Abstract
Lignin peroxidase (LiP) is an important enzyme for degrading aromatic hydrocarbons not only in nature but also in industry. In the presence of H2O2, this enzyme can easily decompose lignin and analogue compounds under mild conditions. In this reaction mechanism, LiP catalyzes the C–C cleavage of a propenyl side chain, being able to produce veratraldehyde (VAD) from 1-(3′,4′-dimethoxyphenyl) propene (DMPP). One of the few and complete proposed mechanisms includes several non-enzymatic reactions. In this study, we performed a computational study to gain insight about the non-enzymatic steps involved in the reaction mechanism of VAD formation from DMPP using LiP as a catalyst. A kinetic characterization of the reaction using the reaction force and the reaction force constant concepts within the density functional theory (DFT) framework is proposed. All theoretical calculations for the reaction pathway were performed using the Minnesota Global Hybrid functional M06-2X and a 6-31++G(d,p) basis set. The complete reaction comprises seven steps (five steps not including LiP as a catalyst), which include radical species formation, bond transformation, water and oxygen addition, atom reordering, and deacetylation. The overall mechanism is an endothermic process with mixed activation energies depending on the four transition states. These results are the first attempt to fully understand the catalytic role of LiP in the degradation of lignin and its aromatic derivative compounds in terms of the electronic structure methods and future hybrid calculation approaches that we have recently been performing.
Highlights
Lignin is a complex, heterogeneous, three-dimensional aromatic polymer containing random dimethoxylated, monomethoxylated, and nonmethoxylated phenylpropanoid subunits [1,2,3]
In our solvation phase molecular models, the reaction started in an forming DMPP as a cation radical species (2)
Steps 1 and 5 involve Lignin peroxidase (LiP) as a catalyst. Those two reaction steps were not studied in our research work research work because energies and information from those steps, due to the catalytic effect because energies and information obtained from obtained those steps, due to the catalytic effect of the enzyme, of the enzyme, would not be accurate enough to be taken into account due to the simplicity of our would not be accurate enough to be taken into account due to the simplicity of our molecular models
Summary
Heterogeneous, three-dimensional aromatic polymer containing random dimethoxylated, monomethoxylated, and nonmethoxylated phenylpropanoid subunits [1,2,3]. From most biological polymers, lignin subunits are linked in a nonlinear way as a consequence of its synthesis reaction mechanism that is mediated by free radicals [3]. The three most abundant lignin monomers are p-coumaryl alcohol, coniferyl alcohol, and sinapyl alcohol, which are synthesized in the cytosol [4]. This polymer encompasses 20–30% of all woody plants and it can be found in woody and vascular tissues making the cell wall more rigid and hydrophobic [1,2,3]. Molecules 2018, 23, 412 secondary cell wall of plants, being within and between the cell walls of tracheids, vessels, and fibers of xylem tissue filling the spaces between the cellulose, hemicellulose, and pectin components [3,4].
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