Calorimetric investigations of phase transitions of sonicated vesicles of dimyristoylphosphatidylcholine.

The process of conversion of large multilamellar vesicles (MLVs) of dimyristoylphosphatidylcholine (DMPC) into the final state of small unilamellar vesicles (SUVs) with an increase in time length of ultrasonic irradiation was investigated by calorimetry and negative-stain electron microscopy. The process was found out to be composed of two stages depending on the primary (near 24 degrees C) and secondary (near 19 degrees C) peaks due to the gel-to-liquid crystal phase (Tm) transition, respectively; a new transition peak for the secondary Tm appears after a maximum broadening of the primary Tm peak is attained. Sonicated vesicles characterized by the primary peak of the broadest shape were observed to be about 200 nm in mean diameter and mostly four or so lamellae, and have an internal aqueous space, in contrast to sonicated SUVs (approx. 40 nm in diameter) characterized by the limiting secondary Tm peak. Thermal data associated with the Tm transition for these two sonicated vesicles were compared with that of the MLV. The enthalpy and entropy changes and cooperative units increased in the order sonicated SUV < sonicated large vesicle < MLV. Furthermore, the enthalpy changes were revealed to fairly differ between the sonicated large vesicle and SUV. Based on the effect of the annealing treatment at -5 degrees C on these vesicles the present result suggested a large contribution of the aggregation state of DMPC molecules to the enthalpy possessed by the vesicles of a gel phase temperature, which is related to the mode of the Tm transitions, primary and secondary.