Structure and Forces in Transfection Related Surfactant Systems

Experiments have been performed on the cationic bilayer and liposome-forming surfactant DODAB, a successful DNA transfection agent. The surface forces apparatus (SFA) was utilized to measure the long-range colloidal and short-range adhesion forces between Langmuir-Blodgett bilayers exposing DODAB in the outer monolayers. Forces were measured in aqueous solutions of varying salt concentrations between 1 and 10 mM, with and without single-stranded DNA added to the solution. These represent the first measurements of the forces between surfactant/DNA assemblies. At low salt concentrations without DNA, the interbilayer forces are repulsive due to their electrostatic interaction, in agreement with DLVO theory. In contrast, in the presence of DNA which adsorbs to the bilayer surfaces, the forces between the bilayers changed to an enhanced and longer ranged repulsive steric-electrostatic interaction. At shorter range. the DNA can be excluded from the gap leading to an enhanced adhesion at contact accompanied by lipid rearrangement.