Structure of self-assembled liposome-DNA-metal complexes

We have studied the structural and morphological properties of the triple complex dioleoyl phosphatidylcholine (DOPC)-DNA-${\mathrm{Mn}}^{2+}$ by means of synchrotron x-ray diffraction and freeze-fracture transmission electron microscopy. This complex is formed in a self-assembled manner when water solutions of neutral lipid, DNA, and metal ions are mixed, which represents a striking example of supramolecular chemistry. The DNA condensation in the complex is promoted by the metal cations that bind the polar heads of the lipid with the negatively charged phosphate groups of DNA. The complex is rather heterogeneous with respect to size and shape and exhibits the lamellar symmetry of the $L_{\alpha }^c$ phase: the structure consists of an ordered multilamellar assembly similar to that recently found in cationic liposome-DNA complexes, where the hydrated DNA helices are sandwiched between the liposome bilayers. The experimental results show that, at equilibrium, globules of the triple complex in the $L_{\alpha }^c$ phase coexist with globules of multilamellar vesicles of DOPC in the $L_{\alpha }$ phase, the volume ratio of the two structures being dependent on the molar ratio of the three components DOPC, DNA, and ${\mathrm{Mn}}^{2+}.$ These complexes are of potential interest for applications as synthetically based nonviral carriers of DNA vectors for gene therapy.