DNA Condensation And Charge Inversion Induced By Organic Monovalent Ions

DNA condensation is the process of DNA from free stretch into order and compact state in good solvent, and it plays an important role in multi-field including life science, polymer physics and applied technology. The research results of DNA condensation for nearly half a century show that DNA condensation and charge inversion usually occur in solution of multivalent counterions. However, we found that the organic monovalent ions can also make DNA condensation.In present study, we show that the organic monovalent ions of tetraphenyl chloride arsenic(Ph4As+) can induce DNA condensation and even invert its electrophoretic mobility by single molecular methods. The morphology of condensed DNA was directly observed by atomic force microscopy (AFM) in presence of low concentration of Ph4As+ in DNA solution. The magnetic tweezers (MT) measurements show that DNA condensation happens at very low concentration (?1?M) of Ph4As+, and the typical step-like structures can be found in the extension-time curves of tethering DNA . However, when the concentration of Ph4As+grows up to 1mM, the steps disappear in the pulling curves and the globular structures can be found in the corresponding AFM images. Electrophoretic mobility measurement shows that charge inversion of DNA induced by the monovalent ions happens at 1.75mM of Ph4As+, which is consistent of 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 condensation by the organic monovalent ion.