Nitration of veratryl alcohol by lignin peroxidase and tetranitromethane

Lignin peroxidase (LiP), from Phanerochaete chrysosporium, in the presence of H2O2 and tetranitromethane (TNM), oxidizes veratryl (3,4-dimethoxybenzyl) alcohol (VA) (I) to veratraldehyde (IV), 4,5-dimethoxy-2-nitrobenzyl alcohol (V), and 3,4-dimethoxy-nitrobenzene (VI). The formation of these products is explained by a mechanism involving the one-electron oxidation of VA by LiP to produce the corresponding cation radical, which loses a proton to generate the benzylic radical. The latter reduces TNM to generate the trinitromethane anion (VIII) and the nitrogen dioxide radical (.NO2). .NO2 couples with the VA cation radical, and the subsequent loss of a proton leads to V. Alternatively, the attack of .NO2 at C-1 of the VA cation radical, followed by aromatization and loss of formaldehyde (VII), yields VI. Isotopic labeling experiments confirm that V is generated by the reaction of .NO2 with the VA cation radical, rather than with the benzylic radical. The nitration of two other LiP substrates, 1,4-dimethoxybenzene (II) and tyrosine (III), also was examined. Product analysis of reactions conducted in the presence of H2O2 with these substrates indicated less nitrated product was formed from 1,4-dimethoxybenzene and no nitrated product was formed from tyrosine. However, significant amounts of nitrated products were formed from 1,4-dimethoxybenzene and tyrosine when glucose and glucose oxidase were used as an H2O2 source. These results suggest that a reductant, either the veratryl alcohol benzylic radical or superoxide, is required in the reaction to reduce TNM to generate .NO2. These results provide further evidence for the formation of the VA cation radical and the first chemical evidence for the formation of the VA benzylic radical in LiP-catalyzed reactions.