Thermodynamics of the proton dissociation of natural polyuronic acids

The thermodynamic properties of the dissociation of natural polyuronic acids in dilute aqueous solution at 25 degrees C are reported. The investigation concerns polygalacturonic acid and alginic acids from different sources, including highly homogeneous mannuronic and gulronic fractions and involved potentiometry, isothermal calorimetry, dilatometry and circular dichroism measurements. The dissociation of polyuronic acids was studied as a function of degree of dissociation, concentration of the polymer and ionic strength. Abnormal behavior upon dissociation was observed only in polyguluronic and polygalacturonic acids and attributed to a pH-induced conformational transition and/or to an interchain disaggregation. Direct molecular weight evidence supports the proposed intramolecular transition of polygalacturonate chain.