Oxidation of Phenolic Arylglycerol β-Aryl Ether Lignin Model Compounds by Manganese Peroxidase from Phanerochaete chrysosporium: Oxidative Cleavage of an α-Carbonyl Model Compound

Manganese peroxidase (MnP) oxidized l-(3, 5-dimethoxy-4-hydroxyphenyl)-2-(4-(hydroxymethyl)-2-methoxyphenoxy)-l, 3-dihydroxypropane (I) in the presence of Mn^IIand 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 Mn^III-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 Mn^IIIto produce a phenoxy radical. This intermediate is further oxidized by Mn^III to a cyclohexadienyl cation. Loss of a proton, followed by rearrangement of the quinone methide intermediate, yields the C_α-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 C_α-C_β 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 Mn^III, subsequently leading to release of vanillyl alcohol (VII). VII and IV are oxidized to vanillin (VIII) and the quinone III, respectively.