E.E. Braudo

Structural studies of the solutions of anionic polysaccharides. IV. Study of pectin solutions by light-scattering☆

Abstract Aqueous solutions of pectins, with a degree of esterification (DE) varying from 0 to 95%, have been studied by light-scattering. A set of samples of similar molecular sizes were prepared by methylation of sodium pectate with diazomethane. The solutions were subjected to ultracentrifugation to ensure the removal of small amounts of gel fraction. The second virial coefficient of pectin was positive and constant for DE values varying between 43 and 95%, but increased by a factor of three when the DE was reduced to zero. This increase is at least partly due to coulombic electrostatic interactions. All of the pectins investigated behaved as semi-rigid-chain polymers. The chain flexibility was a maximum in the DE range 43 to 58%. In pectin solutions with a DE higher than 58% attractive forces exist between pectin macromolecules due to the presence of ester groups. It is expected that the interactions between ester groups contribute to both the chain rigidity and the gel-forming ability of pectins.

Structural characterisation of thermoreversible anionic polysaccharide gels by their elastoviscous properties

Abstract Previously reported results obtained for the elastoviscous properties of some thermoreversible gels formed from anionic polysaccharides (high methoxyl pectin, furcellaran and κ-carrageenan) and also gelatin and maltodextrin are discussed and some conclusions about the structure of the gels are presented. The rate at which the relaxation processes take place in the gel is independent of the polymer concentration suggesting that the gels are structurally inhomogeneous. If the helical conformation of the individual macromolecule is stable the standard enthalpy change on crosslink breakdown is less than 45 kJ mol −1 . A relatively small decrease in standard enthalpy is sufficient for network stability because of the low standard entropy loss on gelation which is typical of semi-rigid chain polymers. If, however, the helical conformation is unstable the gelation process is cooperative and the standard enthalpy change on crosslink breakdown exceeds 200 kJ mol −1 .

Studies of the gel formation of κ-carrageenan above the coil-helix transition temperature range

Abstract The temperatures of gel formation and coil-helix conformational transition of κ-carrageenan have been compared. For a 1·5% solution of the potassium salt of κ-carrageenan the gelation temperature is higher than the coil-helix transition temperature range when the KCl concentration exceeds 3−4 × 10 −2 mole litre −1 . The gelation temperature, T g , and the midpoint temperature of the coil-helix conformational transition, T 1 2 c−h , are linearly increasing functions of the logarithm of the concentration of potassium ions, and δT g δ( lg [ K + ]) > δ(T c− h 1 2 δ( lg [ K + ]) Creep compliance studies on a 1·5% gel of κ-carrageenan in 0·11 m KCl have shown that the standard breakdown enthalpy of gel network crosslinks at temperatures above the coil-helix transition range is about 12 kJ mole −1 . Upon helix formation this value increases 20 times. The relaxation behaviour of the κ-carrageenan gel in the high-temperature region is typical of thermoreversible gels. The network of such gels is generally made up of associates of macromolecules.

The interaction of polyuronides with calcium ions 1 binding isotherms of calcium ions with pectic substances

Abstract The binding isotherms for Ca 2+ with sodium pectate and sodium pectinate (58% esterification) were constructed. To achieve this, a method of determining free Ca 2+ ions using auxiliary ligand, tetramethylmurexide, was employed. It was demonstrated that the Ca 2+ ion binding to the pectate was cooperative exceeding that expected on the basis of purely coulombic interactions. In contrast, the binding of the ions by the pectinate is close to that predicted for coulombic binding.

Formation of thermally reversible maltodextrin gels as revealed by low resolution H-NMR

Abstract Pulsed low resolution proton NMR (20 MHz) was applied to follow the gelation process in different thermally reversible gelling maltodextrin-water systems. Results are expressed in terms of the solidus ratio, which is the ratio of proton systems with high and low relaxation time after the application of a radio frequency pulse. The sol-gel transition process may be followed from the onset until equilibrium is reached after several days. A marked change in the signal occurs on the formation of reversible gels, which depends on the temperature, concentration, time and structural peculiarities. This behaviour is paralleled with the development of the shear modulus as well as ΔH° for melting. A linear relation between the solid-liquid ratio from NMR and X-ray crystallinity suggests that the physical basis of the NMR results is the formation of highly ordered domains as essential constituents of the gel network. Non-gelling solutions of amylopectin as well as of acetylated maltodextrins do not give ‘solid’ signals. Amylose is necessary for initiation and acceleration of the sol-gel transition. However, crystalline amylose in the precipitated, retrograded state gives solid signals too. It is to be concluded that the method applied is suitable for following sol-gel transitions if they are based on a highly ordered (crystalline) structure.

