Liquid crystalline phase metastability of phosphatidylglycerols

Abstract

Aqueous dispersions of dilauroylphosphatidylglycerol (DLPG) and dimyristoylphosphatidylglycerol (DMPG) form stable subgel and liquid crystalline lamellar phases, Lc and Lα, and also metastable gel phases, Lβ′ and Pβ′, at physiologically relevant conditions (neutral pH, <0.2 M salt). The metastable gel phase Lβ′ relaxes to the stable subgel phase upon low-temperature storage, with at least one metastable intermediate in the case of DMPG. The gel–liquid crystalline phase transition takes place at temperature with 15–30°C lower as compared to that of the subgel–liquid crystalline phase transition in these phosphatidylglycerols. Therefore, the lamellar liquid crystalline phase Lα is expected to exhibit metastability in the temperature range between the gel–Lα and subgel–Lα phase transitions, in which the subgel phase is the equilibrium one. Here we demonstrate by using differential scanning calorimetry that crystallisation from the liquid crystalline phase in this temperature interval takes place in DMPG and DLPG. The kinetics of the observed Lα-to-Lc phase relaxation is sensitive to the thermal prehistory of the lipid dispersion. This is possibly related to the different morphologies of the lipid aggregates in the subgel and liquid crystalline phases (Epand et al., 1992) and the slow interconversion between them.