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.     

Physicochemical characterization of konjac glucomannan

Four commercial konjac glucomannan (KGM) samples and a glucomannan derived from yeast were characterized by aqueous gel permeation chromatography coupled with multi angle laser light scattering (GPC-MALLS). Disaggregation of aqueous glucomannan solutions through controlled use of a microwave bomb facilitated reproducible molar mass distribution determination alleviating the need for derivatization of the polymer or the use of aggressive solvents. Further characterization was undertaken by use of capillary viscometry and photon correlation spectroscopy (PCS). The weight average molecular masses (M(w)) determined were in the region of 9.0 +/- 1.0 x 10(5) g mol(-1) for KGM samples and 1.3 +/- 0.4 x 10(5) g mol(-1) for the yeast glucomannan. The values determined for KGM in aqueous solution are in agreement with those reported for KGM in aqueous cadoxen. The degradation of samples observed upon autoclaving has been quantified by GPC-MALLS and intrinsic viscosity determination, allowing comparison with reported Mark-Houwink parameters. Shear flow experiments were undertaken for a range of KGM solutions of concentration 0.05 to 2.0% using a combination of controlled stress and controlled strain rheometers. The concentration dependence of the zero shear specific viscosity was determined by analysis of the data using the Ellis model. The dependence of the zero shear specific viscosity on the coil overlap parameter was defined and interpretation discussed in terms of the Martin and Tuinier equations.     

Stability of xanthan in aqueous sodium chloride at elevated temperature

Aqueous NaCl solutions of dimerized Na xanthan with salt concentrations of 0.005, 0.01 and 0.1 were exposed to 80°C for different time periods t, and their viscosities were determined as a function of t. The measured relative viscosities decreased markedly with t, suggesting that Na xanthan denatured at 80°C undergoes some conformation changes or degradation. The molecular weights of the test samples recovered at different t were estimated by viscometry in cadoxen, a single-coil solvent for xanthan, and were found to decrease monotonically with t. Thus, it was concluded that the observed decreases in relative viscosity are due primarily to degradation of Na xanthan.     

Quantitative analysis of aggregation in dilute solutions of effectively rigid biomacromolecules via the combination of oscillatory flow birefringence and viscoelasticity measurements: example study of aggregation of bovine fibrinogen in aqueous glycerol, and detection of a large aggregate formed on addition of guanidine hydrochloride

Oscillatory flow birefringence (OFB) properties have been measured for dilute solutions of bovine fibrinogen in 65-68% aqueous glycerol with the Miller-Schrag Thin Fluid Layer (TFL) apparatus employing either titanium or stainless steel surfaces in contact with the solutions. The shearing frequency range was 1 to 2500 Hz, the concentrations ranged from 4 to 8 mg/ml, and measurement temperatures were 9.9, 10.0, and 15.8 degrees C. The data showed evidence of significant amounts of aggregation that apparently is caused by the presence of glycerol; contributions from the various aggregates were readily detected since the staggered half-overlap aggregation in this system results in substantial differences in the rotational relaxation times of the various effectively rigid aggregates. The combination of oscillatory flow birefringence and viscoelasticity (VE) data provided sensitive and precise characterization of aggregation in these example systems; all aggregates exhibited the expected positive optical anisotropy. The length of unaggregated fibrinogen in solution was found to be that obtained via electron microscopy. Addition of guanidine hydrochloride to hopefully reduce aggregation did so but also resulted in formation of a very large (2800 to 3500 A), apparently nearly monodisperse, negatively birefringent aggregate, suggesting that this new species might be formed by lateral aggregation.     

