H-2-NMR AND DSC STUDY OF DPPC-DODAB MIXTURES

The phase behaviour of vesicles consisting of binary mixtures of 1,2-d ipalmitoyl-d(62)-sn-glycero-3-phosphocholine (DPPC-d(62)) and dioctade cyl-dimethyl-ammonium-bromide (DODAB) has been investigated using deut erium nuclear magnetic resonance (H-2-NMR), differential scanning calo rimetry (DSC) and freeze-fracture electron microscopy. Using both mult ilamellar vesicle (MLV) and solid supported vesicle (SSV) samples in o ur study we propose partial phase diagrams for each model system and g ain information about the effect of the solid state surface (silica) o n the mixing behavior. DODAB, being a cationic lipid-like amphiphile, exhibits a peculiar phase diagram when mixed with DPPC. An equimolar m ixture shows a higher melting temperature than each of the pure compon ents together with a narrow coexistence region. All other mixing ratio s studied exhibit significantly broader coexistence regions and lower transition temperatures. For the MLV samples freeze fracture electron microscopy shows a reversible reduction in vesicular size by an order of magnitude in going from the fluid to the gel state. For the SSV, we observed a markedly different phase behavior compared to that of the MLV that can be ascribed to the contribution of the Coulomb interactio n between bilayer and solid support to the mixing behavior, possibly l eading to an asymmetry in bilayer composition. As for the MLV, the equ imolar SSV sample shows a remarkable phase transition at high temperat ure connected with the reversible formation of highly curved structure s like vesicular buds or tethers on the SSV surface.