Abstract
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.
Highlights
DNA is one of the most important biological polyelectrolytes, and is highly negatively charged in solution
We introduce the organic monovalent ion Ph4 As+ into a DNA system and find that it provokes the charge inversion of DNA, and leads to DNA compaction, which is the first experimental evidence for DNA compaction induced by monovalent cations
The mechanism of DNA charge reversal is similar to colloid charge inversion, the hydrophobic effect as a driving force for charge inversion
Summary
DNA is one of the most important biological polyelectrolytes, and is highly negatively charged in solution. The related DNA compaction from bulk solution critically depends on the valence of the counterions, and a valence of three or larger is required to overcome the inherently large electrostatic repulsive barrier between the like-charged polyelectrolytