Non-phenolic Lignin Model Compounds Research Articles

Agaricales Mushroom Lignin Peroxidase: From Structure-Function to Degradative Capabilities.

Lignin biodegradation has been extensively studied in white-rot fungi, which largely belong to order Polyporales. Among the enzymes that wood-rotting polypores secrete, lignin peroxidases (LiPs) have been labeled as the most efficient. Here, we characterize a similar enzyme (ApeLiP) from a fungus of the order Agaricales (with ~13,000 described species), the soil-inhabiting mushroom Agrocybe pediades. X-ray crystallography revealed that ApeLiP is structurally related to Polyporales LiPs, with a conserved heme-pocket and a solvent-exposed tryptophan. Its biochemical characterization shows that ApeLiP can oxidize both phenolic and non-phenolic lignin model-compounds, as well as different dyes. Moreover, using stopped-flow rapid spectrophotometry and 2D-NMR, we demonstrate that ApeLiP can also act on real lignin. Characterization of a variant lacking the above tryptophan residue shows that this is the oxidation site for lignin and other high redox-potential substrates, and also plays a role in phenolic substrate oxidation. The reduction potentials of the catalytic-cycle intermediates were estimated by stopped-flow in equilibrium reactions, showing similar activation by H2O2, but a lower potential for the rate-limiting step (compound-II reduction) compared to other LiPs. Unexpectedly, ApeLiP was stable from acidic to basic pH, a relevant feature for application considering its different optima for oxidation of phenolic and nonphenolic compounds.

Selective hydrodeoxygenation of lignin model compound (3,4-dimethoxybenzyl alcohol) by Pd/CNX catalyst

Upgrading of lignin derived bio-oil is an essential step for producing sustainable bio-based chemicals and fuel. Taken into account that α hydroxyl is the abundant functional group in lignin, high effective and selective catalytic alcoholysis for cleaving the Cα-OH linkages would be desirable. However, an in-depth understanding of the reaction mechanisms involved in the cleavage of Caromatic-Cα and Cα-O bonds over a novel catalyst is still needed. Herein, we report an efficient liquid-phase hydrogen transfer strategy for the selective hydrodeoxygenation of a non-phenolic lignin model compound, 3,4-dimethoxybenzyl (veratryl) alcohol, under mild conditions. By employing iso-propanol as solvent and H-donor, and palladium nanoparticles immobilized on nitrogen-doped carbon (Pd/CNX) as efficient multifunctional catalyst, veratryl alcohol dehydroxylation exhibited almost 100% conversion along with very high selectivity for 1,2-dimethoxy-benzene (46%) and 3,4-dimethoxytoluene (54%). Compared with other Pd catalysis, the Pd/CNX has excellent catalytic performances and exhibits higher selectivity for 3,4-dimethoxytoluene under incorporation with 1% HCOOH at 220 °C. The proportion of Pd (0) significantly increases in Pd/CNX catalyst when introduced into N precursor because of its highly dispersed Pd NPs and preventing the reoxidation of Pd (0). The dehydrogenation reaction occurred through the hydrogen generation of a secondary alcohol. Then, the Cα-OH and Caromatic-Cα bonds of veratryl alcohol were selectively cleaved by catalytic transfer hydrogenolysis. The alcoholysis mechanism is supported by dispersion-corrected density functional theory computations.

Technological and biochemical features of lignin-degrading enzymes: a brief review

For many industries, the committed step during the treatment of lignocellulosic materials is the elimination of lignin. This recalcitrant material is commonly removed by many physical/chemical procedures that produce hazardous effluents that poison the environment, and usually the treatment of these effluents are cost-expensive. An eco-friendly alternative to the use of traditional treatments is the degradation of lignocellulosic materials using microorganisms or enzymes, mainly by the use of lignin-degrading enzymes. They are classified as lignin-modifying enzymes (ligninolytic enzymes that catalyze an oxidative cleavage of lignin) and lignin-degrading auxiliary enzymes. The lignin-modifying enzymes include phenol oxidases (such as laccases) and heme-peroxidases (lignin-peroxidases, manganese-peroxidases, versatile-peroxidases and, dye-decolorizing peroxidases), both classes of enzymes are members of the oxidase super-family. These enzymes have biochemical features that point them as excellent tools for being used in several industrial processes. They show a great versatility and can oxidize a wide range of organic and inorganic substrates, such as vanillyl alcohol, catechol, acetosyringone, syringic acid, and guaiacol, several dyes, phenolic and non-phenolic lignin model compounds. Their prospective applications include pulp and paper making, bio-refinery (biofuel), bioremediation and, textile, pharmaceutical and dermatological industries, among others. In this review, we summarize some of their biochemical features and potential industrial applications.

