Structural Characteristics of Xyloglucan – Congo Red Aggregates as Observed by Small Angle X-ray Scattering

Xyloglucan is a type of hemicellulose with a cellulose backbone containing (1→6)-α-xylose or (1→2)-β-galactoxylose as a side chain. It is soluble in water. Its aqueous solution forms a gel or gel-like precipitate by addition of Congo red. Xyloglucan gel structures with various concentrations of Congo red were observed by small angle X-ray scattering (SAXS) at the nano-level. SAXS results indicated that the xyloglucan chains interacted with Congo red, and that an increase of concentration of Congo red induced a characteristic cross-linking domain, which consisted of a flat structure containing stacked xyloglucan chain assemblies. The Congo red molecules are inserted between the xyloglucan chains.     

Lectins and/or xyloglucans/alginate layers as supports for immobilization of dengue virus particles

Formation of stable thin films of mixed xyloglucan (XG) and alginate (ALG) onto Si/SiO(2) wafers was achieved under pH 11.6, 50mM CaCl(2), and at 70 degrees C. XG-ALG films presented mean thickness of (16+/-2)nm and globules rich surface, as evidenced by means of ellipsometry and atomic force microscopy (AFM), respectively. The adsorption of two glucose/mannose-binding seed (Canavalia ensiformis and Dioclea altissima) lectins, coded here as ConA and DAlt, onto XG-ALG surfaces took place under pH 5. Under this condition both lectins present positive net charge. ConA and DAlt adsorbed irreversibly onto XG-ALG forming homogenous monolayers approximately (4+/-1)nm thick. Lectins adsorption was mainly driven by electrostatic interaction between lectins positively charged residues and carboxylated (negatively charged) ALG groups. Adhesion of four serotypes of dengue virus, DENV (1-4), particles to XG-ALG surfaces were observed by ellipsometry and AFM. The attachment of dengue particles onto XG-ALG films might be mediated by (i) H bonding between E protein (located at virus particle surface) polar residues and hydroxyl groups present on XG-ALG surfaces and (ii) electrostatic interaction between E protein positively charged residues and ALG carboxylic groups. DENV-4 serotype presented the weakest adsorption onto XG-ALG surfaces, indicating that E protein on DENV-4 surface presents net charge (amino acid sequence) different from E proteins of other serotypes. All four DENV particles serotypes adsorbed similarly onto lectin films adsorbed. Nevertheless, the addition of 0.005mol/L of mannose prevented dengue particles from adsorbing onto lectin films. XG-ALG and lectin layers serve as potential materials for the development of diagnostic methods for dengue.     

Ultrasound-assisted extraction of xyloglucan from apple pomace

Ultrasound-assisted extraction was evaluated as a simpler and more effective alternative to conventional extraction method for the isolation of xyloglucan from apple pomace. The apple pomace samples were extracted under indirect sonication in an ultrasound cleaning bath. The ultrasound extraction was compared with the conventional method, on the productions of xyloglucan. It was found that the ultrasound-assisted extraction of xyloglucan was about three times faster than the traditional extraction method. Response surface methodology was employed to study the effect of liquid:solid ratio, KOH concentration and ultrasound-assisted extraction time on the yield of xyloglucan from apple pomace. Regression analysis was performed on the data obtained. The most relevant variable was the quadratic of liquid:solid ratio. The coefficient determination (R²) was good for the second-order model. The optimum combination was determined as a liquid:solid ratio of 34.4:1 (v/w), a KOH concentration of 3.3 M and an ultrasound-assisted extraction time of 2.5 h.     

Variation of xyloglucan substitution pattern affects the sorption on celluloses with different degrees of crystallinity

The sorption of xyloglucan (XG) on cellulose is a basic feature of the supramolecular assembly of plant cell walls. The binding to cellulose of xyloglucan fractions from Rubus fruticosus suspension-cultured cells with different substitution patterns was assayed on celluloses having various degrees of crystallinity between 20 and 95%. The primary structure of XGs differing in their Xyl/Glc ratio affected their binding to cellulose. The less substituted XGs gave the highest binding yields. Selective removal of the terminal fucosyl residues of XGs differentially affected the binding depending on the crystallinity of cellulose. The results showed large variations on the way cellulose crystallinity affects the binding interaction of XGs. Interestingly, one of the highest binding capacities was exhibited by the primary cell wall cellulose isolated from the actual R. fruticosus cells which also had the lowest crystallinity. Differences in binding to primary wall cellulose appeared to be inversely related to the global substitution of the glucan main chain of XGs.     

