Native Xanthan Research Articles

The thermodynamic characteristics of the conformational transitions of native xanthan

Xanthan shows a well-known conformational transition in salt free solution at slightly elevated temperature, upon charging. Recently, a second transition was reported at higher temperature, when potentiometric results were analyzed using a Henderson-Hasselbalch representation of the titration curves. The molar fractions of the xanthan monomers in the different conformational states A, B, and C could then be determined as a function of the degree of proton dissociation, θ. Here it is shown that a simple linear dependence of the standard free enthalpy changes (Δ G°) of the transitions on θ adequately describes the changing molar fractions throughout the titration curves. From a temperature series of the first transition ( A → B), the dependence of the changes of standard enthalpy and entropy on θ Δ H°(θ), and Δ S°(θ) are inferred. Surprisingly, irrespective of the degree of dissociation, the A → B transition is dominated by the change of standard entropy of the transition. Beyond θ ≈ 0.3 the B state is entropy stabilized relative to the A state. The dependence of the thermally induced transitions on the polymer charge is illustrated with viscosity data.

On the conformational transitions of native xanthan

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTOn the conformational transitions of native xanthanLeon Bezemer, Job B. Ubbink, Jos A. de Kooker, Maxim E. Kuil, and Jaap C. LeyteCite this: Macromolecules 1993, 26, 24, 6436–6446Publication Date (Print):November 1, 1993Publication History Published online1 May 2002Published inissue 1 November 1993https://doi.org/10.1021/ma00076a021RIGHTS & PERMISSIONSArticle Views237Altmetric-Citations37LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (1 MB) Get e-Alerts Get e-Alerts

Dilute‐solution dynamic viscoelastic properties of xanthan polysaccharide

The frequency dependences of the storage and loss shear moduli, G’ and G‘, of dilute solutions of highly purified xanthan polysaccharide were measured at 20.0 °C using the Birnboim–Schrag multiple‐lumped resonator. The frequency range was 150–8000 Hz and the concentration range was 0.1–0.4 g/l. Three solvents were used, one of which contained 75% by weight glycerol to increase the solution viscosity. Measurements of oscillatory flow birefringence were also made in one solvent over the frequency range from 1–630 Hz and agreed well with the viscoelastic data in the range of overlap. The intrinsic viscosity in both water and 25%/75% water/glycerol (with 0.085 mol/l sodium chloride) was determined as 5200 ml/g. The frequency dependences of G’ and G‘, extrapolated to infinite dilution, could be fitted to a hybrid model for semiflexible rods which was modified to take into account a moderate degree of molecular weight distribution (Mw/Mn=1.4). From the experimental data the persistence length of the native xanthan was estimated at 500–800 nm and Young’s modulus of the rod, modeled as a homogeneous body, was calculated to be about 2×109 dyn/cm2.

Thermal treatment of semi-dilute aqueous xanthan solutions yields weak gels with properties resembling hyaluronic acid

Semi-dilute (ca 2 g/dl) aqueous xanthan (mean molar mass ca 1 x 10(6) g/mol), when heated in the presence of 0.1 M NaCl to a temperature above the order<-->disorder transition temperature, forms highly viscoelastic solutions when returned to room temperature. The steady shear and dynamic rheological behaviour of these solutions discloses a weak gel structure, the viscosity of which is unusually sensitive to the rate of shear. In shear thinning behaviour these heat and salt treated xanthan solutions mimic the properties of the aqueous hyaluronic acid solutions widely used in viscosurgical techniques. The double stranded model of native xanthan is invoked to interpret the observed behaviour of heat and salt treated semi-dilute aqueous xanthan.

Viscous behaviour of xanthan aqueous solutions from a variant strain of Xanthomonas campestris

Xanthan gum produced by a variant strain of Xanthomonas campestris isolated in our laboratory (México) and subjected to a defined downstream process was characterized by low shear viscometry in the dilute regime. The downstream process included an enzymatic treatment. Chemical analysis of the native xanthan were also performed. Xanthan solutions from a commercial source were characterized for comparison purposes. The chemical results indicated that the native xanthan had a higher acetyl/pyruvic ratio. Nevertheless, both products showed to have similar intrinsic viscosities. A linear functionality in the reciprocal of the square root of the ionic strength was observed for the intrinsic viscosity. In general, the results showed good agreement with the literature in terms of the critical concentration ( C∗), linearity of both the Huggins and Kramer equations, and dependence of the intrinsic viscosity on the ionic strength. In the low ionic strength range, deviations from linearity in the Huggins equation were observed and these were explained by considering the polyelectrolyte nature of xanthan solutions. The results suggest that the native xanthan produced with the new strain is comparable to the xanthan from the commercial source.

