Coil-to-helix transition of gellan in dilute solutions is a two-step process

•Evidence of two-step mechanism of coil-to-helix transition of gellan is found by DSC.•Distinct step of heat absorption precedes heat evolution in the course of transition.•Dehydration of gellan is proposed for the mechanism of endothermic step.

Thermodynamics of coil-to-helix transition of natural polysaccharide - gellan was studied in dilute solutions in 130 mM NaCL in concentration range 0.01-1.5% using SETARAM C80 3D Calvet calorimeter operated in a passive scanning mode at low cooling rate (0.04 K/min at the transition temperature). It was shown that the negative values of the total enthalpy of the coil-to-helix transition substantially increased in a step-wise fashion if the concentration of the solution decreased below the threshold of coil overlapping in accordance with the results of dynamic light scattering. The shape of thermograms depended on the concentration of solutions. While in semi-dilute solutions with gellan concentration above 0.25% only one exothermic peak was indicated in the cooling mode, in dilute solutions two distinct peaks of the two-step transition: first - endothermic and second - exothermic were clearly observed. The endothermic and exothermic peaks overlapped for semi-dilute solutions of gellan and became progressively separated as the concentration diminished. The reasons, which underlie the two-step nature of coil-to-helix transition of gellan are discussed based on the origin of molecular interactions involved.

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