When DNA is mixed with positively charged lipid vesicles, complexes form spontaneously. In the hope that they may be used as delivery vehicles in gene therapeutics, these lipoplexes are widely studied for their ability to transfect living cells. Lipoplexes order into phases resembling the inverted hexagonal and lamellar phases of lipids, with DNA intercalated in the regions occupied solely by water in the corresponding lipid phases. The efficiency of gene delivery in vitro has previously been shown to correlate with the structure of these complexes. However, the dense, crowded, osmotically stressed reality of living systems makes it hard to predict how lipoplexes will perform in vivo.

By x-ray diffraction and electron microscopy, the highly ordered lipoplex structures have been shown to depend on lipid composition and on lipid/DNA ratio. These compositional degrees of freedom reflect not only structural characteristics of the lipid component - such as bending rigidity and spontaneous curvature - but also complex stabilization due to entropic gain from release of previously bound counterions.

We learn how lipoplexes respond to a dense environment. By osmotically stressing lipoplexes and following the subsequent structural changes using x-ray and electron microscopy, we can focus on the water compositional degree of freedom and determine the work needed to change the stable phase of the lipoplex. From the equation of state, combining information of applied osmotic pressure versus water volume we are able of determine the work needed to transform one lipid phase (the lamellar) to another (hexagonal). We find that this work is primarily due to the energy needed to bend the lipid membrane around DNA.

Related References

D. Harries, S. May, W. M. Gelbart and A. Ben-Shaul "Structure, Stability and Thermodynamics of Lamellar DNA-Lipid Complexes" Biophys. J. 75, 159-173 (1998). [PDF]

K. Wagner, D. Harries, S. May, V. Kahl, J.O. Raedler and A. Ben-Shaul "Direct Evidence for Counterion Release Upon Cationic Lipid-DNA Condensation" Langmuir 16, 303-306 (2000). [PDF]

S. May, D. Harries and A. Ben-Shaul "The Phase Behavior of Cationic Lipid-DNA Complexes" Biophys. J. 78, 1681-1697 (2000). [PDF]

D. Harries "Electrostatic interactions between macromolecules and lipid bilayers" Ph.D. Thesis, Hebrew University (2001). [PDF]

D. Harries, S. May and A. Ben-Shaul "Curvature and charge modulations in lamellar DNA-lipid complexes" J. Phys. Chem. (to be published, 2003). [PDF]

R. Podgornik, D. Harries, H.H. Strey, V.A. Parsegian. "Molecular interactions in lipids, DNA, and Lipid-DNA complexes" in "Gene Therapy - therapeutic mechanisms and strategies", N. Smyth Templeton edt., 2nd ed. (2003). [PDF] - 194 MB

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