DNA packing in stable lipid complexes designed for gene transfer imitates DNA compaction in bacteriophage

The structure of complexes made from DNA and suitable lipids (lipoplex, Lx) was examined by cryo-electron microscopy (cryoEM), We observed a distinct concentric ring-like pattern with striated shells when using plasmid DNA, T hese spherical multilamellar particles have a mean diameter of 254 nm with repetitive spacing of 7.5 nm with striation of 5.3 nm width. Small angle x- ray scattering revealed repetitive ordering of 6.9 nm, suggesting a lamella r structure containing at least 12 layers. This concentric and lamellar str ucture with different packing regimes also was observed by cryoEM when usin g linear double-stranded DNA, single-stranded DNA, and oligodeoxynucleotide s. DNA chains could be visualized in DNA/lipid complexes. Such specific sup ramolecular organization is the result of thermodynamic forces, which cause compaction to occur through concentric winding of DNA in a liquid crystall ine phase. CryoEM examination of T4 phage DNA packed either in T4 capsides or in lipidic particles showed similar patterns. Small angle x-ray scatteri ng suggested an hexagonal phase in Lx-T4 DNA, Our results indicate that bot h lamellar and hexagonal phases may coexist in the same Lx preparation or p article and that transition between both phases may depend on equilibrium i nfluenced by type and length of the DNA used.