Complexation between DNA and surfactants and lipids: phase behavior and molecular organization

The interaction between DNA and various cationic species, e. g. cationic surfactant (CS), has a broad biological and biotechnological significance. In the cell nucleus as well as in transfection formulations, other species, mainly zwitterionic lipids, are also present but their exact role needs elucidation. A closer investigation of the stability of structures formed as well as the molecular arrangements is hampered by the complexity of the systems with respect to the number of components. A powerful way for reducing the number of components is to base studies on the stoichiometric (1 : 1) compound CSDNA, where the simple (sodium) counterions have been ion-exchanged by a cationic amphiphile ion. CSDNA is typically insoluble in water but is able to form liquid crystalline phases in aqueous mixtures with many additives capable of associating with the amphiphilic counterions (alcohols, non-ionic surfactants, lipids, cyclodextrins, etc.). Mixtures of CSDNA with a number of components have been investigated in detail with respect to phase behavior. The phase diagrams demonstrate a rich liquid crystallinity. The organization of DNA and the surfactant-lipid self-assemblies is controlled by different factors for different cases, mainly (i) the lipophilic characteristics of the components, (ii) the [CSDNA]/[amphiphile] ratio and (iii) DNA packing constraints, due to the large persistence length. A summary of phase diagrams is presented together with structural investigations based mainly on small-angle X-ray scattering. The role of DNA rigidity is illustrated in a comparison with analogous systems based on flexible polyanions.