Difference in adsorbable organic halogen formation between phenolic and non-phenolic lignin model compounds in chlorine dioxide bleaching.

Adsorbable organic halogen (AOX) is generally formed by the reaction of residual lignin in pulps with chlorine dioxide during bleaching. Lignin has a complex structure. Different functional groups and bonds are present in lignin structures. Phenolic hydroxyl is one of the important functional groups in lignin, and it significantly influences the chemical properties and reactivity. To study the effect of phenolic hydroxyl on AOX formation, vanillyl alcohol (VA) was selected as the phenolic lignin model compound, and veratryl alcohol (VE) was selected as the non-phenolic lignin model compound in this study. The kinetics of AOX formation by the reaction of VA or VE with chlorine dioxide was studied. The effects of pH, chlorine dioxide, lignin model compound concentration and reaction temperature on AOX formation are discussed. The activation energies of the reaction of VA and VE with chlorine dioxide are 16 242.47 J mol−1 and 281.34 J mol−1, respectively. Thus, we found that the non-phenolic lignin can react with chlorine dioxide to form AOX more easily than phenolic lignin.

Effect of lignin structure on adsorbable organic halogens formation in chlorine dioxide bleaching.

Adsorbable organic halogens (AOX) are formed in pulp bleaching as a result of the reaction of residual lignin with chlorine dioxide. The natural structure of lignin is very complex and it tends to be damaged by various extraction methods. All the factors can affect the study about the mechanism of AOX formation in the reaction of lignin with chlorine dioxide. Lignin model compounds, with certain structures, can be used to study the role of different lignin structures on AOX formation. The effect of lignin structure on AOX formation was determined by reacting phenolic and non-phenolic lignin model compound with a chlorine dioxide solution. Vanillyl alcohol (VA) and veratryl alcohol (VE) were selected for the phenolic and non-phenolic lignin model compound, respectively. The pattern consumption of lignin model compounds suggests that both VA and VE began reacting with chlorine dioxide within 10 min and then gradually steadied. The volume of AOX produced by VE was significantly higher than that produced by VA for a given initial lignin model compound concentration. In a solution containing a combination of VA and VE in chlorine dioxide, VE was the dominant producer of AOX. This result indicates that the non-phenolic lignin structure was more easily chlorinated, while the phenolic lignin structure was mainly oxidized. In addition, AOX content produced in the combined experiments exceeded the total content of the two separate experiments. It suggested that the combination of phenolic and non-phenolic lignin structure can promote AOX formation.

Structure, chemical reactivity and solubility of lignin: a fresh look

This is a review of historical and modern literature data on the structure versus properties of wood lignin in view of the concepts developed by the authors based on their own research. Changes in the structure of lignin and related changes in its chemical reactivity during alkaline wood pulping are assessed based on the comparison of the structures of lignin at three kinetically distinct stages of delignification: initial, bulk and final. Lignin gradually moves from a solid to a liquid phase during the pulping process; therefore, structures of native, dissolved and residual lignin are elucidated and compared. The emphasis is on changes in the molecular weight distribution and content of alkylarylether bonds, and functional groups, in particular phenolic hydroxyls. For comparison, splitting rates for α- and β-alkylarylether bonds in both phenolic and non-phenolic lignin model compounds are analyzed. Based on the comparative analysis of the experimental data, it is suggested that native lignin in wood consists mainly of three distinct fractions that are different in chemical reactivities of alkylarylether bonds. This phenomenon results in three kinetically distinct stages of the pulping process. Wood delignification is essentially a process of lignin functionalization followed by its dissolution. The functionalization, i.e., formation of additional functional groups in the macromolecule, continues until it reaches the level sufficient for lignin dissolution under chosen conditions, and then, delignification occurs. At the bulk stage of pulping, the rate of delignification is directly proportional to the degree of functionalization. The data characterizing the effect of redox reactions on the structure and chemical reactivity of lignin in alkali–anthraquinone pulping are analyzed in detail in view of their general importance for our understanding of the chemical reactivity of lignin. Results of polarographic studies of numerous representative lignin model compounds (> 70 samples) and lignin samples, including chemically changed lignins, are compiled, and a diagram of reduction potentials of polarographically active functional groups in lignin is drawn. From a comparison of redox properties of lignin and 32 pulping additives, criteria for selection of potential alkaline pulping catalysts are derived. Solubility belongs to basic properties of polymers, lignin included, and all methods of delignification of plant materials can be essentially reduced to solid polymer functionalization followed by its dissolution. Factors contributing to lignin solubility are analyzed, including such characteristics as molecular weight, temperature, liquid–solid ratio and ionic strength. Based on the analyzed data, a uniform scale of solubility for different lignin types is proposed, and formulae for calculating lignin solubility in alkaline media are derived.

