Thermally-induced order-order transition of DNA-cationic surfactant complexes

Polyanionic DNA interacts with cationic amphiphiles to form electrostatic complexes exhibiting rich self-assembled structures. This type of complex has been considered as a nonviral carrier in gene therapy and as a template for nanostructure construction. Here we report a thermally-induced phase transition of the complexes of DNA with the mixtures of a cationic surfactant, dodecyltrimethyl bromide (DTAB), and a neutral lipid, dioleoylphosphatidylethanolamine (DOPE), in fully hydrated state. An order-order transition between a multilamellar (L-alpha(c)) phase and an inverted hexagonal (H-II(c)) phase was found to occur with the transition temperature adjustable by the DTAB-to-DNA base pair molar ratio (x) and DOPE-to-DTAB molar ratio (m). The stability of the L-alpha(c) phase was enhanced at lower m and x, as the L-alpha(c)-to-H-II(c) transition temperature increased with the decreases of these two parameters. The suppression of L-alpha(c)-to-H-II(c) 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.