Solvation and expansion of neutral and charged chains of a carbohydrate polyelectrolyte: Galacturonan in water. A critical revisiting

Experimental and theoretical data have been revisited to shed light onto the aspects of hydration and chain expansion of pectic acid (galacturonan) upon charging. The prediction of the variation of the number of solvation water molecules between the two limit ionization states from theoretical calculations was confirmed to a very high accuracy by the corresponding number evaluated form dilatometric measurements. The relevance of hydration to the mechanism of bonding of calcium ions by sodium pectate is discussed. Characterization of polymer expansion has been obtained by calculating the values of the characteristic ratio and/or the persistence length on the respective populations and comparing the theoretical predictions with experimental data. The results show that a charged chain in typical conditions of ionic strength is more expanded than its neutral counterpart, whereas the ideal limit (31 and 21) helical conformations in the uncharged and totally charged conditions, respectively, share the same value of the linear advance of the helical repeat, when the ionic strength tends to infinite. Total divergence between theoretical predictions and experimental evidence rules out the possibility that carboxylate charge reduction by protonation and by methyl esterification are equivalent in determining the solution behavior of galacturonan.