Temperature dependence of the conformational properties of sodium-type gellan gum in aqueous solutions
The temperature dependence of the conformational properties of sodium-type gellan gum in aqueous solutions was studied by viscometry and osmometry. The osmometry measurements were made at 28, 40, and 45 °C in aqueous solutions at two sodium chloride concentrations (c s = 25 and 50 mM). Viscosity measurements were carried out over the temperature range from 5 to 50 °C in aqueous solutions at three sodium chloride concentrations (c s = 0.0, 12.5, and 25 mM) and four sucrose concentrations (c su = 0.0, 0.05, 0.5, and 1.0 M). The M n values obtained at 40 and 45 °C in aqueous sodium chloride solutions with different c s were almost coincident and the average value (5.7 × 104) was slightly less than half of the M n value (13.6 × 104) obtained at 28 °C, suggesting that association of two molecules and only a partial aggregation of associates take place at 28 °C. On lowering the temperature, the viscosity number, η sp/c, of the sodium chloride solutions increased only slightly over the higher temperature regions but increased drastically below a critical temperature which increased with increasing c s and several degrees below these critical temperatures the η sp/c values increased gradually. For aqueous solutions, in contrast, the η sp/c values showed only a slight increase over higher temperature regions but a steep increase below the critical temperatures. The temperature dependence of η sp/c for the sucrose solutions was similar to that of the aqueous solutions, but the critical temperatures in sucrose solutions increased with increasing c su. These results could be interpreted as showing that, on lowering the temperature, the coil-to-double helix conformational transition of sodium-type gellan molecules and partial aggregation of helices occur below the critical temperatures, and are promoted by increasing the sodium chloride or sucrose concentrations.
Key wordsGellan gum Aqueous solution Conformational transition Osmometry Viscometry
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