Calorimetric investigation of the complex phase behavior of glucocerebroside dispersions

The thermotropic behavior of aqueous dispersions of glucocerebroside from Gaucher's spleen has been investigated by different scanning calorimetry. These results indicate that glucocerebroside undergoes two distinct phase transitions centered at 47 and 83 degrees C, respectively. The high-temperature transition is associated with the main gel-liquid crystalline transition and has an enthalpy change of 13.6 kcal/mol of lipid; this transition is not rapidly reversible and the liquid crystalline phase supercools to a metastable gel phase. The low-temperature transition is exothermic with an enthalpy change of -6.3 kcal/mol and involves a transformation of the metastable gel phase to a more highly ordered gel conformation, without involving changes in the conformational state of the hydrocarbon chains. The behavior of these transitions as a function of the amount of water suggests that the origin of the metastability is related to a hydration-dehydration process of the cerebroside molecule. Experiments with synthetic-D-erythro-N-palmitoylglucocerebroside revealed the same thermotropic behavior. These glucocerebroside transitions are irreversible and together define a unidirectional cycle in which each state of the molecule can only be reached by completing the entire cycle.