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PubMed2002

Pyrolysis of lignin in the presence of tetramethylammonium hydroxide (TMAH): products stemming from beta-5 substructures.

Lignin model compounds, synthetic lignins, and cedar wood have been analyzed by pyrolysis-gas chromatography(-mass spectrometry) in the presence of tetramethylammonium hydroxide (TMAH) to examine the behavior of beta-5 substructures specifically under these conditions. Two model compounds contained a beta-5 linkage and a gamma-CH2OH group. The phenolic model compound produced stilbene products by way of a formaldehyde elimination of the gamma-CH2OH. The nonphenolic model compound underwent dehydration to give arylbenzofuran products. Dehydrogenation polymers of coniferyl alcohol gave a large amount of stilbene products in TMAH/pyrolysis. TMAH/pyrolysis of a Japanese cedar (Cryptomeria japonica) wood yielded a very small amount of stilbene products. The results demonstrated that synthetic lignins are rich in terminal beta-5 substructures, but cedar (a softwood) contains a paucity of the terminal beta-5 substructures.

Gas Chromatography-Mass Spectrometry↗
PubMed2003

Lignin degradation by Agaricus bisporus accounts for a 30% increase in bioavailable holocellulose during cultivation on compost.

The common mushroom Agaricus bisporus is a non-white rot saphrophytic fungus that can degrade lignin to free and utilize holocellulose embedded in fermented straw as present in compost. A new method is described to estimate the actual amount of bioavailable holocellulose in 3.8 kg compost cultures spawned with A. bisporus Horst U1 prior to and during a cultivation with two cycles of mushroom harvesting. The method shows that the initial amount of bioavailable holocellulose per culture, accounting for 130 +/- 22 g, is lower than the total holocellulose consumption by A. bisporus accounting for 182 +/- 15 g. This difference is explained by a 30% increase in bioavailable holocellulose. The increase is caused by the degradation of 95 +/- 3 g of holocellulose-shielding lignin. The results are discussed within the scope of the A. bisporus mushroom yield and lignin degradation by white rot fungi during growth on lignocellulose-containing materials.

Agaricus↗
PubMed2003

Apoplastic pH and monolignol addition rate effects on lignin formation and cell wall degradability in maize.

Monolignol polymerization rate and apoplastic pH and may influence the formation of lignin and its interactions in cell walls. Primary maize walls were artificially lignified by gradual "end-wise" or rapid "bulk" polymerization of coniferyl alcohol at pH 4 or 5.5. Lignification efficiency was greatest for end-wise polymers at pH 5.5 (90-98%), intermediate for bulk polymers formed at either pH (54-82%), and lowest for end-wise polymers at pH 4 (41-53%). End-wise polymers had about 2.2-fold more ether inter-unit linkages and 70% fewer end-groups than bulk polymers. Low pH enhanced the formation of ether linkages in end-wise but not in bulk polymers. Differences in lignin structure did not influence the enzymatic degradability of cell walls, but lowering apoplastic pH from 5.5 to 4.0 during lignification reduced cell wall degradability by 25%. Further studies indicated this pH-dependent depression in degradability was related to cell wall cross-links formed via lignin quinone methide intermediates.

Cell Wall↗
PubMed2004

A comprehensive approach for quantitative lignin characterization by NMR spectroscopy.

A detailed approach for the quantification of different lignin structures in milled wood lignin (MWL) has been suggested using a combination of NMR techniques. 1H-13C heteronuclear multiple quantum coherence and quantitative 13C NMR of nonacetylated and acetylated spruce MWL have been found to have a synergetic effect, resulting in significant progress in the characterization of lignin moieties by NMR. About 80% of side chain moieties, such as different beta-O-4, dibenzodioxocin, phenylcoumaran, pinoresinol, and others, have been identified on the structural level. The presence of appreciable amounts of alpha-O-alkyl and gamma-O-alkyl ethers has been suggested. Although the quantification of various condensed moieties was less precise than for side chain structures, reliable information can be obtained. Comparison of the calculated results with known databases on spruce MWL structure shows that the suggested approach is rather informative and comparable with the information obtained from the combination of various wet chemistry methods. Discrepancies between the results obtained in this study and those previously published are discussed.

Acetylation↗
PubMed2004

Chemical characterization of lignin and lipid fractions in kenaf bast fibers used for manufacturing high-quality papers.