Effects of aromatic ring type on reactions subsequent to the β-O-4 bond cleavage of non-phenolic lignin model compounds under alkaline pulping conditions

The reaction products of an alkaline treatment of non-phenolic β-O-4-type lignin model compounds (C6–C2-type) consisting of p-hydroxyphenyl (H), guaiacyl (G), and/or syringyl (S) nuclei were identified and quantified. This was performed to examine how the type of H, G, or S nucleus affect the reaction product profiles. The major identified and quantified reaction products were phenol derivatives that were liberated from the aryl sides of the β-O-4 ether bonds of the lignin model compounds. Other products included derivatives of phenylethane-1,2-diol (glycol-type), benzaldehyde, and acetophenone, which originated from the alkyl sides of the β-O-4 ether bonds of the lignin model compounds. Although the type of aromatic nucleus of the aryl side of the β-O-4 ether bond of the lignin model compounds did not significantly affect the profile of the reaction products, the type of the alkyl side nucleus was influential. The glycol-type compound was the exclusive major reaction product when the S nucleus was on the alkyl side of the β-O-4 ether bond. On the other hand, when the H or G nucleus was present, a benzaldehyde derivative was the other major reaction product.

A novel and efficient fungal delignification strategy based on versatile peroxidase for lignocellulose bioconversion

BackgroundThe selective lignin-degrading white-rot fungi are regarded to be the best lignin degraders and have been widely used for reducing the saccharification recalcitrance of lignocellulose. However, the biological delignification and conversion of lignocellulose in biorefinery is still limited. It is necessary to develop novel and more efficient bio-delignification systems.ResultsPhysisporinus vitreus relies on a new versatile peroxidase (VP)-based delignification strategy to remove enzymatic recalcitrance of corn stover efficiently, so that saccharification of corn stover was significantly enhanced to 349.1 mg/g biomass (yield of glucose) and 91.5% (hydrolysis yield of cellulose) at 28 days, as high as levels reached by thermochemical treatment. Analysis of the lignin structure using pyrolysis–gas chromatography–mass spectrometry (Py–GC/MS) showed that the total abundance of lignin-derived compounds decreased by 54.0% and revealed a notable demethylation during lignin degradation by P. vitreus. Monomeric and dimeric lignin model compounds were used to confirm the ligninolytic capabilities of extracellular ligninases secreted by P. vitreus. The laccase (Lac) from P. vitreus could not oxidize nonphenolic lignin compounds and polymerized β-O-4 and 5-5′ dimers to precipitate which had a negative effect on the enzymatic hydrolysis of corn stover in vitro. However, the VP from P. vitreus could oxidize both phenolic and nonphenolic lignin model compounds as well as break the β-O-4 and 5-5′ dimers into monomeric compounds, which were measured by high-performance liquid chromatography–electrospray ionization–mass spectrometry (LC–ESI–MS). Moreover, we showed that addition of purified VP in vitro improved the enzymatic hydrolysis of corn stover by 14.1%.ConclusionsFrom the highly efficient system of enzymatic recalcitrance removal by new white-rot fungus, we identified a new delignification strategy based on VP which could oxidize both phenolic and nonphenolic lignin units and break different linkages in lignin. In addition, this is the first evidence that VP could break 5-5′ linkage efficiently in vitro. Moreover, VP improved the enzymatic hydrolysis of corn stover in vitro. The remarkable lignin-degradative potential makes VP attractive for biotechnological applications.

