Structural identification of dehydrotriferulic and dehydrotetraferulic acids isolated from insoluble maize bran fiber

Influence of cross-linked arabinoxylans on the postprandial blood glucose response in rats

Viscous dietary fibers are well established to reduce the blood glucose response to a meal. In this study, arabinoxylans, the most abundant dietary fiber in most cereals, were extracted under alkaline conditions and cross-linked by using laccase. Cross-linking of the arabinoxylans led to gel formation and increased in vitro viscosity almost 100-fold after drying and rehydration. To determine the ability of these cross-linked arabinoxylans to blunt the postprandial blood glucose curve of a meal, arabinoxylans, either native or cross-linked, and either prehydrated or not, were fed to rats as part of a meal, and blood glucose was monitored at intervals after the meal. Cellulose, a nonviscous fiber, served as a control. Cross-linked, but not native, arabinoxylans significantly reduced the area under the blood glucose time curve 5-9% relative to cellulose, indicating that they remained viscous within the gastrointestinal tract, and thus likely provide the health benefits found with other viscous fibers.

Analysis of nonextractable phenolic compounds in foods: the current state of the art

More than 500 phenolic compounds have been reported as present in foodstuffs, and their intake has been related to the prevention of several chronic diseases. Most of the literature on phenolic compounds focuses on those present in the supernatant of aqueous-organic extractions: extractable phenolics. Nevertheless, significant amounts of phenolic compounds remain in the solid residues after such extractions. These nonextractable phenolics are mostly proanthocyanidins, phenolic acids, and hydrolyzable tannins that are closely associated with the food matrix. Studies of this fraction of dietary phenolic compounds are scarce, and the few there are usually refer to particular types of phenolics rather than to the fraction as a whole. The present review reports the state-of-the-art methods that currently exist for analyzing nonextractable phenolic compounds in foods.

Changes in cinnamic acid derivatives associated with the habituation of maize cells to dichlobenil

The habituation of cell cultures to cellulose biosynthesis inhibitors such as dichlobenil (DCB) represents a valuable tool to improve our knowledge of the mechanisms involved in plant cell wall structural plasticity. Maize cell lines habituated to lethal concentrations of DCB were able to grow through the acquisition of a modified cell wall in which cellulose was partially replaced by a more extensive network of arabinoxylans. The aim of this work was to investigate the phenolic metabolism of non-habituated and DCB-habituated maize cell cultures. Maize cell cultures were fed [(14)C]cinnamate and the fate of the radioactivity in different intra-protoplasmic and wall-localized fractions throughout the culture cycle was analyzed by autoradiography and scintillation counting. Non-habituated and habituated cultures did not markedly differ in their ability to uptake exogenous [(14)C]cinnamic acid. However, interesting differences were found in the radiolabeling of low- and high-M(r) metabolites. Habituated cultures displayed a higher number and amount of radiolabeled low-M(r) compounds, which could act as reserves later used for polysaccharide feruloylation. DCB-habituated cultures were highly enriched in esterified [(14)C]dehydrodiferulates and larger coupling products. In conclusion, an extensive and early cross-linking of hydroxycinnamates was observed in DCB-habituated cultures, probably strengthening their cellulose-deficient walls.

Characterization of oligomeric xylan structures from corn fiber resistant to pretreatment and simultaneous saccharification and fermentation

Corn fiber, a byproduct from the corn industry, would be a good source for bioethanol production if the hemicellulose, consisting of polymeric glucoronoarabinoxylans, can be degraded into fermentable sugars. Structural knowledge of the hemicellulose is needed to improve the enzymatic hydrolyses of corn fiber. Oligosaccharides that resisted a mild acid pretreatment and subsequent enzymatic hydrolysis, representing 50% of the starting material, were fractionated on reversed phase and size exclusion material and characterized. The oligosaccharides within each fraction were highly substituted by various compounds. Oligosaccharides containing uronic acid were accumulated in two polar fractions unless also a feruloyl group was present. Feruloylated oligosaccharides, containing mono- and/or diferulic acid, were accumulated within four more apolar fractions. All fractions contained high amounts of acetyl substituents. The data show that complex xylan oligomers are present in which ferulic acid, diferulates, acetic acid, galactose, arabinose, and uronic acids were combined within an oligomer. Hypothetical structures are discussed, demonstrating which enzyme activities are lacking to fully degrade corn glucuronoarabinoxylans.

