Determination of bonding patterns of carbon-13 specifically enriched dehydrogenatively polymerized lignin in solution and solid state

Elucidation of microbial lignin degradation pathways using synthetic isotope-labelled lignin

Pathways by which the biopolymer lignin is broken down by soil microbes could be used to engineer new biocatalytic routes from lignin to renewable chemicals, but are currently not fully understood. In order to probe these pathways, we have prepared synthetic lignins containing ¹³C at the sidechain β-carbon. Feeding of [β-¹³C]-labelled DHP lignin to Rhodococcus jostii RHA1 has led to the incorporation of ¹³C label into metabolites oxalic acid, 4-hydroxyphenylacetic acid, and 4-hydroxy-3-methoxyphenylacetic acid, confirming that they are derived from lignin breakdown. We have identified a glycolate oxidase enzyme in Rhodococcus jostii RHA1 which is able to oxidise glycolaldehyde via glycolic acid to oxalic acid, thereby identifying a pathway for the formation of oxalic acid. R. jostii glycolate oxidase also catalyses the conversion of 4-hydroxyphenylacetic acid to 4-hydroxybenzoylformic acid, identifying another possible pathway to 4-hydroxybenzoylformic acid. Formation of labelled oxalic acid was also observed from [β-¹³C]-polyferulic acid, which provides experimental evidence in favour of a radical mechanism for α,β-bond cleavage of β-aryl ether units.

Probing native lignin macromolecular configuration in Arabidopsis thaliana in specific cell wall types: further insights into limited substrate degeneracy and assembly of the lignins of ref8, fah 1-2 and C4H::F5H lines

The interest in renewable, plant-derived, bioenergy/biofuels has resulted in a renaissance of plant cell-wall/lignin research. Herein, effects of modulating lignin monomeric compositions in a single plant species, Arabidopsis, are described. The earliest stage of putative "AcBr/Klason lignin" deposition was apparently unaffected by modulating p-coumarate 3-hydroxylase or ferulate 5-hydroxylase activities. This finding helps account for the inability of many other studies to fully suppress the reported putative levels of lignin deposition through monolignol biosynthesis manipulation, and also underscores limitations in frequently used lignin analytical protocols. The overall putative lignin content was greatly reduced (circa 62%) in a plant line harboring an H-(p-hydroxyphenyl) enriched lignin phenotype. This slightly increased H-monomer deposition level apparently occurred in cell-wall domains normally harboring guaiacyl (G) and/or syringyl (S) lignin moieties. For G- and S-enriched lignin phenotypes, the overall lignification process appeared analogous to wild type, with only xylem fiber and interfascicular fiber cells forming the S-enriched lignins. Laser microscope dissection of vascular bundles and interfascicular fibers, followed by pyrolysis GC/MS, supported these findings. Some cell types, presumably metaxylem and possibly protoxylem, also afforded small amounts of benzodioxane (sub)structures due to limited substrate degeneracy (i.e. utilizing 5-hydroxyconiferyl alcohol rather than sinapyl alcohol). For all plant lines studied, the 8-O-4' inter-unit frequency of cleavable H, G and/or S monomers was essentially invariant of monomeric composition for a given (putative) lignin content. These data again underscore the need for determination of lignin primary structures and identification of all proteins/enzymes involved in control of lignin polymer assembly/macromolecular configuration.

Levels of plant cell wall structural organization revealed by atomic force microscopy

We used an atomic force microscope to image cell wall isolated from needles of Serbian spruce tree and that synthesized from cell wall components. We also observed the structure of lignin model polymer (DHP), as a best substitute for the natural lignin. A tendency of aggregate formation was observed in all samples. Cell wall was revealed as a laminated fibrous structure. General organization is similar in both isolated and synthesized cell wall samples, with dominating globular motifs arranged regularly as rods and forming cavities. The synthesized cell wall has a more regular structural organization than isolated cell wall. The dimensions of individual globular aggregates and pores differed between the two samples. DHP showed a similar, regular organization, with globular aggregates and holes. Globules and pores are smaller in size than the corresponding structures in both the isolated and synthesized cell walls. Such modular organization of cell walls may have a physiological role in response to the external mechanical stress caused to plant cells.

