Conformational properties of xanthan derivatives in dilute aqueous solution

The Ca²⁺-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.     

Salt-induced extension and dissociation of a native double-stranded xanthan

This study shows that xanthan molecules at room temperature may assume at least three different conformations in 0.1 m NaCl aqueous solutions in which the local structure is ordered: (1) the native compact double helix, (2) the extended double helix, and (3) the extended single helix. Experiments including viscosity, low-angle light scattering and optical rotation measurements have been carried out with a fully pyruvated and fully acetylated native laboratory sample supplied as fermentation broth. Two major conformation changes of the native double helix which were found irreversible in our experimental conditions can be induced by treatments at low ionic strength. After treatment in 10⁻⁴m NaCl, xanthan is still a double helix in 10⁻¹m NaCl, but the backbone of each strand has been extended. After the sample has been in 10⁻⁵m NaCl, the double helix has been dissociated and a single helix sample is obtained. Thus, the denaturing of xanthan is a two-step process. The first step consists of the extension of the two chains inside the double helix, and the second is a dissociation of the native double strand.     

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

Optical 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 (< 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.     

Optical rotation behavior of xanthan in mixtures of water and cadoxen

Optical rotatory dispersion (ORD) data were obtained for a sample of the sodium salt of xanthan gum dissolved in water-cadoxen mixtures at 25°C. The double-helical dimer of the polysaccharide was previously found to dissociate directly to single chains when w_cad (the weight fraction of cadoxen in the mixed solvent) increases from 0·3 to 0·8. ORD data were also obtained for solutions prepared by diluting xathan solutions at low concentration in cadoxen with water to different w_cad values. From previous work these were taken as those for single dissociated chains in the mixed solvent. These sets of data led to the finding that the specific rotation at 300 nm wavelength does not reflect the dissociation of the xanthan double helix in water-cadoxen mixtures, but the Moffitt parameter does. This parameter gave evidence that single coiling chains in cadoxen become intramolecularly ordered to a conformation similar to that of the individual chains in the double helix when the solution is diluted with water to w_cad below 0·4.     

Sodium and calcium counterion activity in the presence of xanthan polysaccharide

This work concerns the activity coefficient determination for sodium and calcium ions in a semi-dilute solution of xanthan when they are mixed in various ratios, and their correlation with the conformational structures. From these values we show that the conformation can be related to the dissociation of the polymer carboxylic acid sites. Below an apparent dissociation coefficient of about 0.29, the conformation is ordered and above 0.55 the xanthan conformation is entirely disordered. The value of the counterion activity coefficient in pure solution of sodium or calcium xanthan salts agrees fairly well with the theoretical ones derived from Manning's and Lifson-Katchalsky's theories if we make the assumption that the Na form is a stretched coil and the Ca form a single fivefold helix.     

A new bacterial polysaccharide (YAS34). I. Characterization of the conformations and conformational transition

This paper concerns the study of the conformational transition of a new exopolysaccharide (YAS34) using experimental techniques such as optical rotation, conductimetric and microcalorimetric measurements as a function of temperature. The behaviors of this polysaccharide in the acid or sodium salt form are compared; a deacetylated sample is also prepared to demonstrate the role of substituents. For the native structure (never heated), a conformational transition is observed but the deacetylated polysaccharide exhibits no ordered conformation. Multidetection size exclusion chromatography (SEC) analyses and conductimetric experiments allowed to determine the nature of each conformation and the molecular dimensions. From these results, it is suggested that the native conformation is a double helix which by heating over T(m) (temperature corresponding to half conformational transition) dissociates into disordered single chains. In the acid and sodium salt forms, by cooling below T(m), an ordered conformation is restored. This conformation seems to be an intramolecular double helix 'hairpin-like turn' (called renatured conformation). Nevertheless an irreversible denaturation is obtained progressively in the sodium salt form when the time of heating over T(m) increases. The conformation of the deacetylated polysaccharide corresponds to that of a single flexible chain (disordered conformation). The conformational transition for the native conformation was studied also in relation to the polyelectrolytic character of the polysaccharide: stability as a function of salt nature and salt and polymer concentrations was investigated for the polymer initially in the sodium and acid forms.     

