Order-Disorder conformation change of xanthan in 0.01M aqueous sodium chloride: Dimensional behavior

Kinetics of denaturation and renaturation processes of double-stranded helical polysaccharide, xanthan in aqueous sodium chloride

Circular dichroism (CD) and small-angle X-ray scattering (SAXS) measurements were made for three xanthan samples, a double helical polysaccharide, in 5 or 10 mM aqueous NaCl after rapid temperature change to investigate the kinetics of the conformational change between the ordered and disordered states. After the rapid heating, the CD signal mainly reflecting the carbonyl groups on the side chains quickly changed (<150 s) while the scattering intensity from SAXS around q (magnitude of the scattering vector) = 1 nm^-1 changed more gradually, reflecting the main-chain conformation. The difference between CD and SAXS implies us the intermediate conformation which can be regarded as a loose double helix. The SAXS profile in the rapid cooling process showed that the loose double helical structure was constructed within 150 s, but the CD signal slowly changed with around 2 days to recover the native tight double helix.

Model for the Temperature-Induced Conformational Change in Xanthan Polysaccharide

Xanthan is an extracellular bacterial polysaccharide. It is manufactured commercially by fermentation of Xanthomonas campestris and used extensively in food and other industries to control the viscosity and texture of various products. Its useful properties stem from its occurrence both as a relatively rigid double-helical polymer and as a branched polymer network presumably crosslinked by the same noncovalent interactions that stabilize the double-helical form. Interconversion of these two forms can be achieved through heating and cooling processes. This paper describes a model for this thermally induced transformation under conditions of very dilute aqueous polymer concentration, where the characteristics of double-helical and crosslinked aggregates can be studied experimentally using light scattering. Because xanthan is a regularly repeating copolymer, there is no requirement for specific registration of the two strands of the duplex structure as is required in naturally occurring nucleic acid double helices. Here, we demonstrate the important role of the resulting xanthan structural degeneracy in dictating the characteristics of the temperature-induced conformational transition.

The principle and effect of transfer agent for the removal of PCE during in situ chemical oxidation

Viscosity remedial technology, which uses a water-soluble polymer mixed with remedial fluids, has been introduced in recent years to improve the removal efficacy of perchloroethylene/tetrachloroethylene (PCE) by improving oxidant coverage (i.e. sweep efficiency). Xanthan gum and hydrolysed polyacrylamide (HPAM) are relatively stable with time and temperature and possess salt and oxidation resistance, indicating that they may be good flooding agents (the former is better than the latter in this work). In this work, we quantified the polymer directly improved oxidation of PCE during transport by using a two-dimensional flow tank. Using a low pore volume (≤3.0), the removal rate of the PCE increased with the polymer concentration before stabilizing at approximately 93.00 and 88.30% for xanthan and HPAM, respectively. In this work, over 80% of PCE was removed via less than 3.0 PV of the SDS solution, whereas complete removal (100%) was achieved with less than 3.0 PV of SDS foam. Furthermore, the new experimental discoveries demonstrate that xanthan is better than HPAM and SDS foam is a better remediation agent than the SDS solution for removing PCE. Graphical abstract (Reaction device, A - inlet device (pump 1#), B - 2D tank, C - outflow device (pump 2#), D - data recording and processing device, E - microscopic expression, E (a) - KMnO₄ flushing, E (b) - polymer solution flushing).

Solution conformations of pectin polysaccharides: determination of chain characteristics by small angle neutron scattering, viscometry, and molecular modeling

The solution behavior of pectin polysaccharides has been investigated by small angle neutron scattering (SANS), viscosimetric, and molecular modeling studies. The samples used in the experimental study were obtained from apple and citrus and had degrees of methylation ranging from 28 to 73%, with a rhamnose content lying between 0.6 and 2.2%. Persistence lengths, derived from intrinsic viscosity measurements, ranged from 59 to 126 A, whereas those derived by SANS were between 45 and 75 A. These values correspond to 10-17 monomer units. The modeling simulations were performed for both homogalacturonan itself and homogalacturonan carrying various degrees of rhamnose inserts (rhamnogalacturonan). This required the evaluation of the accessible conformational space for the eight disaccharides that represent the constituent repeating segments of the homogalacturonan and rhamnogalacturonan polysaccharides. For each dimer, complete conformational analysis was accomplished using the flexible residue method of the MM3 molecular mechanics procedure and the results used to access the configurational statistics of representative pectic polysaccharide chains. For homogalacturonan, an extended chain conformation having a persistence length of 135 A (corresponding to 30 monomers) was predicted. The inclusion of varying amounts of rhamnose units (5-25%) in the model in strict alternating sequence with galacturonate residues (equivalent to the rhamnogalacturonan "hairy region" chains) only slightly reduced the calculated persistence length. The extended overall chain conformation remained relatively unchanged as a consequence of the self-cancellation of the kinking effects of successive paired rhamnose units.

