Purified manganese peroxidase (MnP) from Phanerochaete chrysosporium oxidizes nonphenolic b-1 diarylpropane lignin model compounds in the presence of Tween 80, and in three-to fourfold lower yield in its absence. In the presence of Tween 80, 1-(3¢,4¢-diethoxyphenyl)-1-hydroxy-2-(4¢-methoxyphenyl)propane (I) was oxidized to 3,4-diethoxybenzaldehyde (II), 4-methoxyacetophenone (III) and 1-(3¢,4¢-diethoxyphenyl)-1-oxo-2-(4¢-methoxyphenyl)propane (IV), while only 3,4-diethoxybenzaldehyde (II) and 4-methoxyacetophenone (III) were detected when the reaction was conducted in the absence of Tween 80. In contrast to the oxidation of this substrate by lignin peroxidase (LiP), oxidation of substrates by MnP did not proceed under anaerobic conditions. When the dimer (I) was deuterated at the a position and subsequently oxidized by MnP in the presence of Tween 80, yields of 3,4-diethoxybenzaldehyde, 4-methoxyacetophenone remained constant, while the yield of the a-keto dimeric product (IV) decreased by approximately sixfold, suggesting the involvement of a hydrogen abstraction mechanism. MnP also oxidized the a-keto dimeric product (IV) to yield 3,4-diethoxybenzoic acid (V) and 4-methoxyacetophenone (III), in the presence and, in lower yield, in the absence of Tween 80. When the reaction was performed in the presence of 18 O 2 , both products, 3,4-diethoxybenzoic acid and 4-methoxyacetophenone, contained one atom of 18 O. Finally, MnP oxidized the substrate 1-(3¢,5¢-dimethoxyphenyl)-1-hydroxy-2-(4¢-methoxyphenyl) propane (IX) to yield 3,5-dimethoxybenzaldehyde (XI), 4-methoxyacetophenone (III) and 1-(3¢,5¢-dimethoxyphenyl)-1-oxo-2-(4¢-methoxyphenyl)propane (X). In sharp contrast, LiP was not able to oxidize IX. Based on these results, we propose a mechanism for the MnP-catalyzed oxidation of these dimers, involving hydrogen abstraction at a benzylic carbon, rather than electron abstraction from an aromatic ring.
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