Optico-hydrodynamic properties of high molecular weight DNA. II. Effect of aqueous glycerol solvents

The optical and hydrodynamic properties of T2 bacteriophage DNA have been determined by steady-state flow birefringence and viscosity in glycerol–aqueous buffer solvents at 25°C. Flow birefringence and extinction angle data were obtained over a velocity gradient range of 0.1 to 5 sec^?1 and at concentrations from 3 to 55 μg/ml in solvents containing approximately 30, 42, and 48 vol-% glycerol. Large optical backgrounds were observed in the mixed solvent flow birefringence studies which presented special experimental difficulties; these are described and their effect upon the flow birefringence data are discussed. The data on extinction angle provide no evidence for an internal viscosity effect on the stationary-state hydrodynamic properties of high molecular weight DNA over a range of solvent viscosity from 0.9 to 4.6 cP. Both the optical and hydrodynamic properties under present conditions of measurement appear to be self-consistent in terms of the values for these quantities in neutral aqueous buffer solution. Interpretation of the birefringence is complicated by uncertainties inherent in calculating the form anisotropy of DNA in non-aqueous solvents, but the data imply no large changes in helical structure with increasing glycerol concentration. Both intact and slightly degraded DNA samples were investigated, and no significant polydispersity effects were observed under the experimental conditions described.     

Revisiting the conformation of xanthan and the effect of industrially relevant treatments

The structure and conformation of xanthan in aqueous solution following various processing treatments typically encountered in its application were investigated in this study. Treatments such as heating, autoclaving, high pressure homogenisation and irradiation were subjected to the same sample. Parameters such as weight average molecular weight (M(w)), polydispersity index, root mean square radius of gyration, intrinsic viscosity and Huggins constant were used to monitor the effect of these treatments. Additionally, we have quantified the mass recovery of samples examined by gel permeation chromatography and light scattering to properly account for all fractions present in xanthan solutions. Atomic force microscopy (AFM) images together with height measurements confirmed that xanthan conformation is double helical ordered renatured state (pre-heat treated by the manufacturer) in dilute solution conditions and random coil conformation in very dilute solution. The ordered (renatured) conformation is shown to have partially molten double helix, with more flexibility than the perfectly ordered native double helix. Heat treatment for 2h at 85°C reduces the M(w) of xanthan to half its initial value, and mass recovery measurements indicate that it completely overcomes its associative nature. Thermally treated xanthan solution in the dilute region leads to an order-disorder transition, as determined by contour length per unit mass. Similarly, irradiation of xanthan solution results in an order-disorder transition together with the production of single strand low molecular weight molecules. Autoclaving and high pressure homogenisation treatments cause degradation of xanthan. The results from treated xanthan solutions following high pressure homogenisation and irradiation confirm that xanthan does not reassociate. A revised summary of xanthan conformation in solution together with schematic models following the various treatments are proposed.     

Synthesis and Fungicidal Activity of Diaryl 1,1 -Dichloroethyl-phosphonates

Non-Newtonian behavior and dynamic viscoelasticity of a series of aqueous mixed solutions of xanthan and locust bean gum were measured using a rheogoniometer, and the rheological properties were analysed. A gelation occurred in the mixture at the concentration of 0.2% total gums at room temperature. The flow curves of the mixture solutions showed a yield value and approximated to plastic behavior at 50°C. The maximum dynamic modulus was obtained when the mixing ratio of xanthan to locust bean gum was 1:2, while comparable high moduli were also obtained in the mixing ratio of 1: 3 or 1:4. A mixture of deacetylated xanthan and locust bean gum showed the highest dynamic modulus, about two times that of the mixture of native or Na-form xanthan. The dynamic modulus of the mixtures decreased rapidly with increasing temperature. In contrast, the dynamic viscosity was scarcely changed during increasing temperature in the mixing ratio of 2: 1. The dynamic modulus was decreased by addition of urea (4.0 M), NaCl (0.1%) and MgCl₂. We concluded that the intermolecular interaction between xanthan and locust bean gum might occur between the side chains of the former and backbone of the latter, as in a lock-and-key effect.     

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.     

Transient electric birefringence studies of xanthan solutions

Transient electric birefringence studies have been performed on heat denatured xanthan in 4 m urea. The induced birefringence was positive, the Kerr law was obeyed at low field amplitudes and the birefringence saturated at high fields. The orientation mechanism appears to be mainly induced dipolar in character and the magnitude of the induced dipole moment can be explained on the basis of counterion polarization. The molecules behave as independent rods of mean length 0.65 μm with no evidence for ‘hindered rotation’ in moderately concentrated solutions. The molecular rigidity is attributed to extension of the polyanion due to charge charge repulsions or steric hindrance due to the side chains.     

