Arabinoxylans belong to a group of cell wall polysaccharides named hemicelluloses. In the cell wall, hemicelluloses form the matrix into which cellulose microfibrils are embedded. Arabinoxylans are the dominant hemicelluloses in many monocotyledonous plants including cereals such as wheat, rye, and corn. Although the basic structure of arabinoxylans is comparably simple, their fine structures differ among plant species, organs, tissues, and cell wall layers, demonstrating large diversity. The degree of arabinose substitution, the position of arabinose substitution, the existence of additional side-chains such as 4-O-methyl glucuronic acid, the complexity of arabinose-side chains (oligomeric branches), and the occurrence of various hydroxycinnamoylated side-chains are structural details, which differ much among cell walls of different sources. The physicochemical properties of arabinoxylans are most important for different food processing steps such as dough preparation and bread baking. Depending on their structures, arabinoxylans have different viscosities and, if hydroxycinnamoylated, can be oxidatively cross-linked to form gels. From a nutritional point of view, arabinoxylans are among the quantitatively dominant dietary fiber constituents in large parts of the world, with several suggested health benefits.
Our research on arabinoxylans is focused on the characterization of arabinoxylan fine structure, the development of profiling approaches for arabinoxylans, arabinoxylan modifications, arabinoxylan functionality in food products as well as on arabinoxylans as dietary fiber constituents with health benefits.
Pectins form the middle lamella and are matrix compounds in the primary cell wall where they assemble into a cellulose hemicellulose network. Pectic polysaccharides are among the most complex polymers in nature. Homogalacturonans, rhamnogalacturonans I, and rhamnogalacturonans II are groups, into which pectic polysaccharides are often divided. In addition, other substituted galacturonans such as xylogalacturonans were described in the past. However, an enormous structural variety exists within these groups. This is true for major structural units such as arabinan/(arabino)galactan ratios in rhamnogalacturonans I, but also branching of these polymers and incorporation of less usual monomers into pectic polysaccharides. The molecular assembly of pectic polysaccharides and functions of individual structural units are far from being understood. Pectin structural characteristics affect pectin functions in the plant and may also affect their functionalities in food products and their enzymatic degradability. The impact of homogalacturonan structures on their use of gelling agents in food products is well accepted; however, very little is known about rhamnogalacturonan functionalities in food products and their potential health benefits as part of the dietary fiber complex. Our current research on pectins is focused on the characterization of structural units in pectic polysaccharides and the development of profiling methods to analyze the structural features in pectins from different sources. In addition, we strive to understand how these fines structures affect pectin functionality.
Lignin is a polyphenolic cell wall constituent, which is integrated into the cell wall starting with the deposition of the secondary cell wall. The lignin content and/or structure have been suggested to determine several textural and physiological properties of plant-based foods. For example, lignification of edible tissues of plants is discussed to negatively affect sensory properties of certain vegetables. The interaction of lignified cell walls with specific contaminants such as heterocyclic aromatic amines, however, is considered a positive effect of lignin in plant-based food products. Despite these potential effects of lignins on plant-based food quality and functionality, knowledge about lignins from edible plant organs is rather limited; hardly any structural studies are available. “Quantitative” data are usually gathered from the unspecific “Klason-lignin” methodology. We structurally analyze lignins from a variety of cereal grains, fruits, and vegetables by using mass spectrometric and in-depth NMR approaches to determine the monolignol composition of and linkage types in these lignins. For example, our studies revealed that cereal brans were falsely claimed to be “highly lignified”. Because we questioned the results from the Klason-lignin methodology as applied to food, we have analyzed the composition of Klason-lignins isolated from different dietary fibers and demonstrated that the majority of “Klason-lignin” is made up of non-lignin compounds. In addition, we strive to understand the lignin deposition during ripening and storage of plant based food products and how lignin interacts with (food-borne) contaminants (see also: Interactions of cell wall components with contaminants).
Cell wall cross-links:
The individual cell wall polymers are cross-linked by non-covalent interactions and covalent linkages contributing to the insolubility and strength of the cell wall. Good examples for non-covalent interactions are hydrogen bonding between cellulose microfibrils and matrix hemicelluloses. Also, divalent cations such as calcium can cross-link homogalacturonans via electrostatic interactions between adjacent carboxyl groups, a mechanism which is also used in food products to produce pectin gels in low-sugar products. Our main focus regarding cell-wall cross-links is on covalent linkages formed by oxidative coupling of phenolic cell wall compounds such as hydroxycinnamates. Arabinoxylans from monocotyledonous plants such as cereals and pectins from several plants (especially those which belong to the family of Amaranthaceae) are acylated with ferulic acid and, partially, p-coumaric acid. By oxidative coupling, these ferulates can form oligomers such as dimers, trimers, and tetramers, potentially cross-linking up to four polysaccharide chains. Besides their impact on the cell wall, polysaccharide cross-linking through ferulate oligomers is an important mechanism of gel formation, and both the ferulate (and other hydroxycinnamate) monomers and oligomers were suggested to contribute to the health benefits of cereal grains and cereal grain dietary fiber. Our ongoing research is focused on the structural characterization of hydroxycinnamoylated polysaccharides, the identification and localization of hydroxycinnamate oligomers, their functions in plant physiology, their potential health benefits, and their microbial transformation. We have developed several methodologies to analyze hydroxycinnamate oligomers in the past and strive to optimize our analytical approaches to maximize both information on individual regiosiomers of ferulate (and other hydroxycinnamate) dimers, trimers, and tetramers and sample throughput.
