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

Abstract

The H(2)O(2)-promoted oxidations of the two nonphenolic beta-O-aryl 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 (5) and a trimeric ketone 6 in the oxidation of 1 and a dimeric aldehyde 8 and a trimeric ketone 9 in the oxidation of 2). These products are accompanied by a dimeric aldehyde 7 in the oxidation of 1 and 4-methoxybenzaldehyde (10) in the oxidation of 2. The unexpected formation of these two products has been explained by suggesting that 1.+ and 2.+ can also undergo an intramolecular electron transfer leading to the radical cations 1a.+ and 2a.+ with the charge residing in a monoalkoxylated ring. The fast cleavage of a C-C bond beta to this ring, leading to 7 from 1.+ and to 10 from 2.+, 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 (4), and 3,4-dimethoxybenzyl alcohol (3) has indicated that the turnover number (k(cat)) 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 [Co(III)W]. This suggests a size-dependent interaction of the enzyme with the substrate which may influence the efficiency of the electron transfer.

Iron porphyrins-catalysed oxidation of α-alkyl substituted mono and dimethoxylated benzyl alcohols

“…Moreover 4-methoxybenzaldehyde is the major product in the oxidation of 3 while only small amounts of 3,4-dimethoxybenzaldehyde are formed in the oxidation of 7. These observations are in accord with the results obtained in the oxidation of the trimeric lignin model compounds catalysed by the LiP/H 2 O 2 system [34]. Accordingly, a competition between C a -H and C a -C b bond cleavage was observed in the b fragmentation of dimethoxylated aromatic radical cations whereas monomethoxylated aromatic radical cations underwent only C a -C b bond cleavage.…”

Lignin‐degrading enzymes

“…Unfortunately, the size of the oligomeric lignin model substrates significantly affects the catalytic efficiency of LiP (i.e. the activity of LiP on a β‐O‐4‐linked lignin model trimer is 25‐fold lower than that observed on VA) .…”

Progress and obstacles in the production and application of recombinant lignin-degrading peroxidases

“…Activity assays using lignin as a substrate are time-consuming because both the measurements and data analysis take a long time. The oligomeric nature of lignin model compounds affects the catalytic activity of lignin-degrading enzymes, e.g., the activity of LiP on the model compound b-O-4 lignin was 25-fold lower than on the simple artificial substrate VA. 62 To increase the convenience of protein characterization and engineering approaches, artificial substrates such as ABTS, VA and DMP are used for initial testing and simple phenolic or non-phenolic model compounds are used in subsequent assays. However, the degradation of these simple aromatic compounds may require different catalytic mechanisms, which makes it difficult to extrapolate the results to typical lignin and lignocellulose samples.…”