Physical stability control of vesicle/DNA complexes is a key issue for the development of catanionic vesicles composed of ion pair amphiphile (IPA) as DNA carriers. In this work, physical stability characteristics of the complexes of DNA with positively charged catanionic vesicles composed of an IPA and a double-chain cationic surfactant, dihexadecyldimethylammonium bromide (DHDAB), were explored. It was found that in water, the mixed IPA/DHDAB catanionic vesicles became stable when the mole fraction of DHDAB (xDHDAB) was increased up to 0.5. The improved physical stability of the vesicles with a high xDHDAB could be related to the enhanced electrostatic interaction between the vesicles. When the catanionic vesicles interacted with DNA, excellent physical stability was detected for the vesicle/DNA complexes especially with a high xDHDAB. However, this could not be fully explained by the electrostatic interaction effect, and the role of molecular packing within the vesicular bilayers was apparently important. The corresponding Langmuir monolayer study demonstrated that the molecular packing of mixed IPA/DHDAB layers became ordered with DNA association due to inhibited desorption of the positively charged moiety of the IPA. Moreover, the DNA association-induced improvement in the molecular packing of the mixed IPA/DHDAB layers became pronounced with increased xDHDAB. The results imply that one can fabricate catanionic vesicle/DNA complexes with excellent physical stability through the improved molecular packing in the IPA vesicular bilayers with DHDAB addition and DNA association.
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