Thermally induced coil-to-helix transition of sodium gellan gum with different molar masses in aqueous salt solutions

Using 5 samples of well-purified Na-gellans (Na-gellans G1-G5, weight-average molar mass M-w = 120 x 10(3) - 32 x 10(3) at 40 degrees C), the effects of molar mass on the coil-to-double-helix transition in aqueous solutions with 25 mM NaCl were studied by light scattering and circular dichroism (CD) measurements, viscometry, and differential scanning calorimetry (DSC). From the temperature dependence of M-w, molar ellipticity at 201nm [theta](201), intrinsic viscosity [eta], and DSC exothermic curves, it was found that the coil-to-double-helix transitions for G1-G5 samples took place at almost the same temperature. The [eta] and Mw obtained in the temperature range from 40 to 25 degrees C call be explained by a simple coilldouble-helix equilibrium model using the double-helix contents determined from CD data. The van't Hoff's transition enthalpy Delta H-vH of Na-gellans depended on M-w. It is concluded that the coil-to-double-helix transitions of Na-gellans are all-or-none type transitions, and are accelerated with increasing M-w. (c) 2005 Wiley Periodicals, Inc.