Rheology of concentrated isotropic and anisotropic xanthan solutions: 3. Temperature dependence

The oscillatory rheology of one rodlike and one semiflexible xanthan sample has been investigated as a function of temperature in the range of xanthan concentrations where the polymer forms a lyotropic liquid crystalline phase in aqueous NaCl solutions. Readily observed changes in the rheological observables at temperatures corresponding to phase boundaries permit construction of the biphasic chimney region of the temperature-composition phase diagram. The chimney region leans toward larger values of the polymer concentration with increasing temperature, presumably as a consequence of a reduction in the effective axial ratio of the helical polymer with increasing temperature. The results permit construction of plots of the rheological observables as a function of polymer concentration at temperatures T in the range 20 相似文献   

Conformation of xanthan dissolved in aqueous urea and sodium chloride solutions

Quasielastic light-scattering and other physical-chemical techniques have been used to compare the conformation and intermolecular interactions of xanthan in water, aqueous sodium chloride, and urea solutions. The results showed that xanthan dissolved in 4m urea has a disordered conformation after the solution has been maintained for 3 h at 95° and then cooled to room temperature. This conformation is similar to that previously observed only in solutions having low ionic strength at higher temperatures, following disruption of the ordered, low-temperature form. “Anomalous” behavior is seen for xanthan as a function of ionic strength, in that the hydrodynamic radius increases with increase in ionic strength, whereas a decrease is typical for polyelectrolytes. These observations suggest that aggregation of rod-like chains, similar to that seen for other stiff-chain polymers, occurs for xanthan in salt solutions, where the charged groups of the polyelectrolyte are screened by the salt ions. This aggregation may explain some of the high values reported in the literature for the molecular weight.     

Impact of the extrusion process on xanthan gum behaviour

Processing xanthan gum by extrusion and subsequent drying produces a biopolymer showing particulate, rather than molecular behaviour in aqueous solution. This form of xanthan disperses very readily to give a viscosity that is strongly dependent on salt concentration. On heating above the temperature of the order-disorder transition as determined by calorimetry, there is a viscosity transition that is indicative of the irreversible loss of the particulate structure. It is suggested that the extrusion process melts and aligns xanthan macromolecules. On cooling reordering will occur but in the highly concentrated environment in the extruder ( approximately 45% water w/w), inter-molecular association between neighbouring macromolecules cannot proceed to completion due to kinetic trapping. As a consequence a network structure is created maintained by associations involving ordered regions. A xanthan solution can be prepared from this particulate material by dispersing and subsequent heating far more readily than can be achieved with non-processed xanthan.     

A rheological study of the order-disorder conformational transition of xanthan gum

The rheological properties of a moderately concentrated solution of xanthan gum in both the ordered and the disordered state have been studied. Oscillatory shear, steady shear flow, and extensional flow experiments have been performed at different temperatures, covering the order-disorder transition determined by differential scanning calorimetry (DSC). The principle of time/temperature superposition was applied to the xanthan solutions for the different types of flow. Although a master curve covering six decades of frequency could be obtained for the storage modulus over the entire investigated temperature range, less agreement was found for the other modulus. This indicates that the order-disorder transition reflects changes on the molecular scale and slight modification of the physical network structure. To the authors' knowledge, this is the first time that this transition has been observed using these different rheological techniques.     

Mechanism and dynamics of conformational ordering in xanthan polysaccharide

The thermally induced order-disorder transition of xanthan (extracellular bacterial polysaccharide from Xanthomonas campestris) has been investigated by optical rotation, differential scanning calorimetry, stopped-flow reaction kinetics and low-angle laser light scattering, and the results have been analysed in terms of Zimm -Bragg helix-coil transition theory. The reciprocal of the transition midpoint temperature (Tm) varies linearly with the logarithm of cation (K+) the salt dependence of Tm, is in agreement with Manning polyelectrolyte theory the ordered structure. The associated increase in cation binding, calculated from the salt dependence of tm, is in agreement with the Manning polyelectrolyte theory for one of the candidate structures from X-ray diffraction, a 5(1) single helix stabilized by packing of side-chains along the polymer backbone, but not for the alternative double-helix structure that has also been proposed. At each salt concentration, the two fundamental parameters of the Zimm -Bragg theory, s and sigma, were calculated. The equilibrium constant for growth of the ordered structure (s) is derived directly from calorimetric measurement of transition enthalpy (delta Hcal ), and sigma, which quantifies the relative instability of the helix nucleus, is derived from the ratio of delta Hcal to the apparent transition enthalpy (delta Happ ) obtained by van't Hoff analysis of the optical rotation data. The temperature course of conformational ordering calculated theoretically is in good quantitative agreement with experimental results from both optical rotation and scanning calorimetry. The calculated average length of stable, ordered chain-sequences increases with decreasing temperature, but equals or exceeds the total chain length from light scattering only at temperatures more than approximately equal to 70 K below Tm, suggesting that ordered and disordered regions may co-exist within the same xanthan molecule. Consistent with this interpretation, the observed rate of conformational ordering increases sharply under conditions where the starting solution for dynamic measurements is partially ordered, suggesting that ordered sequences within each chain may act as helix nuclei for adjacent disordered regions, so that helix growth, rather than the slower nucleation process, becomes rate limiting.     