Dynamic simulations of the molecular conformations of wild type and mutant xanthan polymers suggest that conformational differences may contribute to observed differences in viscosity

Xanthan gum is an exopolysaccharide secreted by the bacterium Xanthamonas campestris whose ability to make solutions viscous at low concentrations and over a pH and temperature range have generated much interest in both academic and industrial environments. Mutant Xanthamonas strains have been derived that produce xanthan gums with an altered or variant subunit chemical structure and different measured viscosities when compared with the wild type (wt) form of the polymer. Two variant gums were targeted as potentially interesting in this study, these being the nonacetylated tetramer (natet) and the acetylated tetramer (atet), which both lack a side-chain terminal mannose residue and in one case (natet) lacks an acetate group on an internal mannose residue. Solutions of these tetrameric gums possess viscosities higher (natet) and lower (atet) than the wt gum, and therefore we have attempted to determine whether these molecules possess unique conformational preferences when compared with the wt and with each other. In this manner we can initiate an understanding of how a polysaccharide's conformation contributes to its solution properties. The GEGOP software permits a sampling of the static and dynamic equilibrium states of carbohydrate molecules, and this software was employed to calculate equilibrium states of representative oligosaccharides with chemical structures representative of xanthan-like molecules. Energy minimization techniques revealed similar local minima for all three molecules. Some of these minima are comprised of elongate backbone conformations (A type) in which side chains fold onto backbone surfaces. Other minima with A backbones possessed side chains in less intimate backbone contact especially when calculations were performed with a low dielectric constant. This phenomenon was particularly pronounced in the wt molecule where an increased number of negatively charged side-chain residues experience charge repulsion resulting in reduced side-chain-backbone contact. Metropolis Monte Carlo (MMC) dynamic simulations performed with an elevated temperature factor (1000 K) allowed a better qualitative representation of conformational space than 300 K simulations. Employing a nonhierarchical cluster analysis method (population density profile: PDP) coupled with a classification scheme, it was possible to partition resulting MMC data sets into conformational families. This analysis revealed that in simulations performed with different dielectric constant values (10, 25, and infinity) all molecules possessed primarily A-type backbones. Less elongate, more open helical backbone forms (B, C, D, J, and Flat-a) did occur during the simulations but were populated to a lesser extent. In the natet molecule significantly open helical backbones existed (E, F, G, H, and I) that did not occur in the lower viscosity wt and atet molecules. PDP clustering methods and subsequent conformational classification applied to the first residue (mannose) of the side chain permitted a determination of side-chain orientation. Comparison of all three molecules indicated a larger population of side-chain conformational families in less direct backbone contact for the wt molecule than either of the variant molecules (natet/atet) suggesting that the side chains in the wt are more flexible. Thus, a major conformational difference between the high viscosity natet and the lower viscosities of the wt/atet is the increased amount of open helical backbone in the natet. In addition, the significant difference between the higher viscosity wt and the lower viscosity atet is the increase side-chain flexibility in the wt. We hypothesize that conformational differences of this kind could form a partial explanation of the observed differences in viscosity between these xanthan-like polymers.

Physicochemical properties of aqueous xanthan solutions: static light scattering

The secondary structure of xanthan in solutions of relatively low salt concentration and at room temperature has been investigated using static light scattering experiments. Additional evidence has been found for a dimeric structure at 25 degrees C in 0.01 M NaCl. From the experimental z-average mean square (ms) radius of gyration, a value for the persistence length p has been estimated, taking explicitly into account the polydispersity of the three samples used, which has been established by gel permeation chromatography (GPC) measurements. The experimental particle scattering functions of the three samples are consistent with theoretical estimates for polydisperse systems with the same value of p = 65 +/- 10 nm and the molar mass per unit length for a dimeric structure. This secondary structure remains unaffected by the ionic strength in the 0.005-0.01 M range. Partial aggregation seems to occur at higher NaCl concentrations. Light scattering and GPC data show that heating the xanthan 0.01 M NaCl solutions to about 70 degrees C considerably reduces the Mw of the low molar mass sample (2.3 x 10(5) g.mol-1), contrary to what is observed for the high molar mass sample (1.8 x 10(6) g.mol-1). These experimental findings can be accounted for by a partial temperature-induced dissociation of the xanthan dimers according to an all-or-none mechanism.

Physico-chemical properties of aqueous solutions of xanthan: an n.m.r. study

The conformations of xanthan in aqueous solution as a function of temperature have been studied. Measurements of optical activity indicate that the conformational transition, induced by varying the polymer concentration, is analogous to that induced by changes in ionic strength and pH. Within a certain range of concentrations, the low-temperature conformation has a molecular-weight-dependent stability, which shows the usual sigmoidal melting profile with increase in temperature. The 13C-n.m.r. data reflect the increase of the mobility of C-1 and the side-chain carbon atoms in the transition-temperature region. The 23Na relaxation behaviour changes on melting the ordered xanthan conformation. At least two correlation times are needed in order to describe the field-strength dependence of the longitudinal and transverse 23Na relaxation. At 25 degrees, a value of 6.8 ns is obtained for the largest correlation time for the fluctuation of the electric-field gradient. The high-temperature conformation also generates correlation times of the order of ns. From 17O relaxation measurements, a reduction of the mobility of water molecules in the presence of xanthan chains is also observed.

Thermally induced conformational transition of double-stranded xanthan in aqueous salt solutions

The thermally induced conformational transition of double-stranded xanthans (degree of pyruvate substitution, DSp = 0.45) having Mw = 3.1, 5.7, and 20.3 x 10(5) has been studied in aqueous salt solutions by high-sensitivity differential scanning calorimetry (DSC). The double strandedness of these samples in the ordered conformation was ascertained by the value of mass per unit length, ML = 2090 +/- 270 g mol-1 nm-1, which was determined from the contour length obtained by electron microscopic observations and the molecular weight by light scattering measurements. The temperature at half completion of the transition T 1/2 for these samples increased linearly with the logarithm of the cation (Na+, K+) concentration. The plot of 1/T1/2 vs the natural logarithm of cation (Na+) concentration in mM for the sample with Mw = 5.7 x 10(5) (15-SX) yielded the equation 10(3)/T1/2 = 3.45-0.159 ln [Na+]. The specific enthalpy delta hcal for 15-SX, essentially independent of salt concentration above 20 mM, was 8.31 +/- 0.39 J/g (SD, n = 6). No systematic dependence of molecular weight on the transition temperature and the enthalpy was observed. Application of the Manning polyelectrolyte theory to the system using the DSC data suggested that the separation of the double strand of xanthan into two single chains was not completed at the temperature where the endothermic peak was finished. This suggestion is consistent with recent findings by light scattering measurements as a function of temperature. Our DSC study was extended to include four other samples from various sources. It was found that T1/2 and delta hcal depend on the pyruvate contents of the samples. For example, the t1/2 (t1/2/degrees C = T1/2/K - 237.15) values for samples with high pyruvate content (DSp = 0.9) and depyruvated (DSp = 0.14) in 20 mM aqueous NaCl were 48.8 and 85.3 degrees C, respectively. Two other samples showed relatively broad DSC curves having shoulders, which were resolved into two independent components. Thermodynamic parameters for each component were examined as a function of salt concentration, and the results obtained were interpreted in terms of the heterogeneity of the pyruvate content of the samples.

The reliability of wormlike polysaccharide chain dimensions estimated from electron micrographs

Electron micrographs of alginate, xylinan, xanthan, and scleroglucan were prepared by vacuum-drying aqueous glycerol-containing solutions, and then heavy-metal, low-angle rotary replicated. Quantitative methods for excluding streamlining effects and deformation artifacts were developed and applied to the digitized polymer contours prior to analysis of stiffness. The apparent macromolecular dimensionalities were not obtainable on the basis of the change in the scaling coefficient alpha relating the rms end-to-end distance and the contour length, mean value of r2(1/2) approximately L alpha, for chains subject to the excluded volume effect in two and three dimensions. Using a two-dimensional model, the persistence length of these molecules was estimated to be (9 +/- 1) nm (alginate), (25 +/- 4) nm (xylinan), (30 +/- 4) nm (single-stranded xanthan), (68 +/- 7) nm (double-stranded xanthan), and (80 +/- 10) nm (scleroglucan). Monte Carlo calculations for wormlike chains close to an interacting surface or confined to the region between two surfaces showed that (1) strongly adsorbed molecules are essentially two-dimensional and (2) molecules restricted to the space between two surfaces separated by a distance less than 20% of the persistence length are two-dimensional in their directional correlation. The somewhat low estimates of the persistence lengths obtained from the electron micrographs compared with those reported from solution measurements can be accounted for by the adoption of a strictly two-dimensional model in the analysis, whereas the absorbed polymers are most likely intermediate between the two-and three-dimensional cases. The model calculations and the analysis of the electron micrographs suggest that stiffness parameters are obtainable from the electron micrographs when the proper theoretical description are used in the analysis.