The chemical composition of lignin and lipids of bast fibers from kenaf (Hibiscus cannabinus) used for high-quality paper pulp production was studied. Pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) of fibers showed a lignin with a high syringyl/guaiacyl ratio (5.4) and minor amounts of p-hydroxyphenyl units. Simultaneously, sinapyl and coniferyl acetates were also identified, indicating that this lignin is partially acetylated. p-Hydroxycinnamic acids were found in only trace amounts. The main lipids identified by GC/MS of extracts from kenaf fibers were series of long-chain n-fatty acids, waxes, n-alkanes, and n-fatty alcohols. Free and esterified sterols and triterpenols, steroid hydrocarbons, and steroid and triterpenoid ketones, as well as steryl glycosides, were also found. Finally, the fate of the main constituents of kenaf fibers in alkaline pulping was also investigated.

Gas Chromatography-Mass Spectrometry↗
PubMed1999

Using the acetyl bromide assay to determine lignin concentrations in herbaceous plants: some cautionary notes.

The acetyl bromide assay was developed to provide a rapid and sensitive method for quantifying lignin in woody plant species. The original procedure cautioned against prolonged reaction times and advised keeping the reaction temperature at 70 degrees C to prevent excessive carbohydrate degradation that would skew the absorption spectra. Characterization of the reaction conditions revealed that the acetyl bromide reagent readily degrades xylans, a prominent polysaccharide group within all lignified plants. This degradation results in increased absorbance in the 270-280 nm region that is used to quantify lignin. The degradation of xylans is temperature dependent and is exacerbated by the addition of perchloric acid. Lowering the reaction temperature to 50 degrees C and increasing the reaction time from 2 to 4 h allows complete lignin solubilization but minimizes degradation of the xylans.

Acetates↗
PubMed1999

The DFRC method for lignin analysis. 7. Behavior of cinnamyl end groups.

The behavior of cinnamyl end groups of lignins during the derivatization followed by reductive cleavage (DFRC) procedure has been investigated using lignin model compounds. On AcBr treatment, hydroxycinnamyl alcohols give rise mainly to 1-aryl-1, 3-dibromopropanes from which 1-aryl-3-bromopropanes and arylcyclopropanes are formed by zinc reduction. Arylpropene derivatives are also significant among DFRC products of etherified cinnamyl end-group models. Major monomers from DFRC of hydroxycinnamaldehydes are arylcyclopropyl acetates produced by reductive ring closure of 1-acetoxy-3-aryl-1,3-dibromopropanes. Although the reactions are not as clean as the ether-cleaving reactions that form the basis of the DFRC method, end groups produce diagnostic compounds that provide valuable markers for studying end groups in lignins.

Alcohols↗
PubMed2000

Delignification of maize stems by peroxymonosulfuric acid, peroxyformic acid, peracetic acid, and hydrogen peroxide. 1. Physicochemical and structural characterization of the solubilized lignins.

Water-treated maize stems were subjected to delignification with peroxymonosulfuric acid at 20 degrees C for 144 h, with peroxyformic acid at 80 degrees C for 6 h, with peracetic acid at 50 degrees C for 6 h, and with 2% hydrogen peroxide at 45 degrees C for 12 h at pH 1.5, 4.4, 9.5, 11.5, 12.0, and 12.6, respectively, which solubilized 47.1, 91.3, 33.3, 16.6, 15.9, 17.4, 86.2, 87.7, and 91. 3% of the original lignin, respectively. Substantial lignins were released during the treatment with peroxyformic acid and hydrogen peroxide at pH > or =11.5, whereas an insignificant effect on delignification was observed by using peroxymonosulfuric acid, peracetic acid, and hydrogen peroxide under acidic, natural, and weakly alkaline media conditions. The structures of the isolated lignin preparations were investigated by chemical analysis, gel permeation chromatography, and UV, FT-IR, and (13)C NMR spectroscopy.

Formates↗
PubMed2000

Cross-coupling of hydroxycinnamyl aldehydes into lignins.

Pathways for hydroxycinnamyl aldehyde incorporation into lignins are revealed by examining transgenic plants deficient in cinnamyl alcohol dehydrogenase, the enzyme that converts hydroxycinnamyl aldehydes to the hydroxycinnamyl alcohol lignin monomers. In such plants the aldehydes incorporate into lignins via radical coupling reactions. As diagnostically revealed by long-range (13)C-(1)H correlative NMR, sinapyl aldehyde (3, 5-dimethoxy-4-hydroxy-cinnamaldehyde) 8-O-4-cross-couples with both guaiacyl (3-methoxy-4-hydroxyphenyl-propanoid) and syringyl (3, 5-dimethoxy-4-hydroxyphenyl-propanoid) units, whereas coniferyl aldehyde cross-couples only with syringyl units.

