Thermodynamics of the double helix-coil equilibrium in tetramethylammonium gellan:: High-sensitivity differential scanning calorimetry data

The order-disorder conformational transition in the tetramethylammonium salt of gellan has been studied by high-sensitivity differential scanning calorimetry as a function of concentration of the background electrolyte, Me4NCl (0.0025-0.5 M). The transition temperature and enthalpy increase with increasing salt concentration following a logarithmic relation. At the same time, there is a linear correlation between the transition enthalpy and temperature with a positive slope of 0.18 J g(-1) K-1. According to Kirchhoff's law, the slope can be considered as a heat capacity increment of the transition. This value is much larger than that expected to arise from electrostatic effects. Thus, it has been suggested that the transition is accompanied by an increase in water accessibility to some less hydrophilic fragments of the primary structure of gellan buried in its ordered conformation. The transition has an lambda-like heat capacity profile and can be therefore defined as a second-order phase transition. This assignment is in agreement with predictions of a matching model of the double helix-coil transition. The model fits well to the integral transition curve at 0.5 M Me4NCl, assuming a cooperative length of gellan to be about eight repeating units. This value is close to the size of chain corresponding to the persistent length of gellan in the coil state. The cooperativity parameter in the matching model (sigma = 0.62 +/- 0.01) indicates a small contribution of the stacking effect into the cooperativity of the transition, while a larger contribution apparently originates from the loop factor. The free energy of transition at a reference temperature (273 K) is a logarithmic function of the concentration of the salt, Delta(t)G(C-s), as predicted by the counter condensation theory and the Poisson-Boltzmann (PB) model. The latter is preferable because of the low linear charge density parameter (xi < 1) for the double helix of gellan. Analysis of the Delta(t)G(C-s) function in terms of the PB model showed that the linear charge density of gellan in the coil conformation needs to be higher than that estimated for the fully stretched chain.