THE EFFECTS OF CHLOROPLAST LIPIDS ON THE STABILITY OF LIPOSOMES DURING FREEZING AND DRYING

Chloroplast thylakoids contain four classes of Lipids, monogalactosyld iacylglycerol (MGDG), digalactosyldiacylglycerol (DGDC), sulfoquinovos yldiacylglycerol (SQDG), and phosphatidylglycerol (cpPG). We have inve stigated the effects of these lipids on the stability of large unilame llar vesicles made from egg phosphatidylcholine (EPC), by substitution of different fractions of EPC in the membranes by the various chlorop last lipids. Damage to liposomes after freezing to -18 degrees C was m easured as carboxyfluorescein leakage or fusion between vesicles. The presence of all chloroplast lipids increased leakage. However, the max imum amount of leakage and the concentration dependence were dramatica lly different between the different lipids. Only SQDG induced vesicle fusion, while the non-bilayer lipid MGDG did not. The presence of MGDG in the membranes led to more leakage than the presence of another non -bilayer lipid, egg phosphatidylethanolamine (EPE), In EPE-containing liposomes, leakage was strongly associated with fusion. Combinations o f different chloroplast lipids had an additive effect on leakage induc ed by freezing. Most of the leakage from galactolipid-containing vesic les occurred during the first 15 min of freezing at -18 degrees C. Aft er a 3h incubation period, most leakage occurred between 0 degrees C a nd -10 degrees C. Lowering the temperature to -22 degrees C had only a small additional effect. Incubation of liposomes at -10 degrees C in the presence of 2.5 M NaCl without ice crystallization, approximately the same concentration obtained by freezing to -10 degrees C, resulted in very little leakage. Air drying of liposomes to low water contents resulted in massive leakage, both from pure EPC vesicles and from ves icles containing galactolipids. The latter vesicles showed more leakag e at any given water content than EPC vesicles. (C) 1998 Elsevier Scie nce B.V.