FREEZE-DRYING OF LIPOSOMES WITH FREE AND MEMBRANE-BOUND CRYOPROTECTANTS - THE BACKGROUND OF PROTECTION AND DAMAGING PROCESSES

Studies of the protective effects of different amounts of sucrose and glucose and a carbohydrate directly linked to the liposome surface on large unilamellar vesicles (LUV) built from soybean phosphatidylcholine (SPC) during lyophilization were carried out. Analyses of freeze-dried liposomes were conducted by particle size determination, retention of entrapped water-soluble marker and lipid mixing assay employing resonance energy transfer (RET). The extent of functionality of carbohydrates depends on their concentration and results from spacing mainly preventing fusion at low concentrations, membrane stabilization preventing leakage and the bulk sugar matrix mainly depressing aggregation at higher concentrations. By incorporating hexadecyl-beta-D-galactopyranoside in SPC-LUV as membrane-bound cryoprotectant it could be shown that fixation of the sugar head of galactosides at the membrane surface only leads to prevention of fusion of liposomes. Although the galactoside does not exhibit a membrane stabilizing effect alone, it improves the protective effects of the free carbohydrates hyperadditively. However, this fact is discussed on the basis of sugar-sugar interactions by means of hydrogen bonding.