Conformation of the denatured DNA molecule in solutions of different ionic strengths]

The conformation of the denatured DNA molecule of different molecular weights in the solutions of various ionic composition was studied by the methods of viscometry, light scattering and flow birefringence. Formaldehyde purified from metallic ions with the help of ionites was used for fixation of the denatured state of the DNA molecule. It has been shown that theories developed for flexible macromolecules are in a sufficient accordance with hydrodynamical and optical data. The unperturbed dimensions, equilibrium rigidity of the macromolecule in solutions of different ionic strengths, mu, were determined. In the range of mu greater than or equal to 0.005 the length of Kuhn's segment (A) is equal to approximately 40 A and its value increases with an increase of mu. At mu 0.001 A approximately 60 A and mu 0.0005 A approximately 85 divided by 100 A. A relation between intrinsic viscosity and molecular weight of the denatured DNA molecule was established. Data on the flow birefringence in the solutions of the denatured DNA have shown that the sigh of optical anisotrophy of the macromolecule depends on the ionic strength. The observed dependency may be explained only by assuming that ionic strength influences the equilibrium orientation of nitrogen base planes with respect to the main chain of the macromolecule.     

The anomaly of oxygen diffusion in aqueous xanthan solutions

A membrane-covered polarographic oxygen electrode was used to measure oxygen diffusion coefficients in aqueous polyelectrolyte solutions of xanthan gum, sodium alginate, and sodium carboxymethylcellulose (CMC). In sodium alginate solutions, dilute xanthan solutions, and solutions containing more than 0.3 wt % CMC, oxygen diffusion coefficients decrease with increasing polymer concentrations. Interestingly, in dilute CMC solutions and concentrate xanthan solutions containing more than 0.5 wt % xanthan gum, oxygen diffusion coefficients increase with increasing polymer concentrations, and values exceeding that in pure water are generally observed.     

E.s.r. study of the conformational transition of spin-labelled xanthan gum in aqueous solution

The order to disorder transition of xanthan molecules in aqueous solutions has been studied using e.s.r. spectroscopy. Nitroxide spin-label was covalently attached to carboxyl groups on the xanthan side chains. The e.s.r. spectra obtained for aqueous spin-labelled xanthan solutions at varying ionic strengths contained both isotropic and anisotropic components at room temperature. The anisotropic component was attributed to the association of the side chains with the xanthan cellulosic backbone and was found to be present in greater proportions at increasing ionic strength. The spectra gradually changed with rising temperature and the proportion of anisotropic component decreased. This spectral change reflected the disruption of the side chain association with the backbone during the conformational change. Hysteresis effects were observed following sequential heating and cooling cycles suggesting that chain aggregation occurred.     

Electric and flow birefringence properties of myosin in aqueous urea and sodium pyrophosphate.

The electric dipole moment of rabbit skeletal myosin was estimated from the electric and flow birefringence properties. Myosin formed small polydisperse aggregates (0.2-1.1 microM in length) with an apparent electric dipole moment of 5,000-20,000 Debye in aqueous urea or sodium pyrophosphate at low ionic strength. Permanent dipole moment contributed substantially to the apparent dipole moment. An anti-parallel association of myosin was suggested from the dependence of the apparent dipole moment on myosin concentration. Some interactions between myosin and C-protein were detected in 1 M urea by flow birefringence and analytical ultracentrifugation studies. The apparent dipole moment of myosin aggregates was less dependent on myosin concentration in the presence of C-protein.     

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.     