Molecular modeling of xanthan: galactomannan interactions

X-ray diffraction patterns from stretched fibers of xanthan, guaran and the complex between the two are indicative of good orientation and reasonable crystallinity. The ordered structures of xanthan and guaran appear to be a 5-fold helix of pitch 47.4Aoand a 2-fold helix of pitch 10.3Ao, respectively. The diffraction pattern of the complex is a hybrid of those of the individual components. Both xanthan and guaran in the complex may adopt cellulose-like helices having a slightly longer pitch of 10.5Aoand form a non-coaxial duplex. Alternately, the complex may adopt a xanthan-like, coaxial, 5-fold, double helix in which one strand is xanthan and the other is guaran. The association of a pair of these hybrid helices can take place by direct cross linking of the car☐ylate groups in the side chains of xanthan by divalent ions. The morphologies of these arrangements have now been visualized by computer modeling.     

Application of conductivity studies and polyelectrolyte theory to the conformation and order-disorder transition of xanthan polysaccharide

The conductivity of xanthan (extracellular polysaccharide from Xanthomonas campestris) in the potassium salt form has been studied over the temperature range 5–80°C spanning the order-disorder conformational transition. In salt-free solution data analysis using Manning's polyelectrolyte-conductivity theory gives a charge spacing, b, of 0.58±0.04 nm for the low temperature ordered form, consistent with a single rather than a double helix (b=0.58 and 0.29 nm respectively). In solutions with 0.01 M added KBr the increase in counterion condensation on conformational ordering is found from conductivity studies to be — ^–1= 0.20 ± 0.02, in good agreement with the value 0.20±0.02 using polyelectrolyte-equilibrium theory for the variation of transition-midpoint temperature with added salt determined from opticalrotation data.     

斜顶菌水溶性多糖的构象研究

根据光散射法与粘度法对斜顶菌(Clitopilus Caespitosus PK)多糖在水中与在二甲亚砜中的分子量所进行的对比测定(3:1)以及由多糖水溶液可与刚果红形成络合物的实验结果,推测此多糖在水中是三股螺旋构象,而在DMSO中是单股螺旋。在H_2O—DMSO混合溶剂体系中,体积分数φH_2O降至0.18后,三股螺旋解体为单股螺旋,这种解离具有可逆性。变性剂如脲、酸、碱与高温等条件均因破坏氢键而影响构象的变化。用X射线粉末衍射,确定多糖具有规整有序并有重复性的空间结构。     

Ordering in aqueous polysaccharide solutions. II. Optical rotation and heat capacity of aqueous solutions of a triple-helical polysaccharide schizophyllan

Deuterium oxide solutions of schizophyllan, a triple-helical polysaccharide, undergoing an order-disorder transition centered at 17 degrees C, were studied by optical rotation (OR) and heat capacity (C(p)) to elucidate the molecular mechanism of the transition and water structure in the solution and frozen states. The ordered structure at low temperature consisted of the side chains and water in the vicinity forming an ordered hydrogen-bonded network surrounding the helix core and was disordered at higher temperature. In the solution state appeared clearly defined transition curves in both the OR and C(p) data. The results for three samples of different molecular weights were analyzed theoretically, treating this transition as a typical linear cooperative transition from the ordered to disordered states and explained quantitatively if the molecular weight polydispersity of the sample was considered. The excess heat capacity C(EX)(p) defined as the C(p) minus the contributions from schizophyllan and D(2)O was estimated. In the frozen state it increased with raising temperature above 150 K until the mixture melted. This was compared with the dielectric increment observed in this temperature range and ascribed to unfreezable water. From the heat capacity and dielectric data, unfreezable water is mobile but more ordered than free water. In the solution state, the excess heat capacity originates from the interactions of D(2)O molecules as bound water and structured water, and so forth. Thus the schizophyllan triple helix molds water into various structures of differing orders in solution and in the solid state.     