Microwave-assisted degradation of guaifenesin (GGE) to produce novel compounds in the presence of imidazolium-based ionic liquids

Thermal stabilities and degradation behaviors of guaifenesin (guaiacol glycerol ether = GGE) were studied by means of the differential scanning calorimetry and thermogravimetric (TG/DTG, in nitrogen atmosphere) analyses. To investigate the microwave-assisted pathways of guaiacol production from lignin, guaiacol glycerol ether (GGE) as a nonphenolic lignin model compound was exposed to microwave irradiation in the presence of three 1-butyl-3-methylimidazolium-based ionic liquids ([BMIm]-based ILs). GGE was subjected to microwave irradiation of 300 W in a batch reactor at a temperature of 200 °C, residence times of 5 min and air atmosphere. Based on the distribution of GGE conversion products identified by gas chromatography/mass spectrometry, the most degradation pathways of GGE were discussed. The highest yields of 2-methoxyphenol (guaiacol), 1-(2-methoxy-phenoxy)-propan-2-one, and 2-hydroxymethyl-1,4-benzodioxan were obtained using [BMIm][HSO4]. [BMIm][HSO4] was recovered and reused for three times without any loss of catalytic activity. It was found that it is possible to increase the yield of guaiacol from GGE by controlling of the different parameters such as temperature, molar ratio of water to GGE, acidity and anion effect of [BMIm]-based IL, amount of [BMIm]-based IL, microwave power and irradiation time.

Oxidation of a non-phenolic lignin model compound by two Irpex lacteus manganese peroxidases: evidence for implication of carboxylate and radicals

BackgroundManganese peroxidase is one of the Class II fungal peroxidases that are able to oxidize the low redox potential phenolic lignin compounds. For high redox potential non-phenolic lignin degradation, mediators such as GSH and unsaturated fatty acids are required in the reaction. However, it is not known whether carboxylic acids are a mediator for non-phenolic lignin degradation.ResultsThe white rot fungus Irpex lacteus is one of the most potent fungi in degradation of lignocellulose and xenobiotics. Two manganese peroxidases (IlMnP1 and IlMnP2) from I. lacteus CD2 were over-expressed in Escherichia coli and successfully refolded from inclusion bodies. Both IlMnP1 and IlMnP2 oxidized the phenolic compounds efficiently. Surprisingly, they could degrade veratryl alcohol, a non-phenolic lignin compound, in a Mn2+-dependent fashion. Malonate or oxalate was found to be also essential in this degradation. The oxidation of non-phenolic lignin was further confirmed by analysis of the reaction products using LC–MS/MS. We proved that Mn2+ and a certain carboxylate are indispensable in oxidation and that the radicals generated under this condition, specifically superoxide radical, are at least partially involved in lignin oxidative degradation. IlMnP1 and IlMnP2 can also efficiently decolorize dyes with different structures.ConclusionsWe provide evidence that a carboxylic acid may mediate oxidation of non-phenolic lignin through the action of radicals. MnPs, but not LiP, VP, or DyP, are predominant peroxidases secreted by some white rot fungi such as I. lacteus and the selective lignocellulose degrader Ceriporiopsis subvermispora. Our finding will help understand how these fungi can utilize MnPs and an excreted organic acid, which is usually a normal metabolite, to efficiently degrade the non-phenolic lignin. The unique properties of IlMnP1 and IlMnP2 make them good candidates for exploring molecular mechanisms underlying non-phenolic lignin compounds oxidation by MnPs and for applications in lignocellulose degradation and environmental remediation.

First Report of Lignin Peroxidase Production from Alternaria alternata ANF238 Isolated from Rotten Wood Sample

Lignin peroxidases are considered to be the primary enzyme for oxidative depolymerisation of lignin and are responsible for H2O2-dependent oxidation of both phenolic aromatic substrates and non-phenolic lignin model compounds. The present study was undertaken to study the production of Lignin peroxidase enzyme by Alternaria alternata ANF238 isolated from rotten wood sample using water hyacinth as a lignocellulosic substrate under solid state fermentation. Various production parameters like initial moisture content, various carbon sources, nitrogen sources, inducers and their concentrations were also optimized to further increase the production of this enzyme. Maximum lignin peroxidase yield of 7.6 U/g of dry substrate was obtained with water hyacinth as substrate supplemented with 0.5% mannitol as carbon source, 1% ammonium sulphate as nitrogen source and 3 mM MnSO4 as inducer under optimized conditions. These results indicate that Alternaria alternata ANF238 is a potential candidate for production of lignin peroxidase using solid state fermentation, an enzyme with wide industrial and biotechnological applications. This study also promises effective utilization of water hyacinth as a substrate for lignin peroxidase production. To the best of our knowledge, this is the first report on LiP production by Alternaria alternata.