Phenilpropanoate identification in young wheat plants by liquid chromatography/tandem mass spectrometry: monomeric and dimeric compounds

Metabolomic approach has become a very important tool for determining the actual gene expression and protein activity. Metabolism via the shikimate pathway in plant gives rise to a large number of aromatic compounds, including p-hydroxycinnamates, which are mainly associated with their cell walls in complex structures, ester-linked to heteroxilans. In this work, ferulate dehydrodimers and related compounds were extracted from the cell walls of young lyophilized wheat plants with 2 mol/l NaOH for 4 h at 25 °C under N(2). After solid phase extraction, samples were analysed by high-performance liquid chromatography-quadrupole-linear ion trap tandem mass spectrometry with the electrospray source operating in negative mode. Nineteen dehydrodiferulates, 2 hydrate forms, 3 oxo- forms, 2 decarboxilated forms and 1 dihydroxysinapate, were identified by mass spectra interpretation. In addition, five mixed dehydrodimers of ferulic-sinapic, ferulic-coumaric and ferulic-syringic acids were also identified. Reproducibility of peak areas was usually better than 10% but some minor components which may be procedural artefacts, although relatively mild conditions were used.

Identification of ferulate oligomers from corn stover

BACKGROUND

Cross-links between plant cell wall polymers negatively impact forage digestibility. Hydroxycinnamates and their oligomers act as cross-links between polysaccharides and/or polysaccharides and lignin. Higher ferulate oligomers such as dehydrotrimers were identified in cereal grains but not in vegetative organs of grasses. The aim of this study was to characterize ester-linked hydroxycinnamate oligomers from corn stover with special emphasis on ferulate dehydrotrimers.

RESULTS

With the exception of the 4-O-5-dehydrodiferulic acid all known ferulate dehydrodimers, including the recently described 8-8(tetrahydrofuran) dimer, were identified in the alkaline hydrolyzate of corn stover after chromatographic fractionation. Next to dehydrodimers, 18 cyclobutane dimers made up of ferulic acid and/or p-coumaric acid were identified by GC-MS of the dimeric size exclusion chromatography fraction. Ferulate dehydrotrimers were isolated by using multiple chromatographic procedures and identified by UV spectroscopy, MS and NMR. Four trimers were unambiguously identified as 5-5/8-O-4-, 8-O-4/8-O-4-, 8-8(aryltetralin)/8-O-4-, and 8-O-4/8-5-dehydrotriferulic acids, a fifth tentatively as 8-5/5-5-dehydrotriferulic acid.

CONCLUSION

The formation of ferulate dehydrotrimers is not limited to reproductive organs of grasses but also contribute to network formation in the cell walls of vegetative organs. Although radically coupled hydroxycinnamate dimers and oligomers were in the focus of researchers over the last decade, the earlier described cyclobutane dimers significantly contribute to cell wall cross-linking.

Separation and detection of cell wall-bound ferulic acid dehydrodimers and dehydrotrimers in cereals and other plant materials by reversed phase high-performance liquid chromatography with ultraviolet detection

Ferulate dehydrodimers and the more recently discovered dehydrotrimers play an important role in the cell wall architecture of plant-based foods and forages. High-performance liquid chromatography methods to determine ferulate dimers often lack specificity; methods for trimers did not exist yet. A method for the determination of 11 cell wall-bound ferulate dehydrodimers and -trimers was developed, including the crucial separation of the di/trimers from the often dominating phenolic monomers. Validation parameters for the basic calibration of the dimers and trimers met our acceptance criteria. However, the matrix calibration revealed that lignin-rich matrices lead to problems with precision and accuracy that likely can be addressed by using a more specific detection, that is, mass spectrometric detection, next to improved sample preparation procedures. The method was used to analyze low-lignin fibers from corn, wheat, and rye grains, wild rice, asparagus, and sugar beet. With the exception of wild rice, the 5-5/8-O-4-, 8-O-4/8-O-4-, and 8-8(aryltetralin)/8-O-4-dehydrotrimers were detected in all analyzed samples, however, often in amounts below the limit of quantitation.

