Double-stranded helix of xanthan: Rigidity in 0.01M aqueous sodium chloride containing 0.01 N hydrochloric acid

Six samples of Na xanthan in 0.01M aqueous NaCl containing 0.01 N HCl (pH = 2) were studied by light scattering and viscosity. This study was motivated by the finding that the intrinsic viscosity [η] fairly sharply decreased when the pH of the solvent was lowered from about 6 to 2 by adding HCl to 0.01M aqueous NaCl in which Na xanthan dissolves as rigid dimers having a double-helical structure. The data for weight-average molecular weight, radius of gyration, and [η] showed that Na xanthan at pH = 2 remains a dimer behaving as a semiflexible chain. Data analysis in terms of known theories for unperturbed wormlike chains yielded 0.47 ± 0.02, 2.0 ± 0.6, and 68 ± 7 nm for the contour length h per main-chain residue, diameter d, and persistence length q of the dimer, respectively. these h and d values agreed with the pitch per main-chain residue and the diameter of the double helix of Na xanthan in 0.01 or 0.1M aqueous NaCl. However, the q value, which was close to the intrinsic persistence length q₀ ( = q in the absence of electrostatic interaction) of Na xanthan at pH = 2, was much smaller than the q₀ (106 nm) of this helix. We concluded that the xanthan dimer at pH = 2 assumes a double-helical structure, which is geometrically the same as, but is more flexible than, that at neutral pH.     

Thermally induced conformation change of succinoglycan in aqueous sodium chloride

Static and dynamic light scattering, viscosity, and optical rotation measurements have been made at eight different temperatures between 25 and 75 degrees C on two succinoglycan samples (sodium salt) with weight-average molecular weights M(w) of 7.14 x 10(5) and 3.54 x 10(5) (at 25 degrees C) in 0.01 M aqueous NaCl to investigate the thermally induced order-disorder conformation change of the polysaccharide. Additionally, viscometry and polarimetry have been performed for a sodium salt sample (M(w) = 4.55 x 10(5) at 25 degrees C) whose M(w), z-average radius of gyration (z)(1/2), and hydrodynamic radius R(H) in the aqueous salt had been determined previously. As the temperature increases, M(w), (z)(1/2), R(H), and the intrinsic viscosity for every sample sharply decrease around 55 degrees C where the specific rotation at 300 nm sigmoidally increases. In particular, M(w) at 25 degrees C (i.e., in the ordered helical state) is twice as large as that at 75 degrees C (i.e., in the disordered state). These findings substantiate that the ordered structure is composed of two chains and hence is a double helix. Data analysis shows that this helix at 25 degrees C is characterized by an unperturbed wormlike chain with a helix pitch of about 2 nm (per repeating unit) and a persistence length of about 50 nm and that upon heating, it dissociates directly (i.e., in all-or-none fashion) to disordered chains of a similar contour length but with a much smaller persistence length of about 10 nm. The temperature dependence of the light scattering second viral coefficient is discussed in relation to the association of disordered chains in the cooling process.     

Viscosity behaviour and persistence length of sodium xanthan in aqueous sodium chloride

Zero-shear-rate intrinsic viscosities [eta] of sodium xanthan in aqueous NACl at 25 degrees C were determined for five samples ranging in weight- average molecular weight from 2 x 10(5) to 4 x 10(6) at salt concentrations Cs between 0.005 and 1 M, at which the polysaccharide maintains its double-helical structure. The measured [eta] for every sample was almost independent of Cs, in contrast to usual observations on flexible polyelectrolytes. The persistence length q of sodium xanthan was determined as a function of Cs by use of the theory of Yamakawa et al. for [eta] of an unperturbed worm-like cylinder, and from its Cs dependence the intrinsic persistence length q(o) ( = q at infinite ionic strength) was estimated to be 106 nm. This q(o) value was roughly twice as large as that of double-stranded DNA, indicating a high intrinsic rigidity of the xanthan double helix. The electrostatic contribution ( = q - q(o)) to q was only about 10% even at the lowest Cs of 0.005 M. Thus, it was concluded that above Cs = 0.005 M, the double- helical structure of sodium xanthan is hardly stiffened by electrostatic interactions between charged groups.     

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.     

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.     

Cooperative binding of hexadecyltrimethylammonium chloride and sodium dodecyl sulfate to cytochrome c and the resultant change in protein conformation

The binding of hexadecyltrimethylammonium chloride (HTAC) and sodium dodecyl sulfate (SDS) to cytochrome c was determined by potentiometric titration and the corresponding changes in protein conformation by circular dichroism (CD). The binding isotherms were biphasic; about 20 surfactant cations or anions were bound to cytochrome c in the first phase. Another 30 or so HTA+ ions were bound in the second phase, which was below the critical micelle concentration of the surfactant, but the binding of dodecyl sulfate ions in the second phase increased sharply near the critical micelle concentration. The binding of both surfactants was highly cooperative and was endothermic; the data in the first phase fitted the Hill plot. The corresponding change in the secondary structure of cytochrome c was small; the CD spectra in the ultraviolet region showed a moderate increase in the helicity in HTAC solution and some changes in conformation in SDS solution. However, the CD spectra for the Soret band indicated a marked change in the local conformation around the heme.     