Determination of the microstructure of the fermentation polymer xanthan

AbstractAlthough xanthan has long been the subject of wide‐ranging investigations, it has not yet been possible to establish any clear link between the solution properties and the chemical and steric microstructure. This is due to the fact that large variations in the properties may occur between samples of different manufacturers. Hence more than fifty different samples in liquid and powder form were investigated. This involved the determination of pyruvate and acetate content for all the samples by means of enzymatic analysis and Comparison with results from 1H NMR spectroscopy. In addition, the contents of the side groups are for the first time given in mass and molar percentages.Low angle and multi‐angle laser light scattering as well as viscometry were employed for the determination of the steric microstructure. Here it was ascertained that preliminary treatment of the sample has a crucial influence on the results of light scattering. A method of sample preparation was therefore devised which is aimed at avoiding the formation of microgels and preserving the structure of the native. xanthan. In addition, xanthan is used to illustrate a novel procedure which enables very readily interpretable Zimm plots to be obtained from one individual reading over a few minutes. In a comparison of the scattering curve extrapolated to c=0 with calculated model curves for semiflexible rods, the structure of native xanthan was determined to be more rigid than corresponds with the exponents of previously published [η]‐Mw relationships.

Role of conformation and acetylation of xanthan on xanthan-guar interaction

The synergistic effect obtained by mixing xanthan and guar solutions were examined by low shear viscosity measurements in relation to the temperature. Native and deacetylated xanthan samples were used in mixtures in which the total polymer concentrations were 1 g/litre and 0·5 g/litre. Gelation was observed for temperatures lower than 15°C for the native xanthan-guar system (weight ratio 1 1 ) in 10 −2 m NaCl and at 22–24°C for the same system in water; in this last case, it is known that the xanthan is in the disordered conformation. For a mixture of deacetylated xanthan-guar, gelation was observed at a temperature below 26°C in water. The results confirm that there is a stronger interaction between deacetylated xanthan and guar than native xanthan and guar because of enhanced xanthan-guar gum backbone association in the former case.

Synergistic interaction between xanthan and tara-bean gum

The non-Newtonian behaviour and dynamic viscoelasticity of a series of aqueous mixtures of xanthan and tara-bean gum were measured with a rheogoniometer. At a concentration of 0·2% of total gum, gelation did not occur at room temperature, but at a low temperature (0°C). A much stronger interaction was observed with mixtures containing deacetylated, deacylated, or native xanthan than with depyruvated xanthan. The maximum dynamic modulus was obtained when the ratio of xanthan to tara-bean gum was 1:2. The dynamic viscoelastic parameters for mixtures with deacetylated and deacylated xanthan decreased rapidly at temperatures above 25 and 20°C, respectively. It was concluded that the side chains of the tara-bean gum molecule prevent an intermolecular interaction between xanthan and tara-bean gum. The results obtained support the interaction mechanism between xanthan and locust-bean gum previously proposed.

Synergistic Interaction Between Deacylated Xanthan and Galactomannan

ABSTRACT The dynamic modulus and optical rotation of a mixed solution of denatured xanthan (depyruvated and deacylated) and galactomannan (locust-bean gum and guar gum) were measured with a rheogoniometer and a polarimeter. Gelation occurred in a mixture of native xanthan with locust-bean gum at a concentration of 0.2% total gums at room temperature, but not with guar gum. A mixture of deacylated xanthan and locust-bean gum showed the highest dynamic modulus, about three times as strong ast that of a mixture with depyruvated xanthan. The dynamic modulus of a mixture of deacylated xanthan and locust-bean gum stayed at very small value in the presence of CaCl2 (6.8 mM) and urea (4.0 M). Possible binding sites between deacylated xanthan and locust-bean gum molecules are proposed.

Rheology of a xanthan broth at low stresses and strains

The structure of a concentrated (4·7 wt %) xanthan broth was probed by dynamic oscillatory, steady shear, and stress-relaxation tests on denatured, renatured, and native xanthan solutions. The range of the shear rates extended down to 10 −5 s −1 to test for the existence of a yield stress. Though the viscosities at these very low shear rates were extremely high for polymer solutions (10 5 Pa. s), no evidence of a true yield stress was found. Long range order was indicated by a difference in complex and steady shear viscosity, presumably arising from inter-chain hydrogen bonding and liquid crystal formation. Differences in recoverable strain between the various samples arise from dissimilar structures and long range interactions in the solutions.