Alcohol Oxidoreductases↗
PubMed2004

Biotransformation of lignin polymers derived from beech wood pulping by Sporobolomyces roseus isolated from leafy material.

The ability of the yeast, Sporobolomyces roseus, isolated from leafy material, to modify lignin derived from beechwood pulping was examined by FTIR and 13C NMR spectroscopy, which revealed oxidative cleavage of the Calpha-Cbeta linkages between lignin units. Using veratryl alcohol as a model substrate confirmed that Sp. roseus could oxidize veratryl alcohol into veratric acid. This yeast might be suitable for the pretreatment of lignocellulosic materials and/or for biotransformation of technical lignins.

Basidiomycota↗
PubMed2004

Hydrogen bonding between histidine and lignin model compounds or redox mediators as calculated with the DFT method. Effects on the ease of oxidation.

Using the Density Functional Theory method, the effect of hydrogen bonding between imidazole (IM) and ten benzyl alcohol derivatives (BA) on the ionization potentials of the latter is calculated. IM is used as a model for histidine, which is found in the reaction sites of laccases and lignin peroxidases, and the BA-derivatives serve as lignin model compounds. A marked decrease ([similar]15 kcal mol(-1)) is found for the IP's of the BA-derivatives when paired with IM. This should facilitate the one-electron oxidation of BA in the reaction site of the enzyme. The same effect is found for the known redox mediators violuric acid, 1-hydroxybenzotriazole and N-hydroxyacetanilide which are assumed to enter the reaction site of the enzymes. Furthermore, upon one-electron oxidation the strength of the H-bond from BA to IM is considerably increased and in the case of the mediators this effect is so pronounced that the relevant proton shifts from them to IM. If this occurs in the active site of the enzyme then the oxidized redox mediators are released into the aqueous phase in their neutral form rather than as radical cations (deprotonation of the radical cations). The oxidation power of the neutral radical mediators, however, is too low to initialize oxidation of lignin. A more likely reaction pathway is oxidation of the substrates via hydrogen abstraction. The pertinent bond dissociation energies are similar for the BA-derivatives and the redox mediators, which in principle allows the reaction to occur.

Dimerization↗
PubMed2006

Non-enzymatic reduction of quinone methides during oxidative coupling of monolignols: implications for the origin of benzyl structures in lignins.

Lignin is believed to be synthesized by oxidative coupling of 4-hydroxyphenylpropanoids. In native lignin there are some types of reduced structures that cannot be explained solely by oxidative coupling. In the present work we showed via biomimetic model experiments that nicotinamide adenine dinucleotide (NADH), in an uncatalyzed process, reduced a beta-aryl ether quinone methide to its benzyl derivative. A number of other biologically significant reductants, including the enzyme cellobiose dehydrogenase, failed to produce the reduced structures. Synthetic dehydrogenation polymers of coniferyl alcohol synthesized (under oxidative conditions) in the presence of the reductant NADH produced the same kind of reduced structures as in the model experiment, demonstrating that oxidative and reductive processes can occur in the same environment, and that reduction of the in situ-generated quinone methides was sufficiently competitive with water addition. In situ reduction of beta-beta-quinone methides was not achieved in this study. The origin of racemic benzyl structures in lignins therefore remains unknown, but the potential for simple chemical reduction is demonstrated here.

Benzene↗
PubMed1974

Structural invesiigations on the lignin-carbohydrate complexes of Lolium perenne.

1. Lignin-carbohydrate complexes isolated from leaf blade, leaf sheath and stem tissue of ryegrass by extraction with dimethyl sulphoxide were examined by fractionation procedures. Although the complexes are heterogeneous, heterogeneity is shown only in the ratio of the individual monosaccharide residues and not in the ratio of lignin to carbohydrate. 2. The molecular weight of the complexes is high (>/=150000), but chemical modification by alkaline hydrolysis, borohydride reduction or lead tetra-acetate oxidation does not drastically decrease it. Low-molecular-weight fragments released by alkaline treatment were shown to contain acetic acid, ferulic acid and p-coumaric acid. 3. On the basis of the chemical stability of the complexes, it is postulated that at least three types of bonding may be present between lignin and carbohydrate, namely one cleaved on borohydride reduction, another cleaved by alkali and a linkage resistant to alkali. 4. The carbohydrate portion of the complexes is composed of beta-(1-->4)-linked d-glucose residues (cellulose) and beta-(1-->4)-linked chains of xylose residues. Side chains involving arabinose and galactose residues are linked to C-3 of some of the xylose residues. 5. How the components of the complexes are held together is not certain, but it is suggested that the phenolic acids may act as cross-linking agents.

