Figure 19 Schematic illustration of some possible structures of DNA-mixed lipid (cationic/nonionic) complexes. (A) The sandwichlike (Z4) lamellar complex composed of parallel DNA molecules intercalated between lipid bilayers. (B) The honeycomblike (Hn) hexagonal complex, composed of a hexagonally packed bundle of monolayer-coated DNA strands. (C) Two interpenetrating hexagonal lattices, one of DNA, the other of micelles. (D) Spaghetti-like complex, composed of bilayer-coated DNA. (Reprinted by permission from Ref. 143, Biophysical Society.) See the color insert for a color version of this figure.

thetically derived) cationic lipids often tend to form micelles in solution, helper lipids facilitate the formation of membranes.

In this L°a complex geometry, the DNA strands within each gallery are parallel to each other, exhibiting a well-defined repeat distance d. While d depends on the CL/DNA and CL/ HL concentration ratios, the spacing between two apposed lipid monolayers is nearly constant at ~26 A, corresponding to the diameter of double-stranded B-DNA, ca. 20 A, surrounded by a thin hydration shell. This L°a lamellar (''sandwich'') complex is stabilized by the electrostatic attraction between the negatively charged DNA and the cationic lipid bilayer. Because of strong electrostatic repulsion between the charged bilayers (particularly at low salt conditions), the lamellar lipid phase is unstable without DNA.

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