Synchrotron radiation X-ray diffraction study was performed on short chain DNA/lipid multilayers prepared by drop-casting solutions of DNA and lipid mixtures onto solid surfaces.DOTAP and DOPC mixtures were used in the study with ΦDOTAP denoting the DOTAP content.The DNA contents are such that the systems are at their isoelectric point.The multilayers are formed via self-assembly during evaporation of the solvent.The results indicate that the short chain DNA molecules are embedded randomly in the hydrophilic headgroup regions of the lipid bilayers when ΦDOTAP≤50%

while the DNA molecules form a closely packaged layer inside each lipid bilayers when ΦDOTAP≥90%.The DNA-poor phase and DNA-rich phase coexist at intermediate DNA contents.The bilayers are distorted in the DNA-poor phase

while they are well-ordered in the DNA-rich phase.The phase transition is driven by the lowering of the interfacial energy.