The effect of the cryoprotective sugar, trehalose on the phase behaviour of mixed dispersions of dioleoyl derivatives of phosphatidylethanolamine and phosphatidylcholine

Abstract

The phase behaviour of a mixed dispersion of dioleoyl derivatives of phosphatidylethanolamine and phosphatidylcholine (3:1, by weight) in an excess of water was compared with that in 1·8 M trehalose using dynamic X-ray diffraction, differential scanning calorimetry and freeze-fracture electron microscopy. The phase transitions of the fully-hydrated dispersion observed upon heating, proceeded with lamellar-gel→lamellar liquid crystalline→inverse cubic→inverse hexagonal phase at temperatures of −10°C, 49°C, and 67°C, respectively. Our results confirm the existence of an inverse cubic phase of this system and support the model mechanism for the lamellar/non-lamellar phase transition previously suggested by Siegel (1986, Biophys. J. 49, 1155, 1171). Dispersion of the binary lipid mixture in a 1·8 M solution of the naturally occurring cryoprotective disaccharide, trehalose, inhibited the formation of both lamellar liquid crystalline and inverse cubic phases and resulted in a direct transition from lamellar-gel to inverse hexagonal phase at about −6°C. The effect of trehalose is discussed in terms of a ‘water replacement’ model and Hofmeister effects on water structure. Trehalose is regarded as a kosmotropic agent that may also interact directly with the lipid polar groups. Comparison of the relationship between temperature and the dimensions of the inverse hexagonal phase formed in the presence and absence of trehalose suggests that the osmotic effect of the impermeant trehalose prevents water molecules from being taken up by the hexagonal mesophase which is normally more hydrated that the lamellar phase.