Metastability in the phase behavior of dimyristoylphosphatidylethanolamine bilayers.

A new subgel phase is demonstrated to occur in hydrated dimyristoylphosphatidylethanolamine ( DMPE ) by using dilatometric and calorimetric techniques. The formation of the subgel phase takes place very slowly at temperatures near 0 degree C, but it can still be observed at 25 degrees C. Once formed, the subgel phase melts (delta Hh = 16.0 +/- 0.6 kcal/mol and delta V = 0.085 +/- 0.014 mL/g) directly into the liquid-crystalline phase at a temperature, Th = 56.3 degrees C, that is higher than the gel to liquid-crystalline transition temperature, Tm = 49.6 degrees C. Thus, the gel phase appears to be metastable over its entire temperature range. In this regard, DMPE behaves differently from dipalmitoylphosphatidylcholine and distearoylphosphatidylcholine but similarly to dilaurylphosphatidylethanolamine . This unusual long-lived metastability provides cells an additional option in determining the properties of membranes.