Anna Ebringerová

FT-IR study of plant cell wall model compounds: pectic polysaccharides and hemicelluloses

Pectic polysaccharides and hemicelluloses extracted from plants were studied in highly hydrated films on BaF2 discs. Distinctive absorption band maxima in the mid-infrared region at 1200‐800 cm 21 were shown to be useful for the identification of polysaccharides with different structure and composition. Two series of the hexopyranose and pentopyranose monosaccharides, which are the structural units of the plant cell wall polysaccharides, were also studied by FT-IR spectroscopy in solution (i.e. comparable to the amorphous state of the polymers). Their spectral data showed that the main IR band positions are influenced by the relative position of axial and equatorial (OH) groups on the pyranoid ring. q 2000 Elsevier Science Ltd. All rights reserved.

Xylan and xylan derivatives – biopolymers with valuable properties, 1. Naturally occurring xylans structures, isolation procedures and properties

The availability of xylan-type polysaccharides, representing an immense resource of biopolymers for practical application, is summarized. Xylans constitute 25–35% of the dry biomass of woody tissues of dicots and lignified tissues of monocots and occur up to 50% in some tissues of cereal grains. The most potential sources of xylans include many agricultural crops such as straw, sorghum, sugar cane, corn stalks and cobs, hulls and husks from starch production, as well as forest and pulping waste products from hardwoods, in particular. The structural diversity and complexity of xylans is illustrated and shown to depend on the botanic source. Various extraction procedures suitable for the isolation of xylans from different plant sources are described and compared. It is suggested that certain structural types of xylans like glucuronoxylan, arabinoglucuronoxylan, and arabinoxylan can be prepared from certain plant sources with similar chemical and physical properties. In contrast to structural analyses, the physicochemical properties, including solubility, molecular weight and molecular weight distribution, and rheological properties have been studied only for few xylan types. From the functional properties, the thermophysical and tensioactive properties are described. Finally, the physiological activities of xylans, which represent important dietary fibers as well as the immunological activities of some xylan types, are presented.

Ultrasonic extraction of plant materials––investigation of hemicellulose release from buckwheat hulls

Various one- and two-step extraction procedures with and without a short application of ultrasound at the beginning of the extraction were used to examine the effect of sonication on the extractibility of the hemicellulose components of buckwheat hulls. The polysaccharides recovered from the extracts were characterised by yield as well as composition determined by chemical methods and spectroscopic techniques. They comprised a complex of glucuronoxylan and co-extracted amylose-rich starch in various proportions contaminated with other cell wall components (protein, pectic polysaccharides). The hemicellulose fractions obtained by classical and ultrasound-assisted extraction exhibited significant immunomodulatory activities. The increased yield of ultrasonically extracted hemicelluloses, which have preserved their structural and molecular properties as well as immunological activity, confirmed the importance and great potential of ultrasound-assisted extraction of industrially important polysaccharides from different tissues of plant materials.

Structural and solution properties of corn cob heteroxylans

The main structural features of the l-arabino-(4-O-methyl-d-glucurono)-d-xylan (AGX) from corn cobs were evaluated by means of methylation analysis and 13C-NMR spectroscopy. The heteroxylan consisted of water-soluble and water-insoluble xylan molecules differing in the amount and proportion of non-regularly distributed side chains. Single α-(1 → 2)-linked 4-O-methyl-d-glucopyranosyl uronic acid units and α-(1 → 3)-linked l-arabinofuranose residues, as well as 2-O-β-d-xylopyranosyl-α-l-arabinofuranose moieties constituted the side chains. GPC analysis of the water-soluble fractions coupled with viscometry and light-scattering was used to characterise the molecular weight distribution (MWD) and to establish the Mark-Houwink relationship. The flow properties of aqueous dispersions of the water-soluble and water-insoluble fractions were discussed in relation to their structural properties.

Comparison of classical and ultrasound-assisted extraction of polysaccharides from Salvia officinalis L.

After preparation of medicine tinctures from the herbal plant Salvia officinalis by classical and ultrasound-assisted extraction with aqueous ethanol, the insoluble plant residues were subsequently treated with hot water and dilute alkali to isolate polysaccharide cell wall components. The yields of the hot water extract as well as total extracted polysaccharides were higher in the case of the ultrasound-treated plant in both laboratory and pilot plant experiments. The water-extractable polysaccharide fractions, in all cases, contained glucose, galactose and arabinose as main sugar components, whereas the alkali-extractable fractions were rich in xylans. The fractions also contained considerable amounts of proteins. The water-soluble polysaccharides may contribute to the biological activity of the plant decoction. The results indicate that the addition of a subsequent extraction step during the preparation of the herbal tincture might contribute to a better exploitation of the raw material.

