2H-NMR and DSC study of DPPC-DODAB mixtures

Abstract The phase behaviour of vesicles consisting of binary mixtures of 1,2-dipalmitoyl-d 62 - sn -glycero-3-phosphocholine (DPPC-d 62 ) and dioctadecyl-dimethyl-ammonium-bromide (DODAB) has been investigated using deuterium nuclear magnetic resonance ( 2 H-NMR), differential scanning calorimetry (DSC) and freeze-fracture electron microscopy. Using both multilamellar vesicle (MLV) and solid supported vesicle (SSV) samples in our study we propose partial phase diagrams for each model system and gain information about the effect of the solid state surface (silica) on the mixing behavior. DODAB, being a cationic lipid-like amphiphile, exhibits a peculiar phase diagram when mixed with DPPC. An equimolar mixture shows a higher melting temperature than each of the pure components together with a narrow coexistence region. All other mixing ratios 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 interaction between bilayer and solid support to the mixing behavior, possibly leading to an asymmetry in bilayer composition. As for the MLV, the equimolar SSV sample shows a remarkable phase transition at high temperature connected with the reversible formation of highly curved structures like vesicular buds or tethers on the SSV surface.