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An analysis is given of the theoretical approach to the quantitative description of proton dissociation curves for weak polyacids. The basic model of the counterion condensation theory has been used, with the modification reported in the preceding study (S. Paoletti, A. Cesaro, C. Arce Samper and J.C. Benegas, Biophys. Chem. 34 (1989) 301). In this paper we demonstrate the effect of relaxing the hypothesis of a rigid conformation on the polyelectrolytic properties of weak polyacids. As an application of the present approach, a description is given of the titration curves for two weak polyacids, poly( dl -glutamic acid) (PDLGA) and poly( l -aspartic acid) (PLAA).

U.v. absorption and circular dichroic spectra of lead(II) complexed by uronates and polyuronates in aqueous 0.05 m NaClO4 are presented for the first time. It is shown that not only the carboxylate spectral bands are substantially modified by the interaction, but also the bands of the cation, shifted to higher wavelengths, become optically active. The sign of some bands reflects the stereochemistry of the sugar unit(s) in the complex. On increasing the ion-to-uronate molar ratio, R, the intensity of some bands, changes in a discontinuous mode, suggesting that two types of complex are formed at low and at high values, respectively, of R. The relevance of this structural information to the model of chain dimerization occurring with other divalent cations is discussed.

A statistical model is presented in which an ionic polymer is taken as a linear arrangement of flexible segments (similar to the spring-bead model). This model is used to represent semi-flexible linear ionic polysaccharides in solution. The distribution of end-to-end distances is taken from Monte Carlo calculations of the amylosic chain conformation and combined with Manning's counterion condensation theory of linear polyelectrolytes. The excess thermodynamic properties are then calculated as a function of the degree of ionization and of a number of physical variables. The results show that the statistical average of thermodynamic functions taken over the conformational states is not equivalent to the thermodynamic function of the average conformation. This has important implications for the correct comparison of theoretical prediction with the experimental results.

On the basis of measurements of enthalpy of dissociation and of dilution, an interamolecular conformational transition induced by pH change is shown for pectic acid in aqueous solution. Additional evidence is given by potentiometic, viscometric, and chiroptical results. The transition from a more rigid, probably H-bonded, structure prevailing at low pH to a more extended one at around neutrality is accompanied by a ΔH value of about 500 cal/equiv and a ΔS value of 1.6 cal/equiv K in water at 25°C. The addition of salts increases the stability of the rigid conformation without changing the general features of the phenomenon. Dilatometric measurements suggest that the transition is accompanied by practically no change in the overall solvation of the polymer chain.

Treatment of non-aqueous spherical beads of calcium alginate, or alginic acid gel, with acetic anhydride in pyridine results in acetylation without degradation. The degree and pattern of substitution were investigated using n.m.r. spectroscopy and degradation with specific enzymes. At low degrees of acetylation, substitution occurred almost exclusively on the mannosyluronic acid residues. At high degrees of acetylation, only a minor proportion of the isolated guluronic acid residues was substituted. The presence of O -acetyl groups inhibited depolymerisation by alginate lyases.

Analytical expressions are obtained to evaluate the electrostatic enthalpy of mixing the univalent-salt form of a polyelectrolyte with a neutral salt of the same valency in aqueous solution, following the theory developed by G. S. Manning, avoiding restrictive assumptions regarding the ionic parameters. An endothermic effect is always predicted for the enthalpy of mixing (corrected for the heat of dilution of the polyelectrolyte), which is function of the linear charge density parameter, ξ, of the ratio of neutral salt to the monomoles of polymer, and of the amount of salt initially present in the system. In this way a quantitative method is suggested to test the conformational parameters of polyelectrolytes and their dependence on ion-induced changes, provided that the main non-conformational effect of added salt on the polymer is mostly electrostatic. Specific effects (as site-binding), if present, should be additionally accounted for. However, in the case of the pH-induced conformational transition of polygalacturonate the electrostatic approach is able to give a self-consistent reference for the excess heat effects therein involved. The ionic polysaccharides which have been chosen for confronting the theoretical predictions with the experimental microcalorimetric data are polygalacturonate, polyguluronate, polymannuronate and alginate.

The acid fading of Auramine O (AuO) at 25° in water and in the presence of five different maleic acid copolymers has been studied. The pseudo-first order kinetic constants show that the polyelectrolytes exert quite distinct effects on the fading of AuO. The data are tentatively interpreted in terms of chemical structure and hydrophobicity of the macroions.

An improved method has been used for rapid determination of the ionic strength (I) and the salinity of Apulian groundwaters. The parameters required are the electrolytic conductivity (EC) and the total solute concentration (CT) of the solutions. A simple empirical relation between I, EC and CT is presented. The relation holds over a wide concentration and composition range. The combined use of CT and of the ratio I C T is shown to be effective in defining the salinity of groundwaters.

Physicochemical properties of some ionic derivatives obtained by selective oxidation of amylose, scleroglucan, and cellulose are presented. The study concerns the energetics of proton dissociation of these glycouronans in aqueous solution, by potentiometry and microcalorimetry. In addition, characterization of the samples has been carried out by u.v.-circular dichroism spectroscopy and by dilatometry. The dependence of the apparent dissociation constant and of the enthalpy of dissociation as a function of the degree of dissociation is discussed and compared with that obtained by a simple model based on the polyelectrolytic theory.

