DNA Compaction and Charge Inversion Induced by Organic Monovalent Ions

DNA condensation and charge inversion usually occur in solutions of multivalent counterions. In the present study, we show that the organic monovalent ions of tetraphenyl chloride arsenic (Ph₄As⁺) can induce DNA compaction and even invert its electrophoretic mobility by single molecular methods. The morphology of condensed DNA was directly observed by atomic force microscopy (AFM) in the presence of a low concentration of Ph₄As⁺ in DNA solution. The magnetic tweezers (MT) measurements showed that DNA compaction happens at very low Ph₄As⁺ concentration (≤1 μM), and the typical step-like structures could be found in the extension-time curves of tethering DNA. However, when the concentration of Ph₄As⁺ increased to 1 mM, the steps disappeared in the pulling curves and globular structures could be found in the corresponding AFM images. Electrophoretic mobility measurement showed that charge inversion of DNA induced by the monovalent ions happened at 1.6 mM Ph₄As⁺, which is consistent with the prediction based on the strong hydrophobicity of Ph₄As⁺. We infer that the hydrophobic effect is the main driving force of DNA charge inversion and compaction by the organic monovalent ion.