Rheological and related study of gelation of xyloglucan in the presence of small molecules and other polysaccharides

Interaction between tamarind seed xyloglucan and the other polysaccharides, gellan gum or xanthan investigated by rheology, differential scanning calorimetry, and related methods was discussed. All these three polysaccharides do not form a gel at lower concentrations by itself at the experimental conditions studied but the gelation of xyloglucan occurs in the presence of gellan or xanthan. Gelation of xyloglucan in the presence of a polyphenol, epigallocatechin gallate, is also discussed. Hence the gelation of these mixtures is caused by the synergistic interaction, and the models for the synergistic interaction were discussed. The gelation of polysaccharides by the synergistic interaction is of great value for food and related industries.     

Chemiluminescence accompanying the oxidation of hemicelluloses

On the series of hemicelluloses such as xyloglucans and xylans the effect of O-CH₃ substitution and initial molar mass on chemiluminescence intensity measured under non-isothermal conditions in oxygen and nitrogen from 40 to 220 °C was examined. It was ascertained that the rate constants of oxidation determined from non-isothermal chemiluminescence experiments describe quite well the relative stability of the respective hemicellulose. Similarly as in the case of cellulose and pullulan increasing molar mass has an adverse effect on the intensity of the light emission.     

Dewetting pattern and stability of thin xyloglucan films adsorbed on silicon and mica

Thin polysaccharide films prepared with xyloglucan (XG), a neutral polysaccharide extracted from the seeds of Guibourtia hymenifolia were prepared by spin-coating and drop deposition under pH3, pH5 and pH12, on silicon and mica substrates. Atomic force microscopy (AFM) images show flat nanoporous matrices with additional grain-like structures on both mica and silicon for pH 3 and pH 5. However, X-ray photoelectron spectroscopy (XPS) and Auger spectra of these adsorbed biopolymers prepared under alkaline condition (pH 12) reveal that Na⁺ ions from the solution interact with the mica substrate surface and with XG forming chemical bonds. Both XPS and Auger results suggest XG depolymerisation during adsorption, caused by an alkaline ß-base catalyzed degradation mechanism, which is consistent with the more basic character of the mica surface under these conditions. Thus, the polysaccharide diffusion is inhibited during dewetting due to the surface bonding. On the other hand, the interaction of Na⁺ in solution with the silicon surface is weaker, favoring its interaction with the polysaccharide, conserving the overall polymer structure of XG and allowing the biopolymer to slip and diffuse during dewetting, forming the final branched fractal structure.     

Solvation of xyloglucan in water/alcohol systems by molecular dynamics simulation

Xyloglucan in water solution turns into a gel with addition of alcohol such as methanol and ethanol. In regard to this phenomenon, we investigated the adhesive property of alcohol to xyloglucan and proposed the mechanism of the gelation by molecular dynamics (MD) simulation of a xyloglucan in water, water/methanol, and water/ethanol solution for 10 ns. The alcohol molecules showed its adhesive property to the xyloglucan and made the swelling-shrinking motion of the xyloglucan slow. Alcohol molecules solvated to the xyloglucan mainly in hydrophobic way so as to fill the void of water hydration shell, resulting in reformation of the hydrogen-bond network of water molecules around the solute. We also found that alcohol molecules have strong tendency to hydrogen-bond on xylose O3 in xyloglucan. According to these results, we proposed the gelation mechanism of xyloglucan in water/alcohol solution.     

Gelation of xyloglucan ID water/alcohol systems

Xyloglucan has a cellulose backbone with branched (16)--xylose or (12)--galactoxylose as a side chain. Its aqueous solution yields a gel by adding alcohol. The gel structure of xyloglucan ID various kinds of mono- or polyhydric alcohol/water systems was studied by small-angle X-ray scattering (SAXS). The gelation behavior ID strongly dependent on the type of alcohol. The SAXS from gel with monohydric alcohols indicated that the xyloglucan chains caused random aggregation, as expressed with a Debye–Bueche type scattering function. The type of alcohol added was correlated with the size of the inhomogeneity, as evaluated by SAXS results. The gelation with polyhydric alcohols resulted ID less association, which occurred as side-by-side association with a few xyloglucan chains, rather than as random aggregation.     

Xyloglucan in cellulose modification

Xyloglucans are the principal polysaccharides coating and crosslinking cellulose microfibrills in the majority of land plants. This review summarizes current knowledge of xyloglucan structures, solution properties, and the mechanism of interaction of xyloglucans with cellulose. This knowledge base forms the platform for new biomimetic methods of cellulose surface modification with applications within the fields of textile manufacture, papermaking, and materials science. Recent advances using the enzyme xyloglucan endo-transglycosylase (XET, EC 2.4.1.207) to introduce varied chemical functionality onto cellulose surfaces are highlighted.