Long-term storage of xanthan in seawater at elevated temperature: physical dimensions and chemical composition of degradation products

Commercial xanthan and xanthan from Xanthomonas strain 646 produced in the laboratory have been subjected to heat treatment for various periods of time in oxygen depleted, high salinity, aqueous solutions. Both the viscosity and the carbohydrate content decreased with increasing incubation time at a specified temperature. The losses increased with increasing temperature. Data from electron micrographs and dialysable sugar content indicate that random cleavage of the double-stranded xanthan chain is the main mechanism responsible for the decreasing viscosity. Removal of pyruvate and acetate substituents on the side chains was apparently not related to the change in physical dimensions. The mannose/glucose ratio in the non-diffusible fraction decreased with incubation time, apparently not related to change in physical dimensions. Electron micrographs showed that one of the samples appeared as highly aggregated in the native condition. After 1 month at 80°C, we observed that the aggregates had dissolved and that the viscosity had increased fivefold. This suggests that heat treatment can be used to avoid microgels and to obtain higher viscosifying power of the native xanthan.

Electron microscopy of native xanthan and xanthan exposed to low ionic strength

AbstractOptical rotation data indicate that xanthan can exist both in an ordered and a disordered conformation. Using molecular weights obtained from light scattering measurements and contour length distributions obtained from electron micrographs, we find that a native, filtered xanthan exposed to low salinity (&lt; 10−4M NaCl) and subsequently returned to 0.1M NaCl has a highly elongated structure with a mass per unit length of 1950 ± 200 Dalton/nm. Our data thus suggest that the ordered conformation of this xanthan is double stranded. We find that native, filtered xanthan in 0.1M NH4Ac has a nearly similar structure, but exists in part as aggregates of varying shape and size. Electron micrographs of these xanthans in 10−4M NH4Ac (the disordered conformation) display a mixture of species ranging from unaggregated single‐ or perfectly matched double‐stranded species, to double‐stranded chains branching into its two subunits as well as double‐stranded chains with different degrees of mismatching. This study suggests that the perfectly matched antiparallel or parallel double‐stranded chain constitutes the lowest free energy state of the ordered conformation of xanthan in dilute aqueous solution.

Temperature-induced extension and dissociation of native xanthan

Light scattering and viscosity changes accompanying the temperature-induced chirooptically-recorded denaturation of high weight-average molecular weight ( M ̄ w ) native xanthan coming from a fermentation broth are reported. The M ̄ w and \\η\\ changes as a result of heating the initially ordered native xanthan (0·1 m aq. NaCl, T=25°C) at a temperature above the melting temperature T m then cooling to the initial temperature for reaching a reordered form can be interpreted by a two-step extension-dissociation mechanism. The first step, during which the viscosity increases whilst M ̄ w is unchanged, consists of an extension of the native dimeric structure that is preserved after a return to the initial condition of temperature. The second step is rather slow and requires temperatures higher than that adequate for inducing the optically-detected transition. The molecular weight is roughly halved, therefore indicating a complete dissociation of the dimeric form. The results are compared with those from literature data showing no evidence of a complete dissociation and a tentative explanation is given.

Solution properties of xanthan. 2. Dynamic and static light scattering from semidilute solution

Semidilute solutions of native xanthan (product of Kelco), pyruvate-free xanthan, and acetate-free xanthan have been studied by static and dynamic light scattering up to values of X = 20, where X = A&~,.,C/(~/,, - (In Mw/M,,)/8) - c/c*. The osmotic compressibility (M/RwT)(aa/ac) exhibits a significantly weaker increase for these stiff chains than that predicted by the renormalization theory of Ohta and Oono and that found for flexible chains. The concentration dependence of the cooperative diffusion coefficient is again weaker than for polystyrene chains and other flexible polymers, and it is considerably lower than predicted by a recent theory of Oono et al. The time correlation functions can for each concentration be reduced to angular-independent shape functions {(rt,c) if scaling with respect to r = -8 In g,(t)/at at t = 0 is applied. These shape functions develop an increasingly large tail at long delay times. This tail is considerably more pronounced than that predicted by Doi and Edwards for rigid rods in the semidilute regime. This longer tail is attributed to an enhanced entanglement as the result of the remaining partial flexibility of the xanthan chains.