Molecular-size determination of xanthan polysaccharide

Hydrodynamic chromatographic separations of xanthan polysaccharide of ultrahigh molecular weight have been obtained by using columns packed with 30-μm, non-porous spheres. From calibration curves of the elution volume versus particle size for spherical, polystyrene latexes, it was found that xanthan is eluted at the same volume as a 0.153-μm diameter sphere. Extremely dilute samples of xanthan (70 p.p.m.) were injected to preclude self-association and aggregate formation. Detection at these low concentrations was accomplished by tagging the xanthan with a fluorescein derivative and using a flow-through fluorometer detector. Flow rates of 1 mL/min yielded run times of ～7 min. Comparison of the accepted molecular conformation of xanthan—a rigid rod-like molecule—with the apparent molecular volume from the spherical-latex calibration indicates that the xanthan molecules are substantially oriented by the flow field in the chromatography column.     

Enzymic hydrolysis of the bacterial polysaccharide xanthan by cellulase

In this paper the mechanism of enzymic hydrolysis by a cellulase on xanthan is investigated. It is demonstrated that in salt free solution there is a random breakdown of the main chain when the polysaccharide is in the unordered conformation. The apparent rate of hydrolysis followed by the decrease of the solution viscosity depends on the quality of the solution. In addition, the rate of hydrolysis may be directly correlated with the degree of local order expressed by its specific rotary power. It is shown that there is no hydrolysis on the ordered helical conformation.     

野油菜黄单胞菌低温分泌黄原胶微细结构特征和化学特性

首次发现２８℃培养至对数期且未分泌黄原胶的野油菜黄单胞菌（Ｘａｎｔｈｏｍｏｎａｓ ｃａｍｐｅｓｔｒｉｓ）移 至４℃低温培养仍合成分泌黄原胶。黄原胶低温分泌最微细的结构单位是微丝,多根微丝进一步组成 纤维、类似双螺旋或纤维束等级结构形式。低温分泌的黄原胶与２８℃常温分泌的黄原胶相比有相同 的化学组成与结构,但分子量、丙酮酸含量低。     

Conformation and cooperative order-disorder transition in aqueous solutions of β-1,3-d-glucan with different degree of branching varied by the Smith degradation

β-1,3-d -glucan with different degrees of branching were obtained by selectively and gradually removing side chains from schizophyllan, a water-soluble triple helical polysaccharide, using the Smith degradation. Size exclusion chromatography combined with a multi-angle light scattering detection was performed in aqueous 0.1 M NaCl. The degree of branching decreased after the Smith degradation, while the molar mass distributions were almost unchanged. The molecular conformation of the Smith-degraded β-1,3-d -glucan was analyzed on the basis of the molar mass dependency of the radius gyration, and found to be comparable to the original triple helix of schizophyllan. Differential scanning calorimetry in deuterium oxide–hexadeuterodimethylsulfoxide mixtures was performed to investigate the effects of the degree of branching on the cooperative order-disorder transition. Removal of side chains affects both the transition temperature and transition enthalpy. The ordered structure is formed by the residual side chains in the triplex unit, so that the linear cooperative system of the triplex is maintained after the Smith degradation.     

Renaturation of triple helical polysaccharide lentinan in water-diluted dimethylsulfoxide solution

Triple helical lentinan, a β-(1→3)-d-glucan from Lentinus edodes, was denatured in dimethylsulfoxide (DMSO) into single random coils. The randomly coiled lentinan/DMSO solutions were diluted with pure water to w_H (the weight fraction of water in the mixed solvent) of 95%, and their intrinsic viscosity [η], weight-average molecular weight M_w, radius of gyration R_g, and hydrodynamic radius R_h were investigated at 25 °C after over 5-day storage. The [η] and M_w values, especially the conformation parameter ρ (≡R_g/R_h), of the renatured lentinan were close to those of the originally extracted one, suggesting that random lentinan chains in DMSO were reassembled into triple helical structures. Moreover, the renatured lentinan in 95% water/5% DMSO solution exhibited a unique behavior of triple helical glucans that shear modulus G′ decreased sharply at temperature from 8.4 °C to 13.3 °C with increasing temperature, which was ascribed to the intramolecular conformation transition from ordered triple helical I to disordered triple helical II. The AFM images gave was suggested intuitively evidence that the renatured lentinan mainly existed as rod-like chains, supporting that formation of triple helical structure. The optimal lentinan concentration for triple helical configuration formation was estimated to be over 0.04%. The time dependence of R_h and UV absorption of the water-diluted lentinan/DMSO solution with an indicator of azo dye of Congo red suggested that renaturation of triple helix was a very rapid process. Moreover, the blue-shift of UV-vis absorption spectra suggested that the dye molecules of Congo red were assembled into supramolecular structure in the hydrophobic cavity of the renatured triple helical lentinan. All the results showed that the triple helical structure formed once the randomly coiled lentinan/DMSO was diluted to the final water content of 95%.