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  • 1 Hebrew University-Hadassah Medical School Department of Biochemistry P.O. Box 12272 91120 Jerusalem Israel
  • | 2 Utrecht University for Pharmaceutical Sciences (UIPS) participant in the research school Groningen — Utrecht Institute for Drug Exploration (GUIDE), Faculty of Pharmacy, Utrecht University Department of Pharmaceutics Utrecht The Netherlands
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This study was aimed to investigate the physicochemical changes induced in 200 nm extruded oligolamellar DPPC:DPPG (10:1) liposomes by freezing, followed by γ-irradiation, in the absence and presence of 5 mM stable cyclic nitroxide radicals, 2,2,6,6-tetramethylpiperidine-1-oxyl (Tempo) and 4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl (Tempol). The characterization is based on the use of differential scanning calorimetry (DSC) and was aimed to differentiate the contribution of freezing and γ-irradiation in the presence and absence of nitroxides. Liposomal preparations of DPPC/DPPG which have sub-, pre- and main-phase transitions in the temperature range (0�C<Tm<50�C) were used. Our results show that: (1) freezing modified and induced fusion to MLV as well as fission to SUV, (2) freezing did not fully prevent the radiation-induced changes in the thermotropic characteristics of the liposomes, and (3) Tempo and Tempol did not prevent the changes in thermotropic behavior caused as a result of freezing of the liposomal dispersion. These results demonstrate that DSC is a powerful and sensitive tool in both physical and chemical studies of lipid assemblies.

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