Study of the classical and ultrasound-assisted extraction of the corn cob xylan

Abstract Various two-step extraction procedures with and without application of ultrasound in the first extraction step were used to isolate the xylan component of corn cobs in polymeric form. The obtained immunogenic water-soluble (ws-X) and the biologically inactive water-insoluble (wis-X) xylan fractions were characterised by yield, composition, structural and molecular properties. The yield of ws-X was strongly affected by the alkali concentration and extraction temperature, particularly in the ultrasound-assisted procedures. The results indicate a higher efficiency of the ultrasonic extraction procedures expressed in the shortening of extraction time, and lowering of the alkali concentration and extraction temperature. In contrast to the molecular properties, there were no significant differences between the sugar composition and main structural features of the ws-X fractions obtained by extraction in 5% NaOH with and without application of ultrasound. At similar levels of yield, the biological activity of the ultrasonically extracted ws-X preparations was higher.

Ultrasound-assisted extraction of water-soluble polysaccharides from the roots of valerian (Valeriana officinalis L.).

The insoluble plant residues, obtained after preparation of medicinal tinctures from the roots of valerian (Valeriana officinalis L.) by classical and ultrasound-assisted extraction with aqueous ethanol in a pilot plant, were subsequently treated with hot water to isolate the accessible polysaccharide cell wall components. At almost equal amounts of the hot-water extractable material, the yields of the recovered polysaccharides were lower in the ultrasonical experiment. This is due to the fact that a part of accessible polysaccharides were already solubilised by the aqueous ethanol and recoverable from the medicinal tincture. Therefore, the net yield of extracted polysaccharides was enhanced in the ultrasonical procedure. This fact as well as the sugar composition and structural features of the isolated polysaccharides suggest that ultrasonication have attacked the integrity of cell walls, released and degraded its most accessible polysaccharides (pectic polysaccharides and starch) and increased also the extractibility of its less accessible components--xylan, mannan and glucan. The water-soluble polysaccharide fractions from both the conventional and ultrasonical experiments exhibit significant immunostimulatory activities in mitogenic and comitogenic thymocyte tests.

Quaternized xylans: synthesis and structural characterization

Abstract Quaternization xylans were prepared by treating xylans differing in structural and molecular properties with 3-chloro-2-hydroxypropyltrimethylammonium chloride (CHMAC) in aqueous NaOH under mild reaction conditions. The degree of substitution with quaternary ammonium groups can be monitored by altering the amount of CHMAC at optimum molar ratios of NaOH/CHMAC in the range 1·2-1·6, without changing the structure of the xylan backbones. Hot water pretreatment of the xylans in the alkaline activation step significantly enhances their reactivity. The presence of alkylammonium substituents was revealed by IR and NMR spectroscopy. The applicability of quaternized xylans as a retention aid in papermaking is demonstrated on thermomechanical pulp. Xylans, therefore, represent an important renewable resource for the production of novel cationic biopolymers.

Comparison of conventional and ultrasound-assisted extraction of phenolics-rich heteroxylans from wheat bran

Various two-step extraction procedures with and without a short application of ultrasound in the first step at various reaction conditions (extracting agent, temperature and time), and constant conditions in the second step (5% NaOH, room temperature, 60 min) were used to isolate the hemicellulose component of industrial wheat bran. The polysaccharides recovered from the extracts were characterised by yield, chemical composition and radical scavenging activity. Similar total yields of polysaccharides were achieved by a short ultrasound-assisted extraction (up to 10 min) in the first step using 0.5% or 2% NaOH, when compared to the classical extraction lasting 60 min, what represents a shortening of the process by about 60% and lower consumption of the NaOH. The extracted polysaccharides comprised a complex of phenolics-rich heteroxylans contaminated to various extents with co-extracted amylose-rich starch as well as protein, which can be in part governed by the extraction conditions. The water-soluble fractions from both the classical and ultrasonics experiments were shown to exhibit significant radical scavenging activity in vitro, indicating their potential application as polysaccharide-based antioxidants.

Immunomodulatory activity of acidic xylans in relation to their structural and molecular properties

The structure/function relationship of two acidic heteroxylan types, the arabino-(glucurono)xylan from corn cobs (AGX) and 4-O-methylglucuronoxylans (GXs) from beechwood and three medicinal herbs (Rudbeckia, Altheae, and Mahonia), has been studied. The effect of the molecular mass of AGX, as well as the content and distribution of the 4-O-methylglucuronic acid side chains in GXs on the immunological activity of these xylans was characterized by their biological response in the mitogenic and comitogenic thymocyte in vitro tests. Depolymerization of AGX by ultrasonication resulted in unequivocal decrease of the immunomodulatory activity, whereas already a short treatment by endo-beta-1,4-xylanase brought about a significant increase in its activity when applied in the highest dose. In the case of the GX samples, neither the uronic acid content nor the distribution pattern of the uronic acid side chains was found to be determinant for the expression of their immunomodulatory activity.