Abstract
The DNA-lipid bilayer interaction has been widely studied due to the potential applications in gene therapy. However, the interaction with the lipid monolayer at the air-water interface is still not well studied. In this research, we have studied the mixed monolayers and their interaction with DNA by measuring the pressure-surface area isotherm, neutron and X-ray reflectivity. A synthetic charged lipid, DC-Chol (3β-[N-(dimethylaminoethane)-carbamoyl]cholesterol), was used to mix with the DPPC (1,2-dipalmitoyl-sn-glycero-3-phosphocholine) to form monolayers at the air-liquid interface. The pressure-area isotherm showed that typical plateau of DPPC disappears by adding DC-Chol more than 30%. It was observed by BAM that the domains formed in the plateau region of the isotherm become smaller by adding small amounts of DC-Chol. In-situ neutron reflectivity was used to study the DC-Chol and TC-Chol monolayers at the air-liquid interface in the presence and absence of DNA in the subphase. It was found that the DC-Chol is more effective in adsorbing the DNA than the TC-Chol. From the determined neutron scattering length density profiles, the adsorbed DNA somewhat penetrates into the head group region of the charged lipids. As for the DNA interaction with DPPC monolayer, the reflectivity data showed that the film thickness of the DPPC/DNA LB film is very close to that from the DPPC film which indicates there is very little DNA adsorption by the DPPC monolayer. As for the DNA adsorption by the mixed DPPC/DC-Chol monolayers, the LB film thickness increases as the percentage of DC-Chol increases which indicates that DNA molecules interact more strongly with the DC-Chol due to electrostatic force. The effect of adding divalent ions on the DNA adsorption by cationic lipid monolayer at the air-water interface was also investigated by X-ray reflectivity for Langmuir-Blodgett (LB) films supported on silicon wafers and in-situ neutron reflectivity. It was found that adding divalent ions, such as calcium ions, can enhance the DNA adsorption by the DC-Chol monolayer. The adsorbed DNA layer thickness was found to increase with the increase of divalent ion concentrations. The enhanced DNA adsorption can be attributed to the ion-mediated attractive interaction between the DNA strands. However, no sharp condensation transition was found as in the DNA/bilayer complex. It was also found that pH can also affect the DNA adsorption by the lipid monolayer. By lowering the pH, thicker DNA layer was adsorbed to the DC-Chol lipid monolayer. The enhanced DNA condensation to the DC-Chol monolayer could be due to denatured DNA at low pH. In the denatured state, some hydrophobic parts of the DNA could be exposed and cause DNA condensation.