Targeting expression of a fungal ferulic acid esterase to the apoplast, endoplasmic reticulum or golgi can disrupt feruloylation of the growing cell wall and increase the biodegradability of tall fescue (Festuca arundinacea)

In the cell walls of grasses, ferulic acid is esterified to arabinoxylans and undergoes oxidative reactions to form ferulates dimers, trimers and oligomers. Feruloylation of arabinoxylan is considered important not only because it leads to cross-linked xylans but also because ferulates may act as a nucleating site for the formation of lignin and hence link arabinoxylans to lignin by forming a lignin-ferulate-arabinoxylan complex. Such cross-linking is among the main factors inhibiting the release of fermentable carbohydrates from grasses either for ruminant nutrition or for biofuel production. We have found that significant reductions in the levels of monomeric and dimeric phenolics can be achieved in the growing cell walls during plant development in leaves of Festuca arundinacea by constitutive intracellular targeted expression of Aspergillus niger ferulic acid esterase (FAEA). We propose that this occurred by directly disrupting ester bonds linking phenolics to cell wall polysaccharides by apoplast targeting or by preventing excessive feruloylation of cell wall carbohydrates prior to their incorporation into the cell wall, by targeting to the Golgi membrane system. Plants with lower cell wall ferulate levels, which showed increased digestibility and increased rates of cellulase-mediated release of fermentable sugars, were identified. Targeting FAE to the Golgi was found to be more effective than targeting to the ER, which supports the current theories of the Golgi as the site of feruloylation of arabinoxylans. It is concluded that targeting FAEA expression to the Golgi or apoplast is likely to be an effective strategy for improving wall digestibility in grass species used for fodder or cellulosic ethanol production.

Role of dehydrodiferulates in maize resistance to pests and diseases

Phenolic esters have attracted considerable interest due to the potential they offer for peroxidase catalysed cross-linking of cell wall polysaccharides. Particularly, feruloyl residues undergo radical coupling reactions that result in cross-linking (intra-/intermolecular) between polysaccharides, between polysaccharides and lignin and, between polysaccharides and proteins. This review addresses for the first time different studies in which it is established that cross-linking by dehydrodiferulates contributes to maize's defences to pests and diseases. Dehydrodiferulate cross-links are involved in maize defence mechanisms against insects such as the European, Mediterranean, and tropical corn borers and, storage pest as the maize weevil. In addition, cross-links are also discussed to be involved in genetic resistance of maize to fungus diseases as Gibberella ear and stalk rot. Resistance against insects and fungus attending dehydrodiferulates could go hand in hand. Quantitative trait loci mapping for these cell wall components could be a useful tool for enhancing resistance to pest and diseases in future breeding programs.

Feruloylated arabinoxylans are oxidatively cross-linked by extracellular maize peroxidase but not by horseradish peroxidase

Covalent cross-linking of soluble extracellular arabinoxylans in living maize cultures, which models the cross-linking of wall-bound arabinoxylans, is due to oxidation of feruloyl esters to oligoferuloyl esters and ethers. The oxidizing system responsible could be H2O2/peroxidase, O2/laccase, or reactive oxygen species acting non-enzymically. To distinguish these possibilities, we studied arabinoxylan cross-linking in vivo and in vitro. In living cultures, exogenous, soluble, extracellular, feruloylated [pentosyl-3H]arabinoxylans underwent cross-linking, beginning abruptly 8 d after sub-culture. Cross-linking was suppressed by iodide, an H2O2 scavenger, indicating dependence on endogenous H2O2. However, exogenous H2O2 did not cause precocious cross-linking, despite the constant presence of endogenous peroxidases, suggesting that younger cultures contained natural cross-linking inhibitors. Dialysed culture-filtrates cross-linked [3H]arabinoxylans in vitro only if H2O2 was also added, indicating a peroxidase requirement. This cross-linking was highly ionic-strength-dependent. The peroxidases responsible were heat-labile, although relatively heat-stable peroxidases (assayed on o-dianisidine) were also present. Surprisingly, added horseradish peroxidase, even after heat-denaturation, blocked the arabinoxylan-cross-linking action of maize peroxidases, suggesting that the horseradish protein was a competing substrate for [3H]arabinoxylan coupling. In conclusion, we show for the first time that cross-linking of extracellular arabinoxylan in living maize cultures is an action of apoplastic peroxidases, some of whose unusual properties we report.

