Conformational Transition of Polysaccharide Sodium-Gellan Gum in Aqueous Solutions

Temperature dependences of the conformational properties of sodium-type gellan gum in aqueous solutions were studied by osmometry. The measurements were carried out in the range from 25 to 45 °C for aqueous solutions of three sodium chloride concentrations ( C s = 25, 50, and 75 mmol dm -3 ). It was found that the M n values obtained at 45, 40, 36, 28, and 25 °C with different C s agreed with each other and the average M n values above 36 °C were almost half the values obtained at 28 and 25 °C, suggesting association of two molecules. The values of A 2 obtained at 45, 40, 36, 28, and 25 °C were almost the same at each C s, while these A 2 values decreased with increasing C s . The M n value obtained in solution with C s = 25 mmol dm -3 at 32 °C was slightly higher than those obtained above 36 °C but lower than those obtained below 28 °C, and in solutions with C s = 50 and 75 mmol dm -3, M n values could not be determined due to abnormality of the π/ c vs c plots. This suggests that a unimer should be in simultaneous equilibrium with a dimer. The data obtained at 32 °C were analyzed by employing a simple unimer/dimer equilibrium model. The equilibrium constant K increases with increasing C s, which means that the extent of association increases and the proportion of dimer also increases with increasing C s .