Characterization of methylprednisolone esters of hyaluronan in aqueous solution: conformation and aggregation behavior

Methylprednisolone steroid esters of hyaluronan differing in degree of functionalization and molecular weight were investigated in aqueous solution. Conformation and aggregation phenomena were elucidated by means of circular dichroism, viscometry, rheology, and nuclear magnetic resonance, mainly by (1)H pulsed field gradient (PFG) NMR, which allows the determination of the diffusion coefficient of the species under investigation. The functionalization of hyaluronan with the steroid induces a reduction of the molecular volume, as a consequence of intramolecular hydrophobic interactions. For concentrated samples we have observed the coexistence of unimolecular collapsed chains and of aggregates, the latter disappearing upon dilution. The methylprednisolone ester of lower molecular weight hyaluronan has a larger molecular volume than its higher molecular weight analogue, even though still smaller than the underivatized polymer. This effect can be explained with the reduced flexibility of the polymer backbone probably impairing intramolecular interactions.     

Mid-infrared spectroscopic analysis of saccharides in aqueous solutions with sodium chloride

The infrared spectral characteristics of three different types of disaccharides (trehalose, maltose, and sucrose) and four different types of monosaccharides (glucose, mannose, galactose, and fructose) in aqueous solutions with sodium chloride (NaCl) were determined. The infrared spectra were obtained using the FT-IR/ATR method and the absorption intensities respected the interaction between the saccharide and water with NaCl were determined. This study also focused on not only the glycosidic linkage position and the constituent monosaccharides, but also the concentration of the saccharides and NaCl and found that they have a significant influence on the infrared spectroscopic characterization of the disaccharides in an aqueous solution with NaCl. The absorption intensities representing the interaction between a saccharide and water with NaCl were spectroscopically determined. Additionally, the applications of MIR spectroscopy to obtain information about saccharide–NaCl interactions in foods and biosystems were suggested.     

Yield stress components of waxy corn starch–xanthan mixtures: Effect of xanthan concentration and different starches

Yield stress of 6% (w/w) waxy maize (WXM), cross-linked waxy maize (CLWM), and cold water swelling (CWS) starches in xanthan gum dispersions: 0%, 0.35%, 0.50%, 0.70%, and 1.0% was measured with the vane method at an apparent shear rate of 0.05 s⁻¹. The intrinsic viscosity of the xanthan gum was determined to be: 112.3 dL/g in distilled water at 25 °C. Values of the static (σ_0s) and dynamic (σ_0d) yield stress of each dispersion were measured before and after breaking down its structure under continuous shear, respectively. The WXM and CWS starches exhibited synergistic behavior, whereas the CLWM starch showed antagonistic effect with xanthan gum. The difference (σ_0s − σ_0d) was the stress required to break the inter-particle bonding (σ_b). The contributions of the viscous (σ_v) and network (σ_n) components were estimated from an energy balance model. In general, values of σ_b of the starch–xanthan gum dispersions decreased and those of σ_n increased with increase in xanthan gum concentration.     

Chain conformation and rheological behavior of an extracellular heteropolysaccharide Erwinia gum in aqueous solution

The chain conformation of a heteropolysaccharide Erwinia gum (EG) consisting of Glc, Gal, Fuc, and GlcA in aqueous solution was investigated by using viscometry and static and dynamic light scattering. The Huggins constants k′ ranging from 0.31 to 0.35, and the larger second virial coefficient A₂ of the order of 10⁻⁴ and even 10⁻³ mol g⁻² cm³ for EG fractions having different molecular weights in 0.03 M NaCl aqueous solution at 25 °C, suggested that 0.03 M NaCl aqueous solution is a good solvent for EG polysaccharide. Smidsrød’s ‘B-value’ characterizing chain stiffness was estimated to be 0.028-0.045 for EG fractions indicating that the backbone of EG polysaccharide is semi-stiff having similar stiffness to the semi-stiff Alginate and CMC. The hydrodynamic factor ρ (1.69-1.89), Flory-Fox factor Φ, and the product of ρΦ/N_A (0.16-0.22) also confirmed the semi-stiffness of EG polysaccharide chains. Compared with general flexible polymers, the first remarkable shear-thinning and then Newtonian flowing behaviors in steady shear tests for EG polysaccharides were ascribed to the alignment of extended semi-stiff chains on shearing. The dynamic oscillatory shear experiments indicated that addition of certain amount of NaCl effectively prohibited its gelation in pure water even at high concentration and low temperature for long time, suggesting that 0.03 M NaCl aqueous solution of EG has good stability and ability of antigelation, and thus is a promising additive in food field.     