The Arabidopsis cinnamoyl CoA reductase irx4 mutant has a delayed but coherent (normal) program of lignification

Previous studies have indicated that the Arabidopsis thalianairregular xylem 4 (irx4) mutant is severely lignin-deficient, forming abnormal lignin from aberrant monomers. Studies of lignin structure in dwarfed cinnamoyl CoA reductase (CCR)-downregulated tobacco were also previously reported to incorporate feruloyl tyramine derivatives. The lignin in the Arabidopsis irx4 mutant was re-investigated at 6 weeks and at maturation (9 weeks). Application of (1)H, (13)C, 2D Heteronuclear Multiple Quantum Coherence and 2D Heteronuclear Multiple Bond Coherence spectroscopic analyses to the lignin-enriched isolates from both Arabidopsis wild-type (Ler) and the CCR-irx4 mutant at both developmental stages revealed that only typical guaiacyl/syringyl lignins were formed. For the irx4 mutant, the syringyl content at 6 weeks growth was lower, in accordance with a delayed but coherent program of lignification. At maturation, however, the syringyl/guaiacyl ratio of the irx4 mutant approached that of wild-type. There was no evidence for feruloyl tyramines, or homologues thereof, accumulating as a chemical signature in lignins resulting from CCR mutation. Nor were there any noticeable increases in other phenolic components, such as hydroxycinnamic acids. These findings were further confirmed by application of thioacidolysis, alkaline nitrobenzene oxidation and acetyl bromide analyses. Moreover, in the case of CCR downregulation in tobacco, there were no NMR spectroscopic correlations that demonstrated feruloyl tyramines being incorporated into the lignin biopolymers. This study thus found no evidence that abnormal lignin formation occurs when CCR activity is modulated.

Component analysis of the fluorescence spectra of a lignin model compound

In order to test whether lignin fluorescence originates from discrete fluorophores, fluorescence emission spectra of the lignin model dehydrogenative polymer (DHP) were analyzed by the band deconvolution method and time-resolved analysis of both the excitation and emission spectra. Two series of 22 fluorescence emission spectra of DHP in chloroform/methanol (3:1, v/v) solution, and as a solid suspension in water, were deconvoluted into three fluorescence and one Raman Gaussian components. Emission spectra were obtained by stepwise variation of the excitation wavelength from 360 to 465 nm. Deconvolution was performed by nonlinear fitting of all three Gaussian parameters: area, width and position. Position of all components in a series was treated as a random variable and its approximate probability distribution (APD) calculated from a series of histograms with increasing number of abscissa intervals. A five peak multimodal APD profile was obtained for both series of DHP emission spectra. The mean fluorescence lifetime varied with wavelength both in the emission and the excitation decay-associated spectra (DAS), where four kinetic components were resolved. The shapes of the excitation spectra of the four components were quite different and gradually shifted bathochromically. The multicomponent nature of the DHP emission spectra along with the changes in the mean fluorescence lifetime and the form of the excitation DAS of the four components give evidence of the heterogeneous origin of fluorescent species emitting in the visible.

Study of the lignin model compound supramolecular structure by combination of near-field scanning optical microscopy and atomic force microscopy

In this paper, we present a nanoscale study of the supramolecular structure of the dehydrogenate polymer (ZL-DHP) lignin model compound. The combination of near-field scanning optical microscopy (NSOM or SNOM) and atomic force microscopy (AFM) has been utilized to explore physicochemical properties of the lignin model compound on a scale ranging from individual macromolecules to globular supramolecular assemblies. By utilizing NSOM in transmission mode, the optical inhomogeneity in the lignin supramolecular structure has been observed for the first time. In particular, the transmission-mode NSOM images reveal a combination of hollow and layered supramolecular globular structure in the lignin model compound. Through the paired use of TappingMode and pulsed-mode AFM, we have also confirmed the existence of regions with different rheological properties on the single lignin model compound supramolecular assembly.

