Thermodynamic equilibrium of water and ice in hydrated gliadin and hemoglobin

Crystallization of water and the water-ice equilibrium in the hydrated states (1 g of H2O/g of dry protein) of gliadin and hemoglobin have been studied by differential scanning calorimetry. Water and ice coexist at a thermodynamic equilibrium at all temperatures in the 230-272 K range. Their relative amounts have been determined from 260 to 273 K, and a formalism based on equilibrium thermodynamics has been developed. The temperature dependence of the equilibrium constant of the water <----> ice interconversion does not obey the Gibbs-Helmholtz equation, and this indicates a strong interaction of proteins with water. By using the measured equilibrium constant at different temperatures and the difference between the C-p of the solutions in equilibrium with ice and the ice itself in the hydrated proteins, the DSC scans obtained during cooling have been simulated. The kinetics of crystallization is not determined entirely by the grain-growth process. A double crystallization peak has been observed on cooling hydrated gliadin from 274 K, after the sample had been thermally cycled in the 230-273 K range and annealed at 274 K. Among the nine contributions to the enthalpy and entropy change on cooling and annealing at subfreezing temperatures, the largest contribution remains that from water's crystallization in the hydrated proteins. The amount of water in equilibrium with ice in the two proteins is comparable to that determined for relatively impure proteins. This underscores the importance of H-bond interaction with the protein molecules over that of the effects of impurities.