Lignin peroxidase initiates O₂-dependent self-propagating chemical reactions which accelerate the consumption of 1-(3',4'-dimethoxyphenyl)propene

Lignin peroxidase (LiP) has been used to study the C(α)-C(β) cleavage of the propylene side chain in 1-(3',4'-dimethoxyphenyl)propene (DMPP) to 3,4-dimethoxybenzaldehyde (veratraldehyde, VAD). Under an air atmosphere, LIP oxidized DMPP to VAD (27.8%) and 1-(3',4'-dimethoxyphenyl)propan-2-one (DMPA, 8.7%), after 10 min of incubation. Dissolved O₂ was rapidly consumed during DMPP conversion, of which one-third was converted into superoxide. The remaining two-thirds of the consumed O₂ was involved in C(α)-C(β) cleavage of DMPP to VAD and in self-propagating chemical reactions stimulating the consumption of DMPP. The involvement of peroxyl radicals, in the chemical consumption of DMPP, was confirmed bq using the well-known peroxyl radical reductant Mn²⁺. This metal ion severely inhibited the DMPP consumption rate under air, but did not affect the lower enzymic DMPP consumption rate under N₂. The substoichiometric requirement of LiP for H₂O₂ during DMPP oxidation could be explained in part by dismutation of superoxide, but more importantly by direct chemical reactions of DMPP-derived peroxyl radicals with fresh DMPP. Another VAD-producing route was found by incubating the DMPP oxidation product, DMPA, with LiP. Under air the molar yield of VAD was 29.7%. In the absence of O₂, the C(α)-C(β) cleavage of DMPA to VAD was strongly inhibited and side-chain coupling products (dimers) were formed instead. As a whole, the results suggest two new roles of O₂ in LiP-mediated oxidation of aromatic substrates. First, O₂ is responsible for the formation of reactive peroxyl intermediates, which can directly react with other substrate molecules and thereby accelerate consumption rates. Secondly, O₂ can prevent coupling reactions by lowering the pool of carbon-centred radicals accumulating during LIP catalysis.