Lignin Peroxidase-Catalyzed Oxidation of Nonphenolic Trimeric Lignin Model Compounds: Fragmentation Reactions in the Lignin Model Compounds: Fragmentation Reactions in the Intermediate Radical Cations

The H2O2-promoted oxidations of two nonphenolic lignin model trimers 1 and 2,catalyzed by lignin peroxidase (LiP) at pH 3.5, have been studied. The results have been compared with those obtained in the oxidation of 1 and 2 with the genuine one-electron oxidant potassium 12-tungstocobalt(III)ate. These models present a different substitution pattern of the three aromatic rings, and by one-electron oxidation, they form radical cations with the positive charge, which is localized in the dialkoxylated ring as also evidenced by a pulse radiolysis study. Both the oxidations with the enzymatic and with the chemical systems lead to the formation of products deriving from the cleavage of C-C and C-H bonds in a beta position with respect to the radical cation with the charge residing in the dialkoxylated ring (3,4-dimethoxybenzaldehyde and a trimeric ketone in the oxidation of 1 and a dimeric aldehyde and a trimeric ketone in the oxidation of 2). These products are accompanied by a dimeric aldehyde in the oxidation of 1 and 4-methoxybenzaldehyde in the oxidation of 2. The unexpected formation of these two products has been explained by suggesting that the radical cations of 1 and 2 can also undergo an intramolecular electron transfer leading to the radical cations with the charge residing in a monoalkoxylated ring. The fast cleavage of a C-C bond beta to this ring is the driving force of the endoergonic electron transfer. A kinetic steady-state investigation of the LiP-catalyzed oxidation of the trimer 2, the dimeric model 1-(3,4-dimethoxyphenyl)-2-phenoxy-1-ethanol, and 3,4- dimethoxybenzyl alcohol has indicated that the turnover number (kcat) and the affinity for the enzyme decrease significantly by increasing the size of the model compound. In contrast, the three substrates exhibited a very similar reactivity toward a chemical oxidant [CoIIIW]. This suggests a size-dependent interaction of the enzyme with the substrate which may influence the efficiency of the electron transfer.