A. Cesàro

Copper(II) binding by natural ionic polysaccharides: Part I. Potentiometric and spectroscopic data

Abstract The interaction between copper(II) ions and ionic polysaccharides of natural origin has been studied in 0.05 M NaClO4 aqueous solution at 25°C by means of ion-selective electrode potentiometry, spectrophotometry, and circular dichroism measurements. Many independent variables affect the Cu-polymer interaction: the chemical nature, composition and charge density of the polysaccharide, the copper to polymer ratio, the polymer concentration, and the ionic strength. Moreover, each technique employed seems to show specific features in the binding reaction. A parallel use of different techniques is therefore needed to discriminate between long-range electrostatic and short-range specific modes of interaction.

Transitions and phenomenology of α, α-trehalose polymorphs inter-conversion

The thermal transitions of α,α-trehalose polymorphs have been carefully studied by differential scanning calorimetry (DSC) at different scan rates. Attention has been paid to the correlation of the thermal data with the proper crystalline forms (when present), in order to attribute the observed transitions correctly. In particular, the subtle dehydration process of dihydrate trehalose has been already shown to produce several species, depending on the experimental conditions.Thermodynamic data (i.e., temperatures and enthalpies) for all the known transitions are reported and the results are critically discussed.

Copper(II) binding by natural ionic polysaccharides: Part II. Polarographic data☆

Abstract Differential pulse polarography (d.p.p) was employed to study the interaction of copper(II) ions and carboxylic polysaccharides (pectate, P; algal alginate, AA; bacterial alginate, BA; gul(uronate)-rich oligomers, G; man(nuronate)-rich oligomers, M; alternating man-gul oligomers, MG; and polyacrylate, PAA — the last one for comparison) in 0.05 M NaClO 4 aqueous solution at 25°C. Three main reduction peaks appear in the range of potentials between +0.10 and −0.23 V versus s.c.e. The shapes, the positions of the maxima of the d.p. polarograms, and the areas of the integrated peaks vary with the nature of the polyelectrolyte and with the copper to polymer molar ratio. An attribution of the amount of non-reducible copper ions to a minimum value of bound species is tentatively performed and a comparison with ion selective electrode (i.s.e.) potentiometric results is done. The sequence of copper-binding affinity as determined by d.p.p. is PAA > P > BA ⋍ AA > G > MG > M and it parallels that found potentiometrically using copper ion selective electrodes.

Conformational transition of poly(α-l-glutamic acid): A polyelectrolytic approach

Abstract The charge-induced conformational transition of poly(α- l -glutamic acid) (PLGA) is considered in this paper from the point of view of proton dissociation. Equations for the excess electrostatic Gibbs energy of dissociation (i.e., Δ K a ) are derived as a function of the degree of ionization, α. These analytical equations are used to describe some experimental dissociation curves at different polymer and salt concentrations. The dependence of the calculated Δp K a with respect to the ionic strength for the two conformational states, α-helical and extended coil, respectively, is rather satisfactorily explained. Even more interesting are the predictions which are derived from this approach for the transition point, α tr which is found to be ionic-strength dependent, in full agreement with the experimental results.

Limiting-laws of polyelectrolyte solutions. Ionic distribution in mixed-valency counterions systems. II. A comparison of conductometric data and theoretical predictions

The competitive binding of monovalent and divalent counterions (M+ and M2+, respectively) has been studied by a conductometric procedure as described by De Jong et al. (Biophysical Chemistry 27 (1987) 173) for aqueous solutions of alkali metal polymethacrylates in the presence of Ca (NO3)2 or Mg(NO3)2. The experimentally obtained fractions of conductometrically free counterions are compared with theoretical values computed according to a new thermodynamic model recently developed by Paoletti et al. (Biophysical Chemistry, 41 (1991) 73). For the systems studied, the fractions of free monovalent and divalent counterions can be fairly well described by the theory. In fact, the results support the assumption that under the present conditions the conductometrically obtained distribution parameters (l) and (2) approximate the equilibrium fractions of free monovalent and divalent counterions. For a degree of neutralization of 0.8 and a molar concentration ratio of divalent counterions and charged groups on the polyion up to 0.25, the mean M+/M2+, exchange ratio nu has been found to be 1.39 +/- 0.03 and 1.33 +/- 0.03 for the alkali metal/Ca/PMA and alkali metal/Mg/PMA systems, respectively. These values agree well with the theoretical value, which for this particular case is 1.38.

Iso-ionic and iso-thermal phase transitions in ionic polysaccharides: A calorimetric study.

Abstract 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 Mannings theoretical model, suggests that the transition occurs by a two-step mechanism.

Some aspects of the enthalpy of dilution of biological polyelectrolytes

It is shown that calorimetric experiments on the dilution process of polyelectrolytes, i.e. on the excess enthalpy of electrostatic origin, can give information on the conformational state of the polymer. Analytical expression are obtained relating the enthalpy of dilution to the charge density on the polymer, from Mannings theory. The experimental data reported on some ionic biopolymers are interpreted on the basis of the theoretical prediction of the electrostatic contribution to the enthalpy of dilution in water of a polyelecrolytic salt and of a weak polyacid.The procedure outlined provides a means of calculating the actual charge/per unit length and of disclosing possible non-electrostatic effects arising from conformational transitions.ZusammenfassungEs wurde gezeigt, daß kalorimetrische Versuche an Lösungsprozessen von Polyelektrolyten, d.h. die Überschußenergie elektrostatischen Ursprunges, Informationen über den Konformationszustand des Polymers liefern können. Ausgehend von der Theorie von Manning wurden analytische Ausdrücke entwickelt, welche die Lösungsenthalpie mit der Ladungsdichte am Polymer in Verbindung bringen. Die experimentellen Daten für einige ionische Biopolymere wurden auf der Grundlage der theoretischen Voraussage des elektrostatischen Beitrages zur Lösungsenthalpie eines polyelektrolytischem Salz und einer schwachen Polysäure in Wasser interpretiert.

On the thermodynamics of proton dissociation in weak biological polyacids : an analysis of theoretical and experimental results

A theoretical description of the proton dissociation process of weak polyacids is given. Incorporation of conformational variability in the free energy of a polyelectrolyte system provides quantitative fitting of experimental data. In addition, it extends the validity of the theory to cases in which a cooperative order-disorder transition takes place. Biopolymers considered are: poly(L-aspartic acid), poly(L-glutamic acid), samples of poly(uronic acid) and some carboxylic derivatives of a gelling bacterial polysaccharide.ZusammenfassungEine theoretische Betrachtung der Protonendissoziation schwacher PolysÄuren wird dargelegt. Die Berücksichtigung konformativer Änderungen auf die Grösse der freien Energie eines polyelektrolytischen Sytems ermöglicht eine theoretische Berechnung der Titrationskurven von Poly-L-AsparaginsÄure als auch der Konformationsumwandlungen einiger Biopolymeren wie Poly-L-GlutaminsÄure und carboxylierten Derivative von Polisacchariden.