Phase metastability and supercooled metastable state of diundecanoylphosphatidylethanolamine bilayers

Abstract

Aqueous dispersons of l-α-phosphatidylethanolamine (PE) with identical saturated acyl chains are known to exhibit gel-state metastability. It is also known that the metastability in PE becomes more pronounced with decreasing acyl chain-length. In an attempt to study the metastable phase behavior of PE, we have synthesized diundecanoylphosphatidylethanolamine (diC 11PE) and examined its polymorphic phase behavior. A single endothermic transition at 38° C is detected between 10 and 55° C by DSC for the nonheated sample of diC 11PE in excess water. An immediate second heating scan done after cooling slowly of the same sample from the liquid-crystalline state shows a smaller endothermic transition at a lower temperature, 18°C. However, the high-temperature transition at 38° C can be detected, if the sample which has been heated above 38° C is quench cooled from the liquid-crystalline to a temperature between 18 and 38° C. Furthermore, two endothermic transitions at 18 and 38° C and an exothermic transition at 19° C are recorded for diC 11PE after quench supercooling of the sample from the liquid-crystalline state to an appropriate temperature below 10° C. The gel-state metastability of diC 11PE can be most appropriately explained in terms of changes in interbilayer headgroup-headgroup interactions. It is suggested that the kinetically trapped supercooled metastable state may be a multilamellar structure with melted acyl chains but with strong interbilayer headgroup-headgroup interactions.