Binding Sites↗
PubMed1986

Ligninase of Phanerochaete chrysosporium. Mechanism of its degradation of the non-phenolic arylglycerol beta-aryl ether substructure of lignin.

This study examined the ligninase-catalysed degradation of lignin model compounds representing the arylglycerol beta-aryl ether substructure, which is the dominant one in the lignin polymer. Three dimeric model compounds were used, all methoxylated in the 3- and 4-positions of the arylglycerol ring (ring A) and having various substituents in the beta-ether-linked aromatic ring (ring B), so that competing reactions involving both rings could be compared. Studies of the products formed and the time courses of their formation showed that these model compounds are oxidized by ligninase (+ H2O2 + O2) in both ring A and ring B. The major consequence with all three model compounds is oxidation of ring A, leading primarily to cleavage between C(alpha) and C(beta) (C(alpha) being proximal to ring A), and to a lesser extent to the oxidation of the C(alpha)-hydroxy group to a carbonyl group. Such C(alpha)-oxidation deactivates ring A, leaving only ring B for attack. Studies with C(alpha)-carbonyl model compounds corresponding to the three basic model compounds revealed that oxidation of ring B leads in part to dealkoxylations (i.e. to cleavage of the glycerol beta-aryl ether bond and to demethoxylations), but that these are minor reactions in the model compounds most closely related to lignin. Evidence is also given that another consequence of oxidation of ring B in the C(alpha)-carbonyl model compounds is formation of unstable cyclohexadienone ketals, which can decompose with elimination of the beta-ether-linked aromatic ring. The mechanisms proposed for the observed reactions involve initial formation of aryl cation radicals in either ring A or ring B. The cation radical intermediate from one of the C(alpha)-carbonyl model compounds was identified by e.s.r. spectroscopy. The mechanisms are based on earlier studies showing that ligninase acts by oxidizing appropriately substituted aromatic nuclei to aryl cation radicals [Kersten, Tien, Kalyanaraman & Kirk (1985) J. Biol. Chem. 260, 2609-2612; Hammel, Tien, Kalyanaraman & Kirk (1985) J. Biol. Chem. 260, 8348-8353].

Binding Sites↗
PubMed1988

Veratryl alcohol oxidases from the lignin-degrading basidiomycete Pleurotus sajor-caju.

The basidiomycete Pleurotus sajor-caju mineralizes ring-14C-labelled lignin (dehydrogenative polymer) when grown in mycological broth. Under these conditions, two veratryl alcohol oxidase (VAO) enzymes were found in the culture medium. They oxidized a number of aromatic alcohols to aldehydes and reduced O2 to H2O2. The enzymes were purified by ion-exchange and gel-permeation chromatography. The final step of purification on Mono Q resolved the activity into two peaks (VAO I and VAO II). Both enzymes had the same Mr, approx. 71,000, but their isoelectric points differed slightly, 3.8 for VAO I and 4.0 for VAO II. Their amino acid compositions were similar except for aspartic acid/asparagine and glycine. Both enzymes are glycoproteins and contain flavin prosthetic groups. Their pH optima were around 5, and kinetic constants and specificities were similar. 4-Methoxybenzyl alcohol was oxidized the most rapidly, followed by veratryl alcohol. Not all aromatic alcohols were oxidized, neither were non-aromatic alcohols. Cinnamyl alcohol was oxidized at the gamma position. The VAO enzymes thus represent a significantly different route for veratryl alcohol oxidation from that catalysed by the previously found lignin peroxidases from Phanerochaete chrysosporium. The role of the oxidases in biodegradation might be to produce H2O2 during oxidation of lignin fragments.

Alcohol Oxidoreductases↗
PubMed1997

Cinnamoyl CoA reductase, the first committed enzyme of the lignin branch biosynthetic pathway: cloning, expression and phylogenetic relationships.