Mass spectrometry in the biosynthetic and structural investigation of lignins

Lignin, a resistant cell-wall constituent of all vascular plants that consists of ether and carbon-linked methoxyphenols, is still far from being structurally described in detail. The main problem in its structural elucidation is the difficulty of isolating lignin from other wood components without damaging lignin itself. Furthermore, the high number and variegated forms of linkages that occur between the monomeric units and the chemical resistance of certain ether bonds limit the extent to which analytical and degradation procedures can be used to elucidate the lignin structure. Most of our present knowledge about the molecular structure of lignin is based on the analysis of monomers, dimers or, at the most, tetramers of degraded isolated lignins. Mass spectrometry (MS), which offers advantages in terms of speed, specificity, and sensitivity, has revealed to be a very powerful technique in the structural elucidation of lignins, in combination with the great number of chemical and thermal degradation methods available in the study of lignin. Moreover, the recent development of new ionization techniques in MS-electrospray ionization (ESI)-MS and matrix-assisted laser desorption/ionization (MALDI)-MS-has provided new possibilities to also analyze the undegraded lignin macromolecule.

ZL-DHP lignin model compound at the air-water interface

In this paper we present our surface chemistry studies of enzymatically polymerized, poly-coniferyl alcohol lignin model compound (dehydrogenate polymer a.k.a. ZL-DHP) at the air-water interface. Using the CHCl(3)/MeOH (5:1 v/v) spreading solvent, we found an average molecular area of ZL-DHP of approximately 1200 A(2). The monolayer expresses a high compressibility with a collapsed area of 500 A(2) and collapsed surface pressure of 28 mN m(-1). In the range of applied surface pressures, ZL-DHP polymer have no phase changes, as shown by the very high linearity (R=0.994) of absorbance vs. surface pressure cure. There was no symmetry transitions observed as shown by absence of shifts of absorption peak maximums.

Dirigent proteins and dirigent sites in lignifying tissues

Tissue-specific dirigent protein gene expression and associated dirigent (site) localization were examined in various organs of Forsythia intermedia using tissue printing, in situ mRNA hybridization and immunolabeling techniques, respectively. Dirigent protein gene expression was primarily noted in the undifferentiated cambial regions of stem sections, whereas dirigent protein sites were detected mainly in the vascular cambium and ray parenchyma cell initials. Immunolocalization also revealed cross-reactivity with particular regions of the lignified cell walls, these being coincident with the known sites of initiation of lignin deposition. These latter regions are considered to harbor contiguous arrays of dirigent (monomer binding) sites for initiation of lignin biopolymer assembly. Dirigent protein mRNA expression was also localized in the vascular regions of roots and petioles, whereas in leaves the dirigent sites were primarily associated with the palisade layers and the vascular bundle. That is, dirigent protein mediated lignan biosynthesis was initiated primarily in the cambium and ray cell initial regions of stems as well as in the leaf palisade layers, this being in accordance with the occurrence of the lignans for defense purposes. Within lignified secondary xylem cell walls, however, dirigent sites were primarily localized in the S(1) sublayer and compound middle lamella, these being coincident with previously established sites for initiation of macromolecular lignin biosynthesis. Once initiation occurs, lignification is proposed to continue through template polymerization.

A Comparative Study of Enzymatically and Photochemically Polymerized Artificial Lignin Supramolecular Structures Using Environmental Scanning Electron Microscopy

Environmental scanning electron microscopy images of the self-assembled structures of enzymatically (DHP) and photochemically polymerized (PCP) artificial lignin are herein presented. Differences in the structural organization between DHP and PCP polymer at the supramolecular level were reported. Based on topological information, we proposed a hypothesis about possible new physiological roles of lignin in live plant cells and the ecological significance of possible in-vivo photochemical lignin polymerization. Copyright 2000 Academic Press.

