DNA Compaction at Surfaces

Abstract: The interfacial behavior of DNA and DNA-cationic surfactant has been studied. Particular attention was paid to the type of cationic surfactant and the DNA conformation (single or double stranded) as well as the DNA length. Adsorption of DNA-cationic surfactant complexes is determined by a delicate balance between the DNA-surface, cationic surfactant-surface, and cationic surfactant-DNA-solvent interactions. On negatively charged surfaces, where DNA experiences an effective electrostatic repulsion, adsorption of the DNA-cationic surfactant is favored once the charge density of both the DNA macromolecule and the surface is sufficiently decreased, due to the interaction with cationic surfactant ions. On hydrophobic surfaces, where both DNA and cationic surfactant adsorb, a considerable increase in adsorbed amount and a significant layer compaction is observed for DNA-cationic surfactant complexes. The stability of the mixed adsorbed layer depends largely on the strength of the interaction between DNA and the cationic surfactant, as well as between the surface and the cationic surfactant. The interaction between DNA and small latex particles is similar to that of DNA and macroscopic hydrophobized flat surfaces. Cationic surfactant aggregates that are large and most likely non-spherical were identified within the DNA-cationic surfactant complexes far below the phase separation border. Moreover, the presence of a hydrophobized surface was found to facilitate the DNA compaction by cationic surfactant. Finally, the effect of DNA on an insoluble cationic surfactant film at the air-water interface was investigated. Addition of DNA expands the cationic lipid monolayer with the consequent disruption of condensed aggregates into a featureless liquid-expanded phase.