Grass lignin acylation: p-coumaroyl transferase activity and cell wall characteristics of C3 and C4 grasses

Grasses are a predominant source of nutritional energy for livestock systems around the world. Grasses with high lignin content have lower energy conversion efficiencies for production of bioenergy either in the form of ethanol or to milk and meat through ruminants. Grass lignins are uniquely acylated with p-coumarates (pCA), resulting from the incorporation of monolignol p-coumarate conjugates into the growing lignin polymer within the cell wall matrix. The required acyl-transferase is a soluble enzyme (p-coumaroyl transferase, pCAT) that utilizes p-coumaroyl-CoenzymeA (pCA-CoA) as the activated donor molecule and sinapyl alcohol as the preferred acceptor molecule. Grasses (C3and C4) were evaluated for cell wall characteristics; pCA, lignin, pCAT activity, and neutral sugar composition. All C3 and C4 grasses had measurable pCAT activity, however the pCAT activities did not follow the same pattern as the pCA incorporation into lignin as expected.

Extracellular cross-linking of maize arabinoxylans by oxidation of feruloyl esters to form oligoferuloyl esters and ether-like bonds

Primary cell walls of grasses and cereals contain arabinoxylans with esterified ferulate side chains, which are proposed to cross-link the polysaccharides during maturation by undergoing oxidative coupling. However, the mechanisms and control of arabinoxylan cross-linking in vivo are unclear. Non-lignifying maize (Zea mays L.) cell cultures were incubated with l-[1-(3)H]arabinose or (E)-[U-(14)C]cinnamate (radiolabelling the pentosyl and feruloyl groups of endogenous arabinoxylans, respectively), or with exogenous feruloyl-[(3)H]arabinoxylans. The cross-linking rate of soluble extracellular arabinoxylans, monitored on Sepharose CL-2B, peaked suddenly and transiently, typically at approximately 9 days after subculture. This peak was not associated with appreciable changes in peroxidase activity, and was probably governed by fluctuations in H(2)O(2) and/or inhibitors. De-esterified arabinoxylans failed to cross-link, supporting a role for the feruloyl ester groups. The cross-links were stable in vivo. Some of them also withstood mild alkaline conditions, indicating that they were not (only) based on ester bonds; however, most were cleaved by 6 m NaOH, which is a property of p-hydroxybenzyl-sugar ether bonds. Cross-linking of [(14)C]feruloyl-arabinoxylans also occurred in vitro, in the presence of endogenous peroxidases plus exogenous H(2)O(2). During cross-linking, the feruloyl groups were oxidized, as shown by ultraviolet spectra and thin-layer chromatography. Esterified diferulates were minor oxidation products; major products were: (i) esterified oligoferulates, released by treatment with mild alkali; and (ii) phenolic components attached to polysaccharides via relatively alkali-stable (ether-like) bonds. Thus, feruloyl esters participate in polysaccharide cross-linking, but mainly by oligomerization rather than by dimerization. We propose that, after the oxidative coupling, strong p-hydroxybenzyl-polysaccharide ether bonds are formed via quinone-methide intermediates.