Visualization of artificial lignin supramolecular structures

In this paper we are presenting the results of our environmental scanning electron microscopy (ESEM) investigation of the lignin model compound--enzymatically polymerized coniferyl alcohol, also known as dehydrogenate polymer (DHP). The goals of this study were to visualize the supramolecular organization of DHP polymer on various substrates, namely graphite, mica, and glass, and to explore the influence of substrate surface properties and associated collective phenomena on the lignin self-assembled supramolecular structure. Based on results obtained with ESEM, combined with previously published results based on scanning tunneling microscopy (STM) and electron spin resonance (ESR) technique, we looked at lignin structure ranging from a monomer on a fraction of nanometer scale to a large aggregate on a fraction of millimeter scale, therefore using six orders of magnitude range of size. Herein, we are presenting evidence that there are at least four different levels of the supramolecular structure of lignin, and that its supramolecular organization is well dependent on the substrate surface characteristics, such as hydrophobicity, delocalized orbitals, and surface-free energy.

Biochemical characterization of the suberization-associated anionic peroxidase of potato

The anionic peroxidase associated with the suberization response in potato (Solanum tuberosum L.) tubers during wound healing has been purified and partially characterized at the biochemical level. It is a 45-kD, class III (plant secretory) peroxidase that is localized to suberizing tissues and shows a preference for feruloyl (o-methoxyphenol)-substituted substrates (order of substrate preference: feruloyl > caffeoyl > p-coumaryl approximately syringyl) such as those that accumulate in tubers during wound healing. There was little influence on oxidation by side chain derivatization, although hydroxycinnamates were preferred over the corresponding hydroxycinnamyl alcohols. The substrate specificity pattern is consistent with the natural substrate incorporation into potato wound suberin. In contrast, the cationic peroxidase(s) induced in response to wound healing in potato tubers is present in both suberizing and nonsuberizing tissues and does not discriminate between hydroxycinnamates and hydroxycinnamyl alcohols. A synthetic polymer prepared using E-[8-(13)C]ferulic acid, H(2)O(2), and the purified anionic enzyme contained a significant amount of cross-linking through C-8, albeit with retention of unsaturation.

Biochemical characterization, molecular cloning and expression of laccases - a divergent gene family - in poplar

The nature of the enzyme(s) involved in the dehydrogenative polymerization of lignin monomers is still a matter of debate. Potential candidates include laccases which have recently received attention due to their capacity to oxidize lignin monomers and close spatial and temporal correlation with lignin deposition. We have characterized two H2O2-independent phenoloxidases with approximate molecular masses of 90 kDa and 110 kDa from cell walls of Populus euramericana xylem that are capable of oxidizing coniferyl alcohol. The 90-kDa protein was purified to apparent homogeneity and extensively characterized at the biochemical and structural levels. To our knowledge, this is the first report of a plant laccase purified to homogeneity from a lignifying tissue of an angiosperm. The cDNA clones corresponding to the 90-kDa and 110-kDa proteins, lac90 and lac110, were obtained by a PCR-based approach using specific oligonucleotides derived from peptide sequences. Sequence analysis indicated that lac90 and lac110 encoded two distinct laccases. In addition, heterologous screening using an Acer pseudoplatanus laccase cDNA enabled us to obtain three additional cDNAs (lac1, lac2, lac3) that did not correspond to lac90 and lac110. The five laccase cDNAs correspond to a highly divergent multigene family but Northern analysis with gene-specific probes indicated that all of the genes are exclusively and abundantly expressed in stems. These results highlight the polymorphism of plant laccases by an integrated biochemical and molecular approach, and provide the tools that will enable us to clearly determine the function of these enzymes in plants by molecular and genetic approaches.

The macromolecular aromatic domain in suberized tissue: a changing paradigm

As a structural feature of specialized cell walls, suberization remains an enigma, despite its obvious importance both during normal growth and development and as a stress response in plants. While it is clear that suberized tissues contain both polyaromatic and polyaliphatic domains, and that each of these has its own unique characteristics, whether there is a contiguous macromolecule that can be called suberin is an open question. From a structural perspective, the aromatic domain is unique and distinct from lignin, and is apparently comprised primarily of (poly)hydroxycinnamates, such as amides (e.g., feruloyltyramine). The aliphatic domain is also unique, being quite distinct from cutin in terms of both its chemical composition and cellular location. In the present paper, histochemical, structural and biochemical data, particularly, regarding the polyaromatic domain of suberized tissues, are critically reviewed. A revised description of the polyaromatic domain of suberized tissues, based on the consensus that is emerging from the current data, is presented and especially includes a spatially distinct (poly)hydroxycinnamoyl-containing macromolecule.