Formation rate of benzyl cation intermediate from p-Hydroxyphenyl, guaiacyl, or syringyl nucleus in acidolysis of lignin

The benzyl cation intermediate is believed to be primarily formed in the acid-catalyzed reaction of lignin, and the reaction route of the intermediate determines which reaction products are afforded and whether lignin undergoes depolymerization or condensation. This study aimed to examine the formation rate of the benzyl cation intermediate from phenolic (P) or non-phenolic (N) lignin model compounds with different types of aromatic nuclei, namely p-hydroxyphenyl (H), guaiacyl (G), or syringyl (S). The rate was in the order of H > G > S for both P- and N-type model compounds and of P > N for all H-, G-, and S- type model compounds. The orders were successfully explained by the electron-donating or electron-withdrawing properties of the hydroxy and methoxy groups at the para- and meta- of the benzyl position, which is the reaction center in the formation of the benzyl cation intermediate.

Radicals and molecular products from the gas-phase pyrolysis of lignin model compounds. Cinnamyl alcohol

The experimental results on detection and identification of intermediate radicals and molecular products from gas-phase pyrolysis of cinnamyl alcohol (CnA), the simplest non-phenolic lignin model compound, over the temperature range of 400–800°C are reported. The low temperature matrix isolation – electron paramagnetic resonance (LTMI-EPR) experiments along with the theoretical calculations, provided evidences on the generation of the intermediate carbon and oxygen centered as well as oxygen-linked, conjugated radicals. A mechanistic analysis is performed based on density functional theory to explain formation of the major products from CnA pyrolysis; cinnamaldehyde, indene, styrene, benzaldehyde, 1-propynyl benzene, and 2-propenyl benzene. The evaluated bond dissociation patterns and unimolecular decomposition pathways involve dehydrogenation, dehydration, 1,3-sigmatropic H-migration, 1,2-hydrogen shift, CO and CC bond cleavage processes.

Reaction kinetics of strong nucleophiles with a dimeric non-phenolic lignin model compound with α-carbonyl functionality (adleron) in aqueous alkali solution

Abstract The degradation kinetics of a non-phenolic lignin model compound with α-carbonyl functionality (adlerone) has been studied by varying temperature and concentrations of sodium hydroxide, sodium hydrogen sulfide, and sodium sulfite. The kinetics of adlerone degradation and formation of its reaction products were monitored by UV-Vis spectroscopy and their structures were analyzed by GC/MS. The two step degradation of adlerone was studied in two separate experimental setups. In the first alkali catalyzed step, adlerone is converted to a β-elimination product that reacts further in the second step with hydrogen sulfide or sulfite ion. The Arrhenius kinetic parameters were derived by the KinFit software. The activation energy for the 1st step was 69.1 kJ mol-1, and for the 2nd step with sulfide 42.4 kJ mol-1 and with sulfite ion 35.8 kJ mol-1. The reaction mechanisms presented are in line with those published earlier: β-ether bonds of structures having α-carbonyl functionality do not cleave under soda pulping conditions, whereas in kraft and sulfite pulping the cleavage of β-ether bonds proceeds via nucleophile attack and addition. The combination of hydroxyl and sulfite ions gives the fastest cleavage of β-ether bonds in non-phenolic lignin structures with the α-carbonyl functionality.

Developing energy efficient lignin biomass processing - towards understanding mediator behaviour in ionic liquids.

Environmental concerns have brought attention to the requirement for more efficient and renewable processes for chemicals production. Lignin is the second most abundant natural polymer, and might serve as a sustainable resource for manufacturing fuels and aromatic derivatives for the chemicals industry after being depolymerised. In this work, the mediator 2,2'-azino-bis(3-ethylbenthiazoline-6-sulfonic acid) diammonium salt (ABTS), commonly used with enzyme degradation systems, has been evaluated by means of cyclic voltammetry (CV) for enhancing the oxidation of the non-phenolic lignin model compound veratryl alcohol and three types of lignin (organosolv, Kraft and lignosulfonate) in the ionic liquid 1-ethyl-3-methylimidazolium ethyl sulfate, ([C2mim][C2SO4]). The presence of either veratryl alcohol or organosolv lignin increased the second oxidation peak of ABTS under select conditions, indicating the ABTS-mediated oxidation of these molecules at high potentials in [C2mim][C2SO4]. Furthermore, CV was applied as a quick and efficient way to explore the impact of water in the ABTS-mediated oxidation of both organosolv and lignosulfonate lignin. Higher catalytic efficiencies of ABTS were observed for lignosulfonate solutions either in sodium acetate buffer or when [C2mim][C2SO4] (15 v/v%) was present in the buffer solution, whilst there was no change found in the catalytic efficiency of ABTS in [C2mim][C2SO4]-lignosulfonate mixtures relative to ABTS alone. In contrast, organosolv showed an initial increase in oxidation, followed by a significant decrease on increasing the water content of a [C2mim][C2SO4] solution.

