Compaction and decompaction of DNA dominated by the competition between counterions and DNA associating with cationic aggregates

• The decompaction of DNA largely depends on the counterion species of cationic surfactants.
• The competition between DNA and counterions dominates the conformational changes.
• A reasonable three-step model was promoted.

A systematic work concerning the DNA compaction and decompaction controlled by cationic surfactants, cetyltrimethylammonium with [FeCl3Br]− (CTAFe), Br− (CTABr) and Cl− (CTACl) as counterions, respectively, was performed. We discovered that cationic surfactants with complex counterions, [FeCl3Br]−, cannot promote the decompaction of DNA like those with Br− and Cl− as counterions. The rod-like CTAFe micelles were found to remain free in supernatants and cannot directly promote any redissolution or decompaction of DNA. These interesting findings could provide a better understanding of the interaction behavior of DNA and cationic surfactants. We conclude that the fundamental reason of the DNA decompaction lies upon the electrostatic competition between the counterions and DNA for associating with the cationic aggregates. At a high concentration, the binding of counterions to cationic CTA+ aggregates is promoted, which weakens and screens the electrostatic attraction between DNA and cationic aggregates. This could cause the decompaction of DNA as the cases of CTABr/DNA and CTACl/DNA mixtures. Our data revealed the fundamental reason of the compaction and decompaction behavior of DNA induced by cationic surfactants independently, a reasonable three-step model of the conformational changes of DNA controlled by different amounts of cationic surfactants was presented. The current work could provide a clear guidance in gene delivery, gene therapy and biomedicine fields.

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