Zdenka Hromádková

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.

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.

Structural features of a water-soluble l-arabino-d-xylan from rye bran

A water-soluble L-arabino-D-xylan, obtained from the chlorite holocellulose of de-lipidated, de-starched, and de-pectinated rye bran by ammoniacal extraction, was composed of L-Ara and D-Xyl in the molar ratio 7.8:10 and had Mw 36,500 and Mn 26,950. The backbone of the polysaccharide was shown to comprise (1----4)-linked beta-D-Xylp residues, with approximately 41% unsubstituted, approximately 33% 2- or 3-substituted, and approximately 26% disubstituted. Single alpha-L-Araf groups were attached to the xylan core; only a small proportion was 2-, 3-, or 5-linked.

Structure and mitogenic activities of corn cob heteroxylans

Various structurally different water-soluble (ws) and water-insoluble (wis) heteroxylans have been tested for mitogenic and co-mitogenic activity and shown to differ in their stimulating potency. The ws arabinoglucuronoxylan (ws-AGX) from corn cobs exhibited the highest potency which was comparable to that of the immunomodulator zymosan, whereas the wis-AGX from corn cobs was inactive. Water-soluble derivatives of wis-AGX as well as hydrolytically modified ws-AGX fractions were prepared and tested for their mitogenic and co-mitogenic activity. The results indicate that the disaccharide side chains present only in ws-AGX might be important for expression of the immunostimulatory activity of this xylan.

An overview on the application of ultrasound in extraction, separation and purification of plant polysaccharides

In view of the recent emphasis on non-conventional chemistry, application of ultrasound in isolation of plant polysaccharides represents a viable alternative to traditional extraction processes. This review presents an extensive literature survey of ultrasound-assisted extraction of polysaccharides from different plant materials, particularly herbal plants and secondary agricultural plant sources. Targeted, multistep methods were applied with respect to differences in the types of polysaccharides and their location in plant cell walls. The effectiveness of the methods was evaluated according to yield and properties of the isolated polysaccharides in comparison to classical extraction methods. Substantial shortening of extraction time, reduction of reagent consumption and/or extraction temperature are the most important advantages of the ultrasonic treatment. In combination with sequential extraction steps using different solvents, sonication was shown to be effective in separation and/or purification of polysaccharides. The disadvantages of the sonication treatment, such as degradation and compositional changes of the polysaccharide preparations are discussed as well.

Influence of the drying method on the physical properties and immunomodulatory activity of the particulate (1→3)-β-d-glucan from Saccharomyces cerevisiae

Abstract The particulate (1→3)-β- d -glucan isolated from Saccharomyces cerevisiae cell walls was recovered from the aqueous medium as water-insoluble particles by different drying methods: solvent exchange (GE), lyophilisation (GL), and spray drying (GS). The samples were characterised by optical microscopy, FT-IR spectroscopy, swelling capacity, and rheological behaviour of aqueous dispersions. The immunological activity of the glucan samples was examined using the assay for the mitogenic and comitogenic activities. The drying method affected the microstructure of the glucan particles leading to differences in their physical properties (particle size and shape, swelling capacity, interparticle hydrogen bonding) as well as in the flow and viscoelastic properties. In comparison to GL and GE, the GS particles preserved the ellipsoid shape of yeast cells and exhibited a very low extent of interparticle hydrogen bonds. The rather liquid-like GS dispersion showed a several times lower apparent viscosity than the gel-like GE and GL dispersions. The results suggest that the physical state of the variously dried particulate glucan samples influenced significantly also their immunomodulatory activity which was found to be about twice higher with GS than with the GL and GE samples. The results indicated that for application of the particulate (1→3)-β- d -glucan as immunomodulator/adjuvans in form of aqueous suspension it is important to use spray-dried preparations.

Structural diversity of pectins isolated from the Styrian oil-pumpkin (Cucurbita pepo var. styriaca) fruit

To evaluate the seeded fruit biomass of the Styrian oil-pumpkin in view of its pectin component, a series of acidic polysaccharides were isolated by a six-step sequential extraction using hot water, EDTA, dilute HCl (twice) and dilute and stronger NaOH solutions. Chemical, physicochemical and spectroscopy analyses revealed that the first four fractions comprised partially methyl-esterified and acetylated pectins with varying proportions of rhamnogalacturonan regions ramified with galactose- and arabinose-containing side chains and showed considerable polymolecularity. The alkali-extracted polysaccharides contained lower amounts of pectins with homogalacturonan and arabinose-rich rhamnogalacturonan regions next to hemicelluloses prevailing in the last polysaccharide. Using (1)H-(13)C HSQC and HMBC spectroscopy, the resonances of free and methylesterified galacturonic acid residues in the purified acid-extracted pectin were unambiguously established and various diads formed by both residues identified. The results might serve as a basis for searching technological conditions to produce pectin from the oil-pumpkin fruit biomass.

Structural features of two heteroxylan polysaccharide fractions from wheat bran with anti-complementary and antioxidant activities

Wheat bran is a rich source of bioactive substances ascribed to its arabinoxylan component. Two water-soluble arabinoxylans were sequentially extracted from wheat bran. WB1, released during enzymatic digestion of starch and protein, contained medium-branched arabinoxylan (A/X=0.88) consisting of 3-O-substituted (22%), di-substituted (19.8%) and 58% unsubstituted Xylp residues. It was slightly contaminated with (1→3,1→4)-β-glucan and arabinogalactan, and free of protein. WB2 extracted with 0.5% NaOH contained ∼95% arabinoxylan (A/X=1.09). WB2 and two 5% NaOH-extracted arabinoxylans were rich in protein and phenolic compounds. All radical-scavenging assays indicated a relation with the protein and total phenolics contents. The protein-free WB1 displayed the highest hydroxyl radical scavenging effect indicating the distinct role of phenolic acids. The immunomodulatory activity of WB1 was somewhat lower, whereas, that of WB2 higher in comparison to the immunogenic polysaccharide PMII. The arabinoxylans have the potential as immuno-enhancing and antioxidant additives in functional foods.