THE EFFECT OF THE CRYOPROTECTIVE SUGAR, TREHALOSE ON THE PHASE-BEHAVIOR OF MIXED DISPERSIONS OF DIOLEOYL DERIVATIVES OF PHOSPHATIDYLETHANOLAMINE AND PHOSPHATIDYLCHOLINE

The phase behaviour of a mixed dispersion of dioleoyl derivatives of p hosphatidylethanolamine and phosphatidylcholine (3:1, by weight) in an excess of water was compared with that in 1.8 M trehalose using dynam ic X-ray diffraction, differential scanning calorimetry and freeze-fra cture electron microscopy. The phase transitions of the fully-hydrated dispersion observed upon heating, proceeded with lamellar-gel --> lam ellar liquid crystalline --> inverse cubic --> inverse hexagonal phase at temperatures of -10-degrees-C, 49-degrees-C, and 67-degrees-C, res pectively. Our results confirm the existence of an inverse cubic phase of this system and support the model mechanism for the lamellar/non-l amellar 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, tre halose, inhibited the formation of both lamellar liquid crystalline an d inverse cubic phases and resulted in a direct transition from lamell ar-gel to inverse hexagonal phase at about -6-degrees-C. The effect of trehalose is discussed in terms of a 'water replacement' model and Ho fmeister effects on water structure. Trehalose is regarded as a kosmot ropic agent that may also interact directly with the lipid polar group s. Comparison of the relationship between temperature and the dimensio ns of the inverse hexagonal phase formed in the presence and absence o f trehalose suggests that the osmotic effect of the impermeant trehalo se prevents water molecules from being taken up by the hexagonal mesop hase which is normally more hydrated that the lamellar phase.