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

Abstract

Lignin peroxidase (LiP) has been used to study the Cα-Cβ cleavage of the propylene side chain in 1-(3ʹ,4ʹ-dimethoxyphenyl)propene (DMPP) to 3,4-dimethoxybenzaldehyde (veratraldehyde, VAD). Under an air atmosphere, LiP oxidized DMPP to VAD (27.8%) and 1-(3ʹ,4ʹ-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 Cα-Cβ 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 by 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 Cα-Cβ 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.