Engineering a fungal peroxidase that degrades lignin at very acidic pH.

BackgroundLigninolytic peroxidases are divided into three families: manganese peroxidases (MnPs), lignin peroxidases (LiPs), and versatile peroxidases (VPs). The latter two are able to degrade intact lignins, as shown using nonphenolic lignin model compounds, with VP oxidizing the widest range of recalcitrant substrates. One of the main limiting issues for the use of these two enzymes in lignocellulose biorefineries (for delignification and production of cellulose-based products or modification of industrial lignins to added-value products) is their progressive inactivation under acidic pH conditions, where they exhibit the highest oxidative activities.ResultsIn the screening of peroxidases from basidiomycete genomes, one MnP from Ceriporiopsis subvermispora was found to have a remarkable acidic stability. The crystal structure of this enzyme recently became available and, after comparison with Pleurotus ostreatus VP and Phanerochaete chrysosporium LiP structures, it was used as a robust scaffold to engineer a stable VP by introducing an exposed catalytic tryptophan, with different protein environments. The variants obtained largely maintain the acidic stability and strong Mn2+-oxidizing activity of the parent enzyme, and the ability to oxidize veratryl alcohol and Reactive Black 5 (two simple VP substrates) was introduced. The engineered peroxidases present more acidic optimal pH than the best VP from P. ostreatus, enabling higher catalytic efficiency oxidizing lignins, by lowering the reaction pH, as shown using a nonphenolic model dimer.ConclusionsA peroxidase that degrades lignin at very acidic pH could be obtained by engineering an exposed catalytic site, able to oxidize the bulky and recalcitrant lignin polymers, in a different peroxidase type selected because of its high stability at acidic pH. The potential of this type of engineered peroxidases as industrial biocatalysts in lignocellulose biorefineries is strongly enhanced by the possibility to perform the delignification (or lignin modification) reactions under extremely acidic pH conditions (below pH 2), resulting in enhanced oxidative power of the enzymes.

Quantitative Difference in the Rates of the β-O-4 Bond Cleavage between Lignin Model Compounds with and without γ-Hydroxymethyl Groups during the Alkaline Pulping Process

To examine the effect of the presence of γ-hydroxymethyl groups on the rate of the β-O-4 bond cleavage during the alkaline pulping process, the rates of the β-O-4 bond cleavage of non-phenolic lignin model compounds without a γ-hydroxymethyl group, 2-(2-methoxyphenoxy)-1-(3,4-dimethoxyphenyl)ethanol (G’G) and 2-(2,6-dimethoxyphenoxy)-1-(3,4-dimethoxyphenyl)ethanol (G’S), were compared with those of analogous model compounds with a γ-hydroxymethyl group, 2-(2-methoxyphenoxy)-1-(3,4-dimethoxyphenyl)propane-1,3-diol (GG) and 2-(2,6-dimethoxyphenoxy)-1-(3,4-dimethoxyphenyl)propane-1,3-diol (GS), under alkaline pulping conditions. The disappearance of G’G or G’S was accompanied by the quantitative liberation of 2-methoxyphenol or 2,6-dimethoxyphenol, respectively, indicating that the disappearance resulted from the β-O-4 bond cleavage. The disappearance rate of G’G or G’S was in between those of the erythro and threo isomers of GG or GS, respectively. This result seems to be reasonably explained when the steric repulsions of the three staggered conformations are taken into consideration. The disappearance rate of G’G or G’S increased, but the increment became moderate with increasing hydroxide concentration.