Electric birefringence measurements of native, denatured, and renatured xanthan

Electric birefringence measurements were performed on three xanthan solutions to determine conformational differences between native xanthan, denatured xanthan (heated in 4 M urea to 90°C and cooled), and renatured xanthan (heated and cooled). The birefringence of samples exposed to pulses of 1000 V for 1 ms were analyzed to determine longest relaxation times and steady-state birefringence. The results showed the following order for steady state birefringence, native > renatured > denatured, and longest relaxation time, renatured > native > denatured. The results are interpreted in terms of helix reformation and aggregation upon renaturing.

D-Mannose-specific interaction between xanthan and D-galacto-D-mannan

A gelation occurred in a mixed solution of xanthan and locust-bean gum at room temperature; in contrast, gelation did not occur in a solution of xanthan and guar gum. The maximum dynamic modulus was obtained when the mixing ratio of xanthan and locust-bean gum was 1:2 at 0.2% total gums. A mixture of deacetylated xanthan and locust-bean gum showed the highest dynamic modulus, about twice that of the mixture of native xanthan. We concluded that the intermolecular interaction between xanthan and locustbean gum might occur between the side chains of the former and back-bone of the latter molecules in a lock-and-key arrangement.

Synergistic interaction between xanthan and guar gum

The non-Newtonian behavior and dynamic viscoelasticity of a series of aqueous mixtures of xanthan and guar gum were measured with a rheogoniometer. At a concentration of 0.2% of total gums, gelation did not occur at room temperature but occurred at a low temperature (0°). A much stronger interaction was observed with a mixture of deacetylated xanthan than that with native xanthan. The maximum dynamic modulus was obtained when the ratio of xanthan to guar gum was 2:1. The transition temperatures of dynamic viscoelasticity for mixtures with native and deacetylated xanthan were observed at 25 and 30°, respectively. It was concluded that the side chains of the guar gum molecular prevent an intermolecular interaction with the side chains of the xanthan molecule. An intermolecular interaction between xanthan and guar gum at low temperature might be promoted between the periphery of the side chains of the xanthan molecule and the backbone of the guar gum molecule and dissociation takes place at the transition temperature.

Conformational properties of xanthan derivatives in dilute aqueous solution

The Ca 2+-induced conformational changes of the extracellular microbial polysaccharide xanthan, and of some derivatives, have been investigated by circular dichroism and isothermal microcalorimetric methods. The four polymers studied are native xanthan (NX), acetyl-free xanthan (AFX), pyruvic-free xanthan (PFX) and acetyl-and-pyruvic-free xanthan (APFX), all of about the same molecular weight. Convergent evidence from both techniques indicates that the acetyl group stabilizes the ordered conformation of xanthan, which can be induced by specifically increasing the ionic strength of the dilute aqueous solution by the addition of calcium ions. Pyruvate groups have been shown to have a strong destabilizing effect on the ordered conformation, which is likely to be ascribed to an unfavourable electrostatic contribution. The relative order of stability of the ordered forms was found to be PFX > NX > APFX > AFX; PFX is largely present in the ordered conformation at 25°C even in the calcium-free aqueous solution.

Rheological properties of deacetylated xanthan in aqueous media.

Flow properties of aqueous deacetylated xanthan solutions could be approximated to pseudoplastic behavior at concentrations below 0.1% but to plastic behavior above 0.3%. The flow indices in the power law for the deacetylated xanthan were somewhat different at various concentrations. The apparent viscosity of deacetylated xanthan decreased with increasing temperature at relatively low concentrations from 0.1 to 0.5%, however, it increased with increasing temperature, showed a maximum value at 40°C and decreased gradually at 1.0%. Compared with native xanthan, deacetylated material showed higher dynamic viscoelasticity at high concentrations. The dynamic viscoelasticity of deacetylated xanthan decreased with increasing temperature at various concentrations. The dynamic viscoelasticity of deacetylated xanthan was decreased by addition of urea (4.0 M).This suggests that acetate residues, which are attached to the inner mannose residues of the side chains, contribute to the intramolecular association, and tha...

Gel permeation chromatography of the xanthan gum using a light scattering detector

This work gives the first gel permeation chromatogramms obtained on native xanthan using a refractometric and a light scattering detectors. Results of molecular weights distribution analysis give a polydispersity 1.2 for the native polymer and 1.7 for enzymic partially degraded polymer. The experimental conditions and chromatogramms interpretation are briefly discussed.