Lipid phase dependence of DNA-cationic phospholipid bilayer interactions examined using atomic force microscopy

Magnetic AC mode atomic force microscopy was used to study DNA adsorption onto supported cationic phospholipid (CL) bilayers in the fluid (L-alpha) and gel (L-beta) phases, A L-alpha phase CL bilayer was used as a model system to study DNA-CL complex formation at the single molecule level. Pure and mixed bilayers of dioleoyltrimethylammonium propane (DOTAP), dioleoylphosphatidylethanolamine (DOPE), dioleoylphosphatidylcholine (DOPC), and dipalmitoyltrimethylammonium propane (DPTAP) were investigated. The adsorption of small oligodeoxynucleotide DNA (14 base pairs) and large (1000-3000 base pairs) was examined and revealed that the behavior of the DNA-CL complex depends on the length and density of DNA. Once adsorbed to the bilayer, large DNA induced changes in the bilayer coverage of the mica support, while short DNA did not destabilize the bilayer. There was a strong dependence of the bilayer destabilization on the type of neutral helper lipid mixed with the cationic lipid, with DOTAP/DOPE being much less stable to DNA adsorption than DOTAP/DOPC. The characteristics of pure and mixed DOTAP, DOPE, and DOPC supported phospholipid bilayers were investigated to understand the stability changes induced by DNA. Images of the DNA-free phospholipid bilayer revealed the dependence of supported planar bilayer structure and stability on the chemical nature of the headgroup and on the phase of the bilayer. Finally, it was illuminating to image DNA on DPTAP bilayers as a function of temperature. When the system was examined above the L-alpha-L-beta phase transition, images similar to those recorded for DNA-DOTAP (L-alpha) and DNADOTAP/DOPC (L-alpha) were observed.