Comparison of a series of laccase mediators in the electro-oxidation reactions of non-phenolic lignin model compounds

The electro-oxidations of non-phenolic lignin model compounds in an electrolytic mediator system (EMS) have been investigated with several laccase mediators, including N-hydroxyphthalimide (NHPI), 1-hydroxybenzotriazole (HBT), violuric acid (VLA), 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO), and 2,2′-azinobis-(3-ethylbenzothiazoline-6-sulfonate) (ABTS), under the same reaction conditions. In the bulk electrolyses of the monomeric model compound [1-(4′-ethoxy-3′-methoxyphenyl)ethanol (1G)], oxidation with NHPI gave the corresponding CαO product (2G) in high yield, whereas the oxidations with HBT, VLA, and TEMPO afforded 2G in moderate yields. The highest reaction selectivity for the guaiacyl-units was found in the oxidation conducted in the presence of ABTS, although the yield was low. In the bulk electrolyses of the dimeric model compound [4-ethoxy-3-methoxyphenylglycerol-β-guaiacyl ether (3G)], the oxidation with NHPI gave the corresponding CαO product (4G) in high yield, whereas the oxidations with HBT, VLA, and TEMPO gave 4G in low yields. In contrast, the oxidation with ABTS gave a Cα–Cβ cleavage product (5G) in 5.5% yield. The selectivity of the mediators in the EMS reaction effectively reflected the mechanisms of their reactions, as reported for the laccase mediator system. NHPI was confirmed to be the best mediator in the present system for the selective Cα-carbonylation of the non-phenolic β-O-4 structures in lignin.

Studies on electro-oxidation of lignin and lignin model compounds. Part 2: N-Hydroxyphthalimide (NHPI)-mediated indirect electro-oxidation of non-phenolic lignin model compounds

Abstract The N-hydroxyphthalimide (NHPI)-mediated indirect electro-oxidation of non-phenolic lignin model compounds has been investigated for selective Cα-carbonylation of lignin. A cyclic voltammogram of NHPI in 0.1 M LiClO4/CH3CN with 2,6-lutidine interpreted that NHPI can act as a mediator in the indicated process in the range 0.5–0.8 V vs. Ag/Ag+. The corresponding Cα-carbonyl compounds was obtained in high yields (85–97%) in the case of the monomer 1-(4-ethoxy-3-methoxyphenyl) ethanol in 0.1 M LiClO4/CH3CN or 0.1 M LiClO4/(CH3CN/H2O=7/3) with a small amount of 2,6-lutidine at 0.7 V vs. Ag/Ag+. The processing of the dimeric lignin model compound (4-ethoxy-3-methoxyphenylglycerol-β-guaiacyl ether) also gave the corresponding Cα-carbonyl compound in high yield (88–92%). The reaction proceeds through hydrogen atom transfer in the NHPI-mediated electro-oxidation. On the other hand, the direct electro-oxidation and indirect electro-oxidation mediated by ABTS [2,2′-azinobis(3-ethylbenzothiazoline-6-sulfonate)] of the dimeric compound preferentially gave the corresponding Cα-Cβ cleavage product in low or moderate yields (5–40%). The conclusion is that NHPI is an excellent mediator for selective Cα-carbonylation of non-phenolic β-O-4 structures in lignin in electronic mediator system.

Studies on electrooxidation of lignin and lignin model compounds. Part 1: Direct electrooxidation of non-phenolic lignin model compounds

Abstract The direct anodic oxidation of non-phenolic lignin model compounds was investigated to understand their basic behaviors. The results of cyclic voltammetry (CV) studies of monomeric model, such as 1-(4-ethoxy-3-methoxyphenyl)ethanol, are interpreted as the oxidation for Cα-carbonylation did not proceed in the reaction without a catalyst, but a base promotes this reaction. Indeed, the bulk electrolyses of the monomeric lignin model compounds with 2,6-lutidine afforded the corresponding Cα-carbonyl compounds in high yields (60–80%). It is suggested that deprotonation at Cα-H in the ECEC mechanism (E=electron transfer and C=chemical step) is important for Cα-carbonylation. In the uncatalyzed bulk electrolysis of a β-O-4 model dimeric compound, 4-ethoxy-3-methoxyphenylglycerol-β-guaiacyl ether, the corresponding Cα-carbonyl compound was not detected but as a result of Cα-Cβcleavage 4-O-ethylvanillin was found in 40% yield. In the electrolysis reaction in the presence of 2,6-lutidine (as a sterically hindered light base), the reaction stopped for a short time unexpectedly. These results indicate the different electrochemical behavior of simple monomeric model compounds and dimeric β-O-4 models. The conclusion is that direct electrooxidation is unsuitable for Cα-carbonylation of lignin.