Cinnamoyl CoA:NADP oxidoreductase (CCR, EC 1.2.1.44) catalyzes the conversion of cinnamoyl CoA esters to their corresponding cinnamaldehydes, i.e. the first specific step in the synthesis of the lignin monomers. The cloning of a cDNA encoding CCR in Eucalyptus gunnii (EUCCR) is reported here. The identity of the EUCCR cDNA was demonstrated by comparison with peptide sequence data from purified CCR and functional expression of the recombinant enzyme in Escherichia coli. Sequence analysis revealed remarkable homologies with dihydroflavonol-4-reductase (DFR), the first enzyme of the anthocyanin biosynthetic pathway. Moreover, significant similarities were found with mammalian 3 beta-hydroxysteroid dehydrogenase and bacterial UDP-galactose-4-epimerase, suggesting that CCR shared a common ancestor with these enzymes and can therefore be considered as a new member of the mammalian 3 beta-hydroxysteroid dehydrogenase/ plant dihydroflavonol reductase superfamily. In Eucalyptus gunnii, CCR is encoded by one gene containing four introns whose positions are similar to those of introns I, II, III and V in DFR genes from dicots. In agreement with the involvement of CCR in lignification, the CCR transcript was shown to be expressed in lignified organs, i.e. root and stem tissues, and was localized mainly in young differentiating xylem. On the other hand, its abundance in Eucalyptus leaves suggests that monolignols may be precursors of end products other than lignins. This first characterization of a gene corresponding to CCR opens new possibilities to genetically engineer plants with lower lignin content. This is particularly important for woody plants such as Eucalyptus which are used for pulp making.

Aldehyde Oxidoreductases↗
PubMed2003

Detection and characterization of a novel extracellular fungal enzyme that catalyzes the specific and hydrolytic cleavage of lignin guaiacylglycerol beta-aryl ether linkages.

Cleavage of the arylglycerol beta-aryl ether linkage is the most important process in the biological degradation of lignin. The bacterial beta-etherase was described previously and shown to be tightly associated with the cellular membrane. In this study, we aimed to detect and isolate a new extracellular function that catalyses the beta-aryl ether linkage cleavage of high-molecular lignin in the soil fungi. We screened and isolated 2BW-1 cells by using a highly sensitive fluorescence assay system. The beta-aryl ether cleavage enzyme was produced by a newly isolated fungus, 2BW-1, and is secreted into the extracellular fraction. The beta-aryl ether cleavage enzyme converts the guaiacylglycerol beta-O-guaiacyl ether (GOG) to guaiacylglycerol and guaiacol. It requires the C alpha alcohol structure and p-hydroxyl group and specifically attacks the beta-aryl ether linkage of high-molecular mass lignins with addition of two water molecules at the C alpha and C beta positions.

Catalysis↗
PubMed1999

New routes for lignin biosynthesis defined by biochemical characterization of recombinant ferulate 5-hydroxylase, a multifunctional cytochrome P450-dependent monooxygenase.

The enzymes and genes of the lignin biosynthetic pathway have been studied for several decades, but the gene encoding ferulate 5-hydroxylase (F5H) was cloned only 3 years ago by T-DNA tagging in Arabidopsis. To characterize the enzyme in detail, we have expressed F5H in yeast. According to current models of the phenylpropanoid pathway, F5H catalyzes the hydroxylation of ferulate to 5-hydroxyferulate; however, our studies indicate that the enzyme also uses coniferaldehyde and coniferyl alcohol as substrates. Unexpectedly, the K(m) values measured for the latter two substrates are three orders of magnitude lower than that measured for ferulic acid, suggesting that in lignifying tissues, syringyl monomers may be derived from their guaiacyl counterparts by hydroxylation and subsequent methylation. Thus, F5H may function later in the lignin biosynthetic pathway than was originally proposed. To further test this model, recombinant F5H was incubated together with ferulic acid, coniferaldehyde, or coniferyl alcohol in the presence of native or recombinant Arabidopsis caffeic acid/5-hydroxyferulic acid O-methyltransferase and [(14)C]S-adenosylmethionine. In all cases, the corresponding radiolabeled sinapyl derivatives were synthesized, indicating that the necessary enzymes required for this pathway are present in Arabidopsis. Taken together, these data suggest that the previously accepted pathway for lignin biosynthesis is likely to be incorrect.

Arabidopsis↗