexadienyl cations are attacked by water. Subsequent alkyl-phenyl cleavage yields the hydroquinone IV and the phenoxypropanal V from I, and IV and the phenoxypropanoic acid X from II, respectively. The initial phenoxy radical also can undergo Cα-Cβ bond cleavage, yielding syringaldehyde (VI) and a C6-C2-ether radical from I and syringic acid (IX) and the same C6-C2-ether radical from II. The C6-C2-ether radical is scavenged by O2or further oxidized by MnIII, subsequently leading to release of vanillyl alcohol (VII). VII and IV are oxidized to vanillin (VIII) and the quinone III, respectively.
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