Effects of molar mass on the coil to helix transition of sodium-type gellan gums in aqueous solutions

The effects of molar mass on the coil to helix transition of sodium-type gellan gums in aqueous solution with 25 mmol NaCl were studied. Six samples of well purified sodium-type gellan gums, ranging in weight average molar mass Mw from 120×103 to 17×103 at 40 °C, were used for light scattering and circular dichroism measurements, viscometry, and differential scanning calorimetry. These measurements were carried out in the temperature range from 5 to 90 °C. It was found that on lowering temperature, the coil to double-helix transition for G1–G5 samples (Mw=120×310−32×310) took place at the same temperatures, and the coil to double-helix transition is accelerated with increase of Mw. The G6 sample (Mw=17×103) did not form a double-helix at 25 °C. It is concluded that the lowest molar mass below which no helix is formed at 25 °C in aqueous solutions with 25 mmol NaCl lies between Mw=32×103 and Mw=17×103.