Ferulate-coniferyl alcohol cross-coupled products formed by radical coupling reactions

Radical coupling reactions between ethyl ferulate (Et-FA), a simple model for feruloyl polysaccharides in planta, and coniferyl alcohol (CA), a monolignol, were studied in order to better understand the polymer cross-coupling interactions among polysaccharides and monolignols or lignin, mediated by ferulate (FA), in plant cell walls. Cross-coupled FA/CA dimers produced in an aqueous buffer (pH 5.0) containing peroxidase/hydrogen peroxide were isolated and characterized by NMR. The total coupling products were characterized by 2D (13)C-(1)H correlation (HSQC) NMR spectroscopy and GC-MS. Results from this study showed that ferulate readily cross-couples with coniferyl alcohol through free radical coupling mechanisms producing a series of cross-coupled FA/CA dimers with beta-O-4-, beta-5-/8-5-, and 8-beta-linkages; the syntheses and isolation of beta-5- and 8-5-cross-coupled dimers are reported here. The transformation from 8-beta-coupled FA/CA hydroxyl esters into lactones through intramolecular transesterification is demonstrated for the first time and mechanisms behind these transformations are discussed. The finding of both beta-5- and 8-5-cross-coupled dimers in this study suggests that analogs of both may be present in plant cell walls. Finally it is suggested that ferulates in plants indeed react with monolignols through free radical mechanisms producing a more diverse array of cross-coupled dimers than previously reported.

Evidence for intra- and extra-protoplasmic feruloylation and cross-linking in wheat seedling roots

The sub-cellular feruloylation and oxidative coupling sites of cell wall polysaccharides were investigated in planta by monitoring the kinetics of appearance of arabinosyl- and feruloyl-radiolabelled polysaccharides in the protoplasmic compartment and their secretion in the wall either in the presence or absence of brefeldin A (BFA). By using root apical segments excised from wheat seedlings (Triticum durum Desf.), incubated with trans-[U-(14)C]cinnamic acid, we demonstrated that [14C]ferulate, likely [14C]diferulate, as well as trimers and larger products of ferulate are incorporated into the protoplasmic polysaccharides very rapidly within 1-3 min of [14C]cinnamate feeding. This agrees with the assumption that (glucurono)arabinoxylans [(G)AX] feruloylation and oxidative coupling occur intracellularly, likely in the Golgi apparatus. Simultaneously, polymer bound radioactive hydroxycinnamic acids appeared to be incorporated into the cell wall of root apical segments as early as 2 min after trans-[U-(14)C]cinnamic acid feeding. On the contrary, starting from L-[1-(14)C]arabinose as tracer, the secretion of the pentose-containing polymers into the wall was between 5 to 10 min. These results indicated that (G)AX feruloylation and oxidative coupling occur both intra-protoplasmically and in muro. The occurrence of in muro feruloylation and oxidative coupling was confirmed by the use of BFA a well known inhibitor of secretion. The drug caused a strong inhibition of the synthesis and secretion into the wall of the 14C-pentosyl-labelled polymers as well as of 14C-feruloyl-polymers. In spite of this, the total amount of 14C-feruloyl-polymers incorporated into the wall was only slightly affected by BFA. This indicates the existence of a mechanism involved into secretion of the activated hydroxycinnamoyl precursors to the wall, alternative to that involved in polysaccharide secretion.

Peroxidase-catalyzed oligomerization of ferulic acid esters

Valuable information about possible types of linkages, reaction mechanisms, and sequences for oxidative coupling of phenolic compounds in planta is available from in vitro model systems. Ferulate oligomers were generated in a system using ethyl ferulate, peroxidase, and hydrogen peroxide under various conditions. A molar ferulate/H2O2 ratio of 1:1, an ethanol level of 30% in an aqueous sodium phosphate buffer (pH 6.0), and a reaction time of 10 min were considered to be ideal to produce maximal proportions of ferulate trimers and tetramers from ethyl ferulate as starting material. The dominant trimer and tetramer were each isolated from the reaction mixture and identified as 8-O-4/8-5(cyclic)-dehydrotriferulic acid triethyl ester and 8-5(cyclic)/4-O-5/8-5(cyclic)-dehydrotetraferulic acid tetraethyl ester. The structure of the 8-O-4/8-5(cyclic)-dehydrotriferulic acid triethyl ester revealed that a third ferulate unit is bound to a preformed 8-O-4-diferulate dimer, a surprising reaction sequence considering the dominance of 8-5-coupled dimers among dehydrodiferulates in H2O2/peroxidase-based model reactions. As 4-O-5-coupling is not favored in the dimerization process of ferulates, the main tetramer isolated in this study is probably formed by 4-O-5-coupling of two preformed 8-5(cyclic)-diferulates, a logical step in analogy with reactions occurring in lignin biosynthesis.

