Electrophoretic properties of complexes between DNA and the cationic surfactant cetyltrimethylammonium bromide

Abstract

Electrophoretic properties of complexes between DNA and the cationic surfactant cetyltrimethylammonium bromideWe use agarose gel electrophoresis to characterize how the monovalent catioinic surfactant cetyltrimethylammonium bromide (CTAB) compacts double-stranded DNA, which is detected as a reduction in electrophoretic DNA velocity. The velocity reaches a plateau at a ratio R = 1.8 of CTAB to DNA-phosphate charges, i.e., above the neutralization point, and the complexes retain a net negative charge at least up to R = 200. Condensation experiments on a mixture of two DNA sizes show that the complexes formed contain only one condensed DNA molecule each. These CTAB-DNA globules were further characterized by time-resolved measurements of their velocity inside the gel, which showed that CTAB does not dissociate during the migration but possibly upon entry into the gel. Using the Ogston-model for electrophoresis of spherical particles, the measured in-gel velocity of the globule is quantitatively consistent with CTAB having two opposite effects, reduction of both the electrophoretic charge and DNA coil size. In the case of CTAB the two effects nearly cancel, which can explain why opposite velocity shifts (globule faster than uncomplexed DNA) have been observed with some catioinic condensation agents. Dissociation of the complexes by addition of anionic surfactants was also studied. The DNA release from the globule was complete at a mixing ratio between anionic and cationic surfactants equal to 1, in agreement with equilibrium studies. Circular DNA retained its supercoiling, and this demonstrates a lack of DNA nicking in the compaction-release cycle which is important in DNA transfection and purification applications. IntroductionComplexation with cationic lipids is one strategy for delivery of DNA to cells. Binding of the lipids leads to compaction of the DNA coils and to a reduction of its surface charge. Both effects are believed to contribute to the facilitated uptake of the nucleic acids through the cellular membrane [1][2][3][4][5]. Methods to monitor size and charge of the complexes are therefore helpful tools in the process of screening protocols for packaging the DNA before in vivo experiments are performed. Commonly a titration series is made where the ratio R between concentration of charges from cationic lipid and DNA-phosphate groups is varied, usually in a range that includes the point R = 1 where the number of added positive and negative charges is equal. It is widely accepted that surfactants display a very strong associative binding with DNA inducing its compaction [6][7][8], aggregation [9], and precipitation [8,10,11]. The associative phase behavior presented by these systems and the absence of redissolution with the addition of an excess of surfactant lead to the assumption that the formed complexes in solution are neutral [8,12].Electrophoresis has been used to study the properties of lipoplexes because the mobility reflects their net charge. Mobilities in free solution have been used [13,14] to q...

DNA Compaction Induced by a Cationic Polymer or Surfactant Impact Gene Expression and DNA Degradation

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Interaction between DNA and Cationic Surfactants: Effect of DNA Conformation and Surfactant Headgroup

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