Fourier-transform infrared and Raman spectroscopic evidence for the incorporation of cinnamaldehydes into the lignin of transgenic tobacco (Nicotiana tabacum L.) plants with reduced expression of cinnamyl alcohol dehydrogenase

Xylem from stems of genetically manipulated tobacco plants which had had cinnamyl alcohol dehydrogenase (CAD; EC 1.1.1.195) activity down-regulated to a greater or lesser degree (clones 37 and 49, respectively) by the insertion of antisense CAD cDNA had similar, or slightly higher, lignin contents than xylem from wild-type plants. Fourier-transform infrared (FT-IR) microspectroscopy indicated that down-regulation of CAD had resulted in the incorporation of moieties with conjugated carbonyl groups into lignin and that the overall extent of cross-linking, particularly of guaiacyl (4-hydroxy-3-methoxyphenyl) rings, in the lignin had altered. The FT-Raman spectra of manipulated xylem exhibited maxima consistent with the presence of elevated levels of aldehydic groups conjugated to a carbon-carbon double bond and a guaiacyl ring. These maxima were particularly intense in the spectra of xylem from clone 37, the xylem of which exhibits a uniform red coloration, and their absolute frequencies matched those of coniferaldehyde. Furthermore, xylem from clone 37 was found to have a higher content of carbonyl groups than that of clone 49 or the wild-type (clone 37: clone 49: wild-type; 2.4:1.6:1.0) as measured by a degradative chemical method. This is the first report of the combined use of FT-IR and FT-Raman spectroscopies to study lignin structure in situ. These analyses provide strong evidence for the incorporation of cinnamaldehyde groups into the lignin of transgenic plants with down-regulated CAD expression. In addition, these non-destructive analyses also suggest that the plants transformed with antisense CAD, in particular clone 37, may contain lignin that is less condensed (cross-linked) than that of the wild-type.

Hydroxycinnamic acid-derived polymers constitute the polyaromatic domain of suberin

Suberin is an abundant, complex, intractable, plant cell wall polymeric network that forms both protective and wound-healing layers. Its function is, therefore, critical to the survival of all vascular plants. Its chemical structure and biosynthesis are poorly defined, although it is known to consist of both aromatic and aliphatic domains. While the composition of the aliphatic component has been fairly well characterized, that of the phenolic component has not. Using a combination of specific carbon-13 labeling techniques, and in situ solid state 13C NMR spectroscopic analysis, we now provide the first direct evidence for the nature of the phenolic domain of suberin and report here that it is almost exclusively comprised of a covalently linked, hydroxycinnamic acid-derived polymeric matrix.

Magic-angle 13C NMR analysis of hard wheat flour and dough

Samples of hard wheat flour and dough are analyzed by magic-angle spinning 13C NMR spectroscopy. Cross-polarization magic-angle spinning (CPMAS) 13C NMR spectra of the dry flour allow its starch and protein content to be accurately measured. These two components are phase-separated. Spectra of hydrated hard wheat doughs are collected under both CPMAS and single-pulse carbon with low-power 1H decoupling conditions. The former report on the macromolecular components of the dough, while the latter reveal small molecules which are solubilized by the water. Results of the present study are interpreted as indicating that the protein is largely unaffected by the added water and remains phased-separated from the starch, while water causes significant changes in polymer dynamics of the starch component.

Monitoring Biosynthesis of Wheat Cell-Wall Phenylpropanoids in Situ

Lignins and suberins are complex plant cell-wall macromolecules that are composed mainly of phenylpropanoid residues derived from L-phenylalanine. Lignins and suberins are considered to be covalently linked to carbohydrates and to lipids, respectively. The bonding of these important structural materials within cell walls has never been established. By feeding specifically labeled [(13)C] ferulic acid over extended durations to seedlings of Triticum aestivum L. and by using solid-state carbon-13 nuclear magnetic resonance techniques, the major resonances due to specific carbons in the propanoid side chains of these cell-wall polymers have been identified in situ. The signals were found to differ significantly from those of synthetic lignins, which have usually been considered to be good approximations of natural lignin structure.