Abstract
Polyanionic DNA binds to cationic amphiphiles such as lipids and surfactants to form electrostatic complexes exhibiting rich self-assembled structures. This type of complexes has been considered as a non-viral carrier in gene therapy. Extensive efforts have been dedicated to the relationship between the trnasfection efficiency and the molecular and self-assembled structures of the complexes. In addition to the biomedical application, DNA-cationic amphiphile complexes have also been considered as nanostructured templates due to the characteristic nanostructuring of DNA and the amphiphiles. For non-biomedical applications, cationic surfactant would be a more advantageous complexing agent compared with cationic lipid due to its lower cost and wider availability. However, the self-assembly behavior of the complexes of DNA with cationic surfactants still requires detailed investigations. In this dissertation, we systematically studied the lyotropic and thermotropic phase behaviors of the complexes of DNA with a cationic surfactant, dodecyltrimethyl bromide (DTAB), mixed with a neutral lipid, dioleoylphosphatidylethanolamine (DOPE), in fully hydrated state as a function of DTAB-to-DNA base pair molar ratio (x) and DOPE-to-DTAB molar ratio (m). Lyotropically, it was observed that the addition of DOPE induced a phase transition from a condensed multilamellar phase (L□c) to an inverted hexagonal phase (HIIc) at large m (m > ~ 6). The DNA chains confined between the hydrophobic layers in the L□c phase formed ordered arrays with the interhelical distance affected by both x and m. A thermally-induced order-order transition between L□c phase and HIIc phase was found to occur with the transition temperature adjustable by x and m. The stability of L□c phase was enhanced at lower m and x. The suppression of L□c-to-HIIc transition at lower x was attributed to the lower entropic gain from the counterion release due to the presence of uncomplexed DNA in the bulk solution. In addition to the self-assembled structure in the bulk state, we also studied the structure of the spin-coated thin film formed by the complex of DNA with a cationic surfactant, cetyltrimethylammonium bromide (CTAB). A highly uniform network structure was fond to develop in the thin film. The formation of the network structure was ascribed to the viscoelastic phase separation complexed with dewetting effect during the solvent evaporation.