LIQUID-CRYSTALLINE PHASE METASTABILITY OF PHOSPHATIDYLGLYCEROLS

Aqueous dispersions of dilauroylphosphatidylglycerol (DLPG) and dimyri stoylphosphatidylglycerol (DMPG) form stable subgel and liquid crystal line lamellar phases, L-c and L-alpha, and also metastable gel phases, L-beta, arid P-beta, at physiologically relevant conditions (neutral pH, <0.2 M salt). The metastable gel phase L-beta, relaxes to the stab le subgel phase upon low-temperature storage, with at least one metast able intermediate in the case of DMPG. The gel-liquid crystalline phas e transition takes place at temperature with 15-30 degrees C lower as compared to that of the subgel-liquid crystalline phase transition in these phosphatidylglycerols. Therefore, the lamellar liquid crystallin e phase L-alpha is expected to exhibit metastability in the temperatur e range between the gel-L-alpha and subgel-L-alpha phase transitions, in which the subgel phase is the equilibrium one. Here we demonstrate by using differential scanning calorimetry that crystallisation from t he liquid crystalline phase in this temperature interval takes place i n DMPG and DLPG. The kinetics of the observed L-alpha-to-L-c phase rel axation 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. (C) 1997 Elsevier Sci ence Ireland Ltd.