KINETIC FEATURES OF HYSTERESIS OF THE MAI N PHASE-TRANSITION IN DILAUROYLPHOSPHATIDYLETHANOLAMINE MULTILAMELLAR VESICLES

The main phase transition in dilauroylphosphatidylethanolamine multila mellar vesicles was studied by means of sound velocity measurements (7 ,4 MHz) in a temperature-scanning mode. Hysteresis loop width (0,30+/- 0,05 K) was found to be independent of the scan rate for the rates ran ged within 0,05-2 K/min. If, on the course of cooling, temperature was stopped at any point within the transition zone (at the midpoint, e.g .), fraction of solid state did not tend to shift towards the opposite branch of the loop (obtained upon heating) at least during one hour. Furthermore, if, starting from this point, temperature increased again , the fraction of solid almost was not changed, until the cheating, br anch of the loop was reached. Only then the fraction of solid started to reduce, and further on it followed the <<heating>> branch exactly. A similar but not the same behavior was observed when the starting sta te was the solid one. These observations mean that melting and freezin g proceed via quite different paths: the main phase transition is not a local-equilibrium process and therefore the equilibrium-thermodynami c approach is not applicable to molecular interpretation of the shape of the transition curves. Also it has been concluded that creation of a critical-sized nucleus is not the rate-limiting stage of the main ph ase transition in these vesicles. A hypothesis on the possible mechani sm of the irreversibility is suggested.