Calorimetric studies of lipid tubule formation from ethanol-water solutions

We have used differential scanning calorimetry to systematically investigate the thermal formation of hollow cylindrical crystalline microstructures or 'tubules' upon cooling a diacetylenic phosphatidylcholine (1,2-bis(10,12-tricosadiynoyl)-sn-glycero-3-phosphocholine) dispersed in varying volume fractions of ethanol/water. Tubule formation is characterized by a large exothermic event, observed upon cooling the lipid in 60-80% ethanol. The enthalpy of the transition was observed to be highest in this window of tubule formation (128-138 J/g) which is significantly higher than previously reported values for the enthalpy of tubule formation in water (90 -95 J/g). The enthalpy associated with the formation of tubules in 70% ethanol was also found to be strongly dependent on the efficiency of tubule formation and decreased as the number density of tubules decreased. A significant decrease in tubule number density could be brought about by increasing the lipid concentration of the 70% ethanol solution. Tubule number density was maximized at lipid concentrations between 0.5 and 2 mg/ml in 70% ethanol. Examination of the C-H stretch region from infrared spectra of the lipid below the phase transition, indicate that the intramolecular chain order-disorder is similar, regardless of the fraction of ethanol. The higher transition enthalpy for the melting of tubules in 60-80% ethanol (compared to water) implies that the high-temperature phase from which the tubules form in ethanol is more disordered than the lamellar liquid crystalline phase from which tubules form in water.