Control of diferulate formation in dicotyledonous and gramineous cell-suspension cultures

Primary cell wall polysaccharides of some plants carry ester-linked feruloyl groups that can be oxidatively dimerised both within the protoplast and after secretion into the apoplast. Apoplastic dimerisation has been postulated to form inter-polysaccharide cross-links, contributing to wall assembly, but this role remains conjectural. By feeding cultured cells with [14C]cinnamate, we monitored the kinetics of polysaccharide-binding and subsequent dimerisation of 14C-labelled feruloyl groups. Cultured maize and spinach cells took up [14C]cinnamate more rapidly than barley, Arabidopsis, Acer, tomato and rose cultures. Maize and spinach cells rapidly formed [14C]feruloyl-polysaccharides and, simultaneously, low-Mr [14C]feruloyl esters. When all free [14C]cinnamate had been consumed, there followed a gradual recruitment of radiolabel from the low-Mr pool into the polysaccharide fraction. A proportion of the [14C]feruloyl-polysaccharides was sloughed into the culture medium, the rest remaining wall-bound. Some of the polysaccharide-bound [14C]feruloyl groups were coupled to form dehydrodiferulates. At least six putative isomers of [14C]dehydrodiferulate were formed both rapidly (thus intra-protoplasmically) and gradually (thus mainly apoplastically). These data do not support the hypothesis that intra-protoplasmic dimerisation yields predominantly one isomer (8-5'-dehydrodiferulate). In maize, apoplastic coupling was much more extensive in 7-day old than in 2-day-old cultures; indeed, in 2-day-old cultures apoplastic coupling could not be evoked even by exogenous H2O2, suggesting strong control of peroxidase action by apoplastic factors. When apoplastic coupling was minimised by exogenous application of peroxidase-blockers (iodide, dithiothreitol and cysteine), a higher proportion of the secreted [14C]feruloyl-polysaccharides was sloughed into the medium. This observation lends support to the hypothesis that feruloyl coupling contributes to wall assembly.

Feruloyl esterases as biotechnological tools: current and future perspectives

Feruloyl esterases represent a diverse group of hydrolases catalyzing the cleavage and formation of ester bonds between plant cell wall polysaccharide and phenolic acid. They are widely distributed in plants and microorganisms. Besides lipases, a considerable number of microbial feruloyl esterases have also been discovered and overexpressed. This review summarizes the latest research on their classification, production, and biophysicochemical properties. Special emphasis is given to the importance of that type of enzyme and their related phenolic ferulic acid compound in biotechnological processes, and industrial and medicinal applications.

Characterization of Arabinoxylan−Dehydrogenation Polymer (Synthetic Lignin Polymer) Nanoparticles

Coniferyl alcohol (G monomer) and a mixture of coniferyl alcohol/sinapyl alcohol (GS monomers, 1/1 ratio) were polymerized to dehydrogenation polymers (DHPs) in presence of two structurally related heteroxylans (HX) differing only in their phenolic substitution patterns. One (HX-40) was enriched in ferulate (FA) while the other (HX-90) was almost devoid of FA. The morphology of the resulting nanoparticles was studied by transmission electron microscopy whereas formation of particles was followed by size exclusion chromatography with online multiangle laser light scattering. HX-40-DHP-G- and HX-40-DHP-GS-derived particles display complex morphological patterns whereas HX-90-DHP-G and HX-90-DHP-GS present rather spherical shapes. The determination of particle sizes and molar masses showed that HX-90 samples formed denser particles than HX-40 ones. These differences are discussed in relation to the ferulate substitution level.