Polysaccharides counterions binding

The equilibrium properties in dilute aqueous solution of weakly ionized polysaccharides, e.g. carboxylated natural poly= saccharides, have not been so thoroughly investigated in comparison with other natural and synthetic polyelectrolytes. For instance, a detailed thermodynamic characterization of acid ionization and of counterion binding in terms of combined experimental potentiometric, calorimetric and volumetric data has not been achieved so far for the above types of polysaccharides. Such a description, however, is of obvious relevance for a better understanding of structure-conformation dependent solution properties for this important class of biopolymers. 

Chemical and enzymatic studies on dermatan sulphate from whale intestine have revealed that sulphate groups are located solely at 0-4 of 2-acetamido-2-deo -D-galactopyranosyl residues. L-Idopyranosyluronic acid residues are assumed to be distributed uniformly in the backbone of the polysaccharide chain but D-glucopyranosyluronic acid residues are located only in the linkage region of the protein core. Na N.m.r. studies on dermatan sulphate have provided information on counterion binding and quadrupole coupling constants. ... 

A. Dilute solution properties of ionic-polysaccharides, in particular a thermodynamic approach to counterions binding in i-car-rageenan (segments) aqueous solutions. ... 

The ( >p pjj values needed for the computation of the Vp/Op values associated with the linear polyion, DxS, were obtained by interpolation of the ( )p versus a/b plots accessible for ionic polysaccharides (Fig. 12) where the b values were calculated with Eq. (16). Tlie log Kc and the log K, values determined for the commercially available DxS sample (Pharmacia), with a DS value of ca. 2, were also used at under various NaCl concentration levels. The Kf- values were determined in complex-ation studies facilitated by potentiometric measurements, whereas the Kj values due to Mattai and Kwak [46], were obtained by a dye method. In spite of the difference in valence of the counterions and the binding constants, the Vp/np values resolved for both systems are consistent with... 

In the preceding discussion of the relationship of the Donnan potential and the Donnan phase volume terms to the structural parameter of a linear polyion, a/b (A ), it has been shown that the elearostatic effect on the binding equilibria of ionic polysaccharides can be predicted quantitatively by use of the universal curves shown in Figs. 13 and 14. Also, by separating the overall binding equilibria into two processes, i.e., (1) the concentration of counterion in the polyion domain, territorial binding. 

On the other hand data reported in Fig. 8 Indicate that the higher charge-density of Xanthan compared to PS-10 shows up in a higher heat of Cu ion binding and in a lower activity coefficient of Na counterions. From the initial slope of the Qg against [Cu2tl /N plots of Fig. 6 one estimates for Xanthan in water a differential enthalpy of Cu2+ binding of about 1 Kcal per mole of copper bound. In conclusion, as found with the sulfated polysaccharides (Fig. 4) as well as with different synthetic poly-carboxylates ( ), chelation of Cu2+ ions by polyanions is a process systematically driven by the entropy. 

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