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 phosphatidylcholin e (SPC) during lyophilization were carried out. Analyses of freeze-dri ed liposomes were conducted by particle size determination, retention of entrapped water-soluble marker and lipid mixing assay employing res onance energy transfer (RET). The extent of functionality of carbohydr ates depends on their concentration and results from spacing mainly pr eventing fusion at low concentrations, membrane stabilization preventi ng leakage and the bulk sugar matrix mainly depressing aggregation at higher concentrations. By incorporating hexadecyl-beta-D-galactopyrano side in SPC-LUV as membrane-bound cryoprotectant it could be shown tha t fixation of the sugar head of galactosides at the membrane surface o nly leads to prevention of fusion of liposomes. Although the galactosi de does not exhibit a membrane stabilizing effect alone, it improves t he protective effects of the free carbohydrates hyperadditively. Howev er, this fact is discussed on the basis of sugar-sugar interactions by means of hydrogen bonding.