Low molecular weight plant secondary metabolites are involved in the interactions between plants and their environment. Many plant secondary metabolites are involved in the plant´s defense mechanisms against biotic and abiotic stress, they attract pollinators, and various other functions are suggested. However, specific functions of many secondary metabolites in the plant are not known yet. As constituents of plant-based foods, they have many functional and potential physiological effects. Depending on the phytochemicals, they act as colorants in food products, delay lipid oxidation improving shelf-life, interact with enzymes in food products and humans, show anti-inflammatory effects in humans etc. Activity guided fractionation is as a well-known approach that can be applied to identify plant secondary metabolites with specific functions. In the past, we used this concept to identify, for example, amylase and lipase inhibiting phytochemicals. Furthermore, we are interested in the fate of specific phytochemicals during food processing operations and develop methods to identify and quantitate specific phytochemicals, their conjugates, and degradation products.
Interactions of cell wall components with contaminants and other low molecular weight compounds:
Due to their varying structures, cell wall components have very different physicochemical properties, which are also reflected by their interactions with non-cell wall components. Several health benefits of cell wall constituents are suggested to be based on their interactions with specific compounds such as bile acids, toxicants etc. In the past, we were mostly interested in the interactions of cell wall constituents with bile acids and heterocyclic aromatic amines, common food-borne, mutagenic contaminants, which are mostly found in heated animal based products. To study these interactions, we do not only use natural sources of dietary fiber, but also cell walls (“dietary fiber”) from cell suspension cultures to manipulate fiber compositions for mechanistic studies. By using such models, we were able to demonstrate that neither lignin contents nor composition are relevant factors for the interactions between dietary fiber and bile acids, whereas the interactions of dietary fiber and specific heterocylic aromatic amines are strongly related to both the lignin content and, to a lesser degree, the lignin composition. Ongoing research studies are dealing with the interactions of fiber constituents with other contaminants and also non-toxic low molecular weight food constituents.
While we use UV/vis and fluorescence spectroscopy mostly as detection tools for liquid chromatography separations and less often for compound identification, mass spectrometry (MS) and nuclear magnetic resonance (NMR) spectroscopy are used for both identification and quantitation. Our mass spectrometers are coupled to liquid and gas chromatographs allowing for the identification and quantification of volatile and non-volatile compounds. Regarding our liquid chromatography-MS couplings, a linear ion trap mass spectrometer is mostly used for characterization purposes (MSn) of oligosaccharides and other natural products, whereas a triple quadrupole mass spectrometer is used for quantitation purposes, often in the multiple reaction monitoring mode, also applying stable isotope dilution techniques. Our gas chromatographs are coupled to single quadrupole mass analyzers. Different from MS, NMR spectroscopy allows for unambiguous, often independent structural characterization. One- and several two-dimensional (e.g. H,H-COSY, NOESY, TOCSY, HSQC, HSQC-TOCSY, HMBC) (and, if necessary, three-dimensional) NMR-experiments are ideal tools for a detailed structural characterization of both cell wall polymers such as non-starch polysaccharides and lignins and low-molecular weight phytochemicals. In addition, we strive to apply and develop NMR approaches for (semi) quantitative purposes in food chemistry and phytochemistry.
We use a wide range of available chromatography techniques for the separation of our target molecules: reversed-phase (RP) and normal phase (NP) chromatography, hydrophilic interaction chromatography (HILIC), ion chromatography (IC), size exclusion/gel permeation chromatography (SEC/GPC), and gas chromatography (GC). RP-high-performance liquid chromatography (HPLC) using mostly standard stationary phases is used for the separation of most phytochemicals of interest on both analytical and preparative scale. Specific (more polar) phytochemicals may require the application of HILIC. Our routine carbohydrate analysis (for example, monomers of cell wall polysaccharides) is performed by applying high-performance anion exchange chromatography (HPAEC) coupled to a pulsed amperometric detector (PAD). SEC and GPC (low and medium pressure) is often but not exclusively used for the separation of oligosaccharides and polysaccharides as well as hydroxycinnamic acid oligomers etc. Standard applications of our NP-flash chromatography system include the purification of synthetic raw products. GC is most often used after analyte derivatization, for example to analyze partially methylated alditol acetates (PMAAs) as diagnostic end products of the methylation analysis to study glycosidic linkages. In addition, we use more specialized stationary phases such as porous graphitic carbon for selected applications. Our liquid chromatographs are coupled to various detectors such as photodiode array, fluorescence, evaporative light scattering, refractive index, and PAD detectors as well as to mass spectrometers.
Plant cell cultures:
Plant cell cultures are widely used for entirely different purposes such as the large scale production of phytochemicals to be used as, for example, pharmaceuticals, for plant genetic engineering or to study biochemical pathways in the plant. Because cultured plant cells proliferate indefinitely in an in-vitro system and because biological processes can be studied under reproducible and easy to manipulate conditions plant cell cultures are an ideal resource in plant chemistry research. Callus cultures, which represent an amorphous mass of parenchyma cells growing on the surface of solid culture media, grow slowly and are used in our lab to maintain our plant cell lines for long periods of time. Cell suspension cultures, represented by suspensions of rapidly dividing cells in a liquid medium, are initiated of a callus culture and used for our studies. In the past, we have used suspension cell cultures, for example, to modify plant cell walls and to study the conjugation of mycotoxins.
Many natural products are not commercially available as standard compounds for analytical methods or to perform biological and other studies. Independent synthesis of identical standard compounds or of standard compounds with specific structural elements also supports unambiguous structural characterization of phytochemicals or of individual structural elements within polymers. Incorporation of stable labels such as 13C or deuterium into standard compounds provides the basis for stable isotope dilution techniques.