The kinetics of formation of ionic polysaccharide gels was studied in oscillatory flow conditions by using a coaxial cylinder rheometer. Homogeneous gels of alginate and pectate with divalent ions were prepared in situ by proper procedure. The stress response was expanded in a Fourier series and the variations of the components of the fundamental harmonic with time during the gelation tests were examined. From the temperature dependence of the rate of development of the storage modulus the conclusion is reached that the elasticity of gels is essentially energetic in origin.

An extension of the counterion-condensation (CC) theory of linear polyelectrolytes has been developed for the case of a system containing a mixture of counterions of different valency, i and j. The main assumption in the derivation of the model is that the relative amount of the condensed counterions of the type i and j is strongly correlated and it is determined by the overall physical bounds of the system. The results predicted by the model are consistent, in the limiting cases of single species component, with those of the original CC theory. The most striking results are obtained for the cases of low charge density and excess of counterion species: in particular, an apparent positive "binding" cooperativity of divalent ions is revealed for small, increasing additions of M2+ ions to a solution containing a swamping amount of monovalent salt and a polyelectrolyte of low charge density. Apparent "competitive binding" of mono- and divalent ions derives as a bare consequence of the electrostatic interactions. Theoretical calculations of experimentally accessible quantities, namely single-(counter) ion activity coefficients, confirm the surprising predictions at low charge density, which qualitatively agree with the measured quantities.

The thermoreversible gelation of syndiotactic poly(methyl methacrylate) in toluene has been investigated in detail. The two-step mechanism, proposed in an earlier publication, has been confirmed and illustrated in detail through the combination of FTIR investigations and rheological and NMR observations. The basic steps of this gelation mechanism are the equilibrium coil-to-single helix transition (first step) and the helix association into dimers (second steps) and larger multichain association units. The coil-to-helix transition is a true equilibrium process that can be detected separately through kinetic trapping. The comparative analysis of the infrared and rheological data and the temperature dependence of the line widths of the 1 H signals support the validity of this proposition.

Weight-average (Mw) molar mass data were obtained by light scattering on aqueous solutions of a sample of κ-carrageenan with a relative molar mass of about 157 000. They were compared with literature data of several samples, having Mw values from 35 000 to 720 000, using a variety of methods. Temperature and concentration of supporting 1:1 electrolyte were chosen so as to span across the disorder-to-order conformational transition, monitored by the change of optical activity. The present comparative study indicates that the single helix is the fundamental ordered (secondary) conformation of κ-carrageenan in aqueous solution. This conclusion appears to be universally valid on the basis of a very large number of cases, including polymer samples of different average molar mass, in the presence of different counterions and co-ions, and at different temperatures. The single-helical structure is the conformational “building-block” of a higher (“tertiary”) ordered structure that stems from the association of (...

Agar polymers were extracted with and without alkali pre-treatment from the red seaweed Gracilaria dura , growing in the northern Adriatic Sea. Chemical analysis combined with 1 H and 13 C NMR spectroscopy showed that agar polysaccharides characterised by low methoxyl and fairly high pyruvate contents were obtained. Alkali pre-treatment markedly reduced the molecular weight and significantly increased the polydispersity of the extracted polymers. The native and alkali-treated agars exhibited comparable gelling and melting temperatures; however, the alkali-treated sample had higher gel strength than that of the native one. Chemical structure, molecular weight and rheological properties of the extracted galactans are discussed with regard to the algal source.

Homogeneous networks of an ionic polysaccharide (namely pectate) in the presence of divalent ions were prepared in situ by proper procedure and the kinetics of the gel formation was studied by means of a coaxial cylinder rheometer in oscillatory flow conditions. The stress response was expanded in a Fourier series and the variation of the components of the fundamental harmonic with time during the gelation process were examined. A model is proposed for the correlation between kinetic parameters such as the concentration of the species active during the gel formation and the rheological parameters that can be experimentally determined, like G′ and G′′. Considerations are developed concerning the mechanism that governs the gelation process under dynamic conditions.

The process of conformational transition of K-carrageenan in aqueous solution containing cesium chloride was studied by microcalorimetry. The experimental evidence as well as the comparison of data collected with the prediction of Manning's theoretical model, suggests that the transition occurs by a two-step mechanism.

The interaction of some divalent cations (Ca2+, Cu2+, Pb2+, Zn2+ and Cd2+) with two polyuronates (alginate and polygalacturonate) has been studied by thermodynamic and structural methods. From the thermodynamic results, the enthalpy of interaction is claimed to reveal the changes in the conformation and/or association of polymeric chains, while the dilatometric results are related with the solvent release due to ion binding. The degree of cooperativity of the ion-binding process (with Ca and Pb) is evidenced by the calorimetric and dilatometric data and, more quantitatively, by the Scatchard plot of the single-ion activity data (with Cu). Theoretical evaluation of the electrostatic energy change accompanying the process of mixing polyuronates with ions and/or of the conformational energy of the polysaccharide is carried out to discriminate the cases in which the binding process is complicated by other effects (chain aggregation or conformational change). Further structural information is obtained by studying the perturbation on the circular dichroic bands of the chromophore arising from the binding process.