Polymorphism of DNA–anionic liposome complexes reveals hierarchy of ion-mediated interactions

doi:10.1073/pnas.0502416102

Polymorphism of DNA–anionic liposome complexes reveals hierarchy of ion-mediated interactions

^†Departments of Materials Science and Engineering, Physics, and Bioengineering, University of Illinois at Urbana–Champaign, Urbana, IL 61801; and ^‡Laboratory of Physical and Structural Biology, National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, MD 20892

Edited by David Chandler, University of California, Berkeley, CA, and approved June 13, 2005 (received for review March 23, 2005)

Abstract

Self-assembled DNA delivery systems based on anionic lipids (ALs) complexed with DNA mediated by divalent cations have been recently introduced as an alternative to cationic lipid–DNA complexes because of their low cytotoxicity. We investigate AL–DNA complexes induced by different cations by using synchrotron small angle x-ray scattering and confocal microscopy to show how different ion-mediated interactions are expressed in the self-assembled structures and phase behavior of AL–DNA complexes. The governing interactions in AL–DNA systems are complex: divalent ions can mediate strong attractions between different combinations of the components (such as DNA–DNA and membrane–membrane). Moreover, divalent cations can coordinate nonelectrostatically with lipids and modify the resultant membrane structure. We find that at low membrane charge densities AL–DNA complexes organize into a lamellar structure of alternating DNA and membrane layers crosslinked by ions. At high membrane charge densities, a new phase with no analog in cationic lipid–DNA systems is observed: DNA is expelled from the complex, and a lamellar stack of membranes and intercalated ions is formed. For a subset of the ionic species, high ion concentrations generate an inverted hexagonal phase comprised of DNA strands wrapped by ion-coated lipid tubes. A simple theoretical model that takes into account the electrostatic and membrane elastic contributions to the free energy shows that this transition is consistent with an ion-induced change in the membrane spontaneous curvature, c ₀. Moreover, the crossover between the lamellar and inverted hexagonal phases occurs at a critical c ₀ that agrees well with experimental values.

Footnotes

↵ § To whom correspondence should be addressed. E-mail: gclwong{at}uiuc.edu.
Author contributions: G.C.L.W. designed research; H.L. and D.H. performed research; H.L., D.H., and G.C.L.W. analyzed data; and H.L., D.H., and G.C.L.W. wrote the paper.
This paper was submitted directly (Track II) to the PNAS office.
Abbreviations: AL, anionic lipid; CL, cationic lipid; SAXS, small angle x-ray scattering; DOPG, 1,2-dioleoyl-sn-glycero-3-[phospho-rac-(1-glycerol)]; DOPC, dioleoyl phosphatidyl choline; D/L, DNA-to-lipid charge ratio.