Study On The C_?-OH Bio-oxidation Enhancing The Formic Acid Catalytic Depolymerization Of Lignin

Lignin is the main natural biopolymer composed of aromatic compounds.Its depolymerization products have broad potential application in energy and chemical industry,but the complex resistance in structure limits the efficient depolymerization of lignin.Recent studies have found that chemical oxidation of lignin C?-OH can enhance its depolymerization efficiency.Ligninolytic enzymes play a key role in the lignin carbon cycle on earth.If ligninolytic enzyme can be used to selectively oxidize C?-OH,it is expected to provide a new strategy for green mild lignin depolymerization.In this study,three ligninolytic enzymes including laccase?Pv-Lac?,versatile peroxidase?Pv-VP?and manganese peroxidase?Et-MnP?obtained from Physisporinus vitreus and Echinodontium taxodii were systematically compared.C?-OH oxidation of phenolic?-O-4 dimer model compounds by three ligninolytic enzymes was studied.C?-OH oxidation by Pv-Lac enhancing the formic acid catalytic depolymerization of lignin was further evaluated.Pv-Lac,Pv-VP and Et-MnP were isolated and purified from the extracellular fluids of P.vitreus and E.taxodii,respectively.Their enzymatic characterization and reaction kinetics were studied under different pH conditions.The results showed that the optimum pH of Pv-Lac,Pv-VP and Et-MnP were 2.0,4.5 and 4.0 respectively.Unlike peroxidase,the optimum pH of Pv-Lac reaction and enzyme stability was very different.Under acidic conditions,Pv-Lac had strong catalytic activity but low stability.Pv-Lac showed higher stability and substrate affinity at pH 7.0,but its catalytic activity was low under this condition.H₂O₂concentration was a key factor affecting Pv-VP catalysis.Pv-VP can oxidize high redox potential substrates that cannot be oxidized by Et-MnP,but the catalytic efficiency for low redox potential substrates was lower than that of Et-MnP.The lignin dimer model compound was successfully synthesized,and the oxidation law of side chain C?-OH was studied by phenolic?-O-4 dimer.The results showed that Pv-Lac had the highest catalytic efficiency for phenolic?-O-4 dimer at pH 7.0,and C_?-OH of the dimeric side chain was selectively oxidized to C?=O.When pH was 3.0 or 4.5,the catalytic efficiency of the phenolic?-O-4 dimer by Pv-Lac was reduced.In addition to the slight oxidation of C?-OH,the phenolic?-O-4 dimer was repolymerized to form a C-C bonded tetramer.High concentration of H₂O₂ enhanced the catalytic rate of Pv-VP,but led to rapid inactivation of peroxidase.Et-MnP and Pv-VP catalyzed the phenolic?-O-4 dimer with the slight oxidation of C?-OH,and simultaneously the phenolic?-O-4 dimer was repolymerized to form C-C bonded tetramers.The effect of C?-OH oxidation by Pv-Lac on the catalytic depolymerization of lignin by formic acid was further studied.The results showed that,Pv-Lac oxidizing phenolic?-O-4dimer could be depolymerized by formic acid,and the depolymerization rate increased significantly from 6.80 wt%to 36.33 wt%.The main depolymerization products were guaiacol,vanillin,vanillic acid and guaiacyl propane diketon.The two lignin polymers milled wood lignin?MWL?and alkali lignin were oxidized by Pv-Lac,followed by formic acid depolymerization,and the depolymerization rates were 84.80 wt%and 19.60 wt%,respectively,increasing 24 wt%and 9 wt%,compared with no laccase treatment.In summary,laccase was used to oxidize C?-OH in lignin for the first time,which enhanced the depolymerization efficiency of lignin by formic acid.This study lays a foundation for constructing a new strategy of lignin depolymerization by coupling bio-modification with chemical oxidation.