Conformational transition of gellan gum of sodium, lithium, and potassium types in aqueous solutions

The effects of cationic ions on the conformational properties of gellan gum in aqueous solutions with and without salts are studied using highly purified lithium, sodium, and potassium-type gellan gum samples by measuring the osmotic pressure, intrinsic viscosity, and circular dichroism (CD). The Mn values of Li, Na, and K-gellan obtained at 45°C were nearly the same. The Mn values of Na-gellan or Li-gellan obtained at 45°C in aqueous solutions with added salts were almost half or slightly less than half the value of that obtained at 28°C, respectively. The temperature dependence of θ201 and (ηsp/cp)for Li, Na, and K-gellan solutions with and without added salts showed drastic changes at the transition temperatures. These results could be interpreted as the occurrence of the coil to double-helix transition and partial aggregation of helices at the transition temperatures on lowering temperature. In the aqueous solutions without added salts, the transition temperatures for Li, Na, and K-gellans determined by CD are almost coincident with those obtained from viscometry. The transition temperatures for Li, Na, and K-gellan solutions increased with increasing polymer concentration but almost independent of the chemical species of countercations. In the aqueous solutions with added salts, the transition temperature of Li, Na, and K-gellan depends not only on polymer concentration but also the concentration of added salt and the chemical species of countercations. The observed transition temperature for K-gellan solution with KCl is higher than that for the solutions of Li-gellan with LiCl and Na-gellan with NaCl.