Structural study of the solutions of acidic polysaccharides. II. Study of some thermodynamic properties of the dilute pectin solutions with different degrees of esterification

Abstract Light-scattering and surface tension measurements have been used to study the structure and thermodynamic properties of aqueous-salt solutions of pectins, with the degree of esterification (DE) varying from 5 to 100%. Pectin macromolecules were found in the form of aggregates with M w = 5 × 10 5 to 1·1 × 10 6 D . The second virial coefficient of pectin was negative in these poor solvents and independent of the DE. Pectins exhibited surface activity at the air-water interface, the magnitude of this activity also being independent of the DE.

Correlation between the temperatures of formation/breakdown of the gel network and conformational transitions of agarose macromolecules

Abstract The melting temperature of a 1% agarose gel in a solvent consisting of equal weights of water and dimethylsulphoxide, exceeds the temperature of completion of the helix-coil transition by 25°C. It is concluded that the agarose gel network may be retained even when the polysaccharide is in the disordered conformation.

Structural study of the solutions of acidic polysaccharides. I. Study of the solutions of acidic polysaccharides by measuring the activity coefficients of counterions

Abstract The activity coefficient of sodium ions in the presence of acidic polysaccharides (alginates, pectins and κ-carrageenan) has been measured potentiometrically. The activity determined in limit-dilute solutions normally exceeded the values calculated from the Manning theory. The difference (ΔγNa+) was regarded as a measure of the chain flexibility. The change in ΔγNa+ when a polysaccharide is transferred from water to 8 m urea solution gave information about the contribution of hydrogen bonds to the stabilisation of the polysaccharide conformation in aqueous solutions. The function n is independent of polymer concentration at high concentration. n = 1 − (γ Na + exp ) (γ Na + NaCl ) where γNa+exp is the experimentally determined value of the activity coefficient of the counterions and γNa+NaCl is the activity coefficient of sodium ions in a NaCl solution of the same equivalent concentration as the polymer solution. This suggests that the polysaccharide solution has a microheterogeneous structure. The results of the viscosity measurements suggest that there is agreement between equilibrium thermodynamic and rheological evaluation criteria for the structure of polymer solutions.

Studies on the mechanisms of gelation of kappa-carrageenan and agarose

It has been established that hydrogen bonds control both gelation and helix formation completely in the case of agarose and partially in the case of kappa-carrageenan, the major role belonging in the latter case to the interactions of a polysaccharide with metal ions. Na+ and K+ ions form contact ion pairs with sulphate groups of kappa-carrageenan. It is supposed that an increase in the number of contact ion pairs together with association of macromolecules having unordered conformation, a decrease in the second virial coefficient, and a decrease in the refraction index increment (i.e., an increase in the solvation degree of dissolved particles) is a necessary condition for forming the kappa-carrageenan gel netwórk. A sufficient condition of kappa-carrageenan gelation is the intermolecular coordination binding of ions such as K+ ions, promoting gelation. The coil-to-helix transition of the polysaccharide is controlled by shielding the charge of kappa-carrageenan-sulphate groups. Hydrophobic interactions proved to be unessential for gelation of either agarose or kappa-carrageenan.

Conformational transition and aggregation of ϰ-carrageenan macromolecules in solution

SummaryConsideration has been given to the processes of conformational transition and aggregation in the aqueous solutions of ϰ-carrageenan depending on its concentration, the temperature of the system as a whole and the content of the potassium ions by the methods of polarimetry and spectroturbidimetry. It has been established that one and the same degree of mutarotation can be attained on variation of the concentration of both ϰ-carrageenan and potassium ions. The aggregation of the ϰ-carrageenan macromolecules occurs under the action of the potasssium ions and is independent of temperature.