Bovine serum albumin partitioning in an aqueous two-phase system: Effect of pH and sodium chloride concentration

The partitioning of bovine serum albumin (BSA) in a polyethylene glycol 3350 (8% w/w)–dextran 37 500 (6% w/w)–0.05 M phosphate aqueous two-phase was investigated at different pHs, at varying concentrations of sodium chloride at 20°C. The effect of NaCl concentration on the partition coefficient of BSA was studied for the PEG–dx systems with initial pH values of 4.2, 5.0, 7.0, 9.0, and 9.8. The NaCl concentrations in the phase systems with constant pH value were 0.06, 0.1, 0.2, 0.3, and 0.34 M. It was observed that the BSA partition coefficient decreased at concentrations smaller than 0.2 M NaCl and increased at concentrations greater than 0.2 M NaCl for all systems with initial pHs of 4.2, 5.0, 7.0, 9.0, and 9.8. It was also seen that the partition coefficient of BSA decreased as the pH of the aqueous two-phase systems increased at any NaCl salt concentration studied.     

Conformation of amylose–iodine–iodide complex in aqueous solution

The conformational characteristics of the amylose–iodine–iodide complex in aqueous solution, particularly for a rapidly mixed system, were studied by resonance polarized scattering measurements using a He-Ne laser at low concentrations of the complex and by viscosity measurements at high concentrations of the complex. For the scattering measurements, the following results were obtained: the depolarization ratios ρ_u and ρ_v showed a pronounced increase with the degree of saturation of the bound iodine (q) in amylose, depending on KI concentration. At q ? 0.7, the increase in these values appeared to be suppressed. However, the ρ_h value was approximately 1, irrespective of q. Additionally, the dissymmetry Z decreased appreciably with increasing q. The conformational change of the complex with q was characterized by the changes in the contour and persistence lengths of the chain and in the optical anisotropy of the scattering segments, which were obtained from numerical computations based on the polarized scattering equation for a wormlike-chain model with a restriction by the entropy force of the chain. The viscosity of the complex solution decreased with increasing q; above q ? 0.7 it increased strikingly. The conformational change of the complex with q was characterized by the change in exponent α in the Houwink-Mark-Sakurada equation [η] = KM^α. It was concluded that the iodine-saturated complex has the characteristics of a rod, regardless of the complex concentration.     

Fluorescence depolarization studies of conformation changes in pyrene-butyryl–fumarase

Measurements of fluorescence depolarization on fumarase labeled with the dye pyrene-butyryl were used to test for previously reported structural changes in this enzymes. These apparent conformation changes were of interest because they seemed to correlate with variation in catalytic activity provoked by changing temperature or pH, or by the presence of a competitive inhibitor. In the present studies, the bound dye pyrene-butyryl and the enzymes were investigated systematically to ensure that simple interpretation of fluorescence depolarization results would be meaningful. This analysis showed that carefully controlled experimental condition were necessary to eliminate a dye component with a short fluorescence lifetime and that it was essential to allow for small variations of lifetime with temperature. Contrary to the previous report, a constant rotational relaxation time of the magnitude expected for a nearly spherical molecule of fumarase was found. No changes were detectable by fluorescence depolarization in the size or shape of pyrene-butyryl–fumarase under the solution conditions tested that caused variation in enzyme activity.     

Molecular simulation study of structure and dynamical properties of nitrate anion in sodium chloride aqueous solution

The molecular dynamics simulations have been performed for the sodium nitrate solution at various concentrations in the 1.1 mol% sodium chloride aqueous solution, which is a model of the sea water. The structure, the velocity correlation functions and the frequency-dependent diffusion coefficients have been obtained. The share viscosity has also been obtained at various concentrations using the Green–Kubo formula, which agrees well with the experimental data. The complex formation and the microscopic charge neutrality in the solution have been discussed in relation to the ionic conductivity estimated by the Nernst–Einstein relation.     

Different behavior of type I and type I-trimer collagen in neutral sodium chloride solutions

Type I and type I-trimer collagen, isolated from ductal infiltrating carcinoma of the human breast, have been tested for their behavior in neutral NaCl solutions. Evident diversities in their rate of precipitation at different saline concentrations have been found, since type I-trimer collagen precipitates at low NaCl molarity while type I collagen is mostly recovered in 2.6-3.6 M NaCl solutions. The native conformation of homotrimer collagen is proved by its ability to produce segment long-spacing crystallites and native-type fibrils.