Thermodynamic study of the crystallization of sickle-cell deoxyhemoglobin (hemoglobin S solubility/saturation concentration/enthalpy of crystallization/entropy of crystallization).

The solubility of crystalline deoxygenated sickle-cell hemoglobin has been determined by a new turbidometric technique. In the absence of the allosteric effector, inositol hexaphosphate, the solubility of sickle deoxyhemoglobin crystals is about 30% less than that of gels. The dependence of the solubility of crystals on temperature at pH 7.1 is very much the same as that of gels at pH 6.48. The change in Van't Hoff enthalpy with crystallization is positive but small (3.11 kcal mol^?1) and essentially independent of temperature below 30 °C. The presence of inositol hexaphosphate cuts the solubility almost in half. The entropy change when crystals form, just under 40 cal K^?1 mol^?1, is larger than the values of ΔS ° found for gels. None of the measurements reported required the estimation of pellet volumes and compositions.