Anti-juvenile Hormone Effects of an Imidazole Compound (KK-42) in Different Larval Instars of Bombyx mori

The non-Newtonian behavior and dynamic viscoelasticity of solutions of the Ca salt of xanthan were measured with a rheogoniometer. The Ca salt of xanthan showed pseudoplastic behavior < 0.1 % but was plastic >0.3%. Compared with native, Na, and K salts of xanthan, the Ca salt had higher dynamic viscoelasticity at high concentrations. The apparent viscosity of Ca salt of xanthan was very large at low temperatures and decreased with increasing temperature. These suggest very strong intermolecular association of the xanthan (Ca salt) molecules, probably due to the formation of ionic force between adjacent charged trisaccharide side-chains via Ca²⁺ on different molecules. Possible structures for quaternary association including intramolecular association of xanthan molecules (Ca and K salts) in aqueous solution were proposed.     

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   

Dynamic light scattering investigation of sodium caseinate and xanthan mixtures

Aqueous solutions of sodium caseinate and xanthan at pH 7 and containing 0.1 M NaCl, and their mixtures were investigated using dynamic light scattering. Sodium caseinate solutions showed a bimodal distribution of relaxation rates; with the aggregate peak distribution predominating. Xanthan solutions showed a single distribution at low concentrations (≤0.06 wt.%) and a bimodal distribution at higher concentrations. The sodium caseinate–xanthan mixture modes were independent of the total biopolymer concentration, and behaved as a superposition of sodium caseinate solution alone and xanthan solution alone. This indicates that there is no interaction between xanthan and sodium caseinate in the range of concentrations considered in this study.     

Genetic engineering of polysaccharide structure: production of variants of xanthan gum in Xanthomonas campestris.

Xanthan gum is an extracellular heteropolysaccharide produced by the bacterium Xanthomonas campestris. Xanthan has wide commercial application as a viscosifier of aqueous solutions. Previously, through genetic engineering, a set of mutants defective in the xanthan biosynthetic pathway has been obtained. Certain mutants were shown to synthesize and polymerize structural variants of the xanthan repeating unit and thus produce "variant xanthans". Initial studies of solution viscosities of these polymers, presented here, indicate that the variants have rheological properties similar to, but not identical with, xanthan. These results indicate that acetylation and pyruvylation can affect the viscometric properties of xanthan. Specifically, the presence of pyruvate increases viscosity, whereas acetate decreases viscosity. In addition, the elimination of sugar residues from xanthan side chains also has a major effect on viscosity. Compared to wild-type xanthan, polymer lacking the terminal mannose (polytetramer) is a poor viscosifier. In contrast, polymer lacking both the terminal mannose and glucuronic acid (polytrimer) is a superior viscosifier, on a weight basis. There is a negative effect of acetylation on the viscosity of polytetramer xanthan, but there is seemingly no effect of acetylation on polytrimer xanthan viscosity. The further study of these materials should provide insight into the relationship between xanthan structure and rheological behavior.     

Electric and hydrodynamic properties of polypeptides in solution. 3. Aggregation of poly(L-glutamic acid) in aqueous methanol solvents studied by electric birefringence

The rise and decay of electric birefringence for poly(L -glutamic acid) (PLGA) in aqueous solvents containing 20 and 10 vol % methanol have been found to be unusual. The decay curves have been analyzed on the assumption that there exist two kinds of particles, namely, one (component I) with a shorter relaxation time exhibiting positive birefringence and the other (component II) with a longer relaxation time exhibiting negative birefringence at low fields. From the field strength dependence of the steadystate birefringence the permanent dipole moment, the anisotropy of electric polarizability, and the saturation value of birefringence have been determined for each component. Furthermore, from the relaxation time the length of component I and the diameter of component II have been computed on the models of cylindrical rod and oblate ellipsoid, respectively. The dipole moment, the anisotropy of electric polarizability, and the relaxation time of component II are much larger than those of component I. Both the anisotropy of electric polarizability and the optical anisotropy factor are positive in sign for component I and negative for component II. It is concluded that component I is the helical PLGA molecule itself and component II is the side-by-side (antiparallel) aggregate composed of many helical PLGA molecules. The optical anisotropy factor of each component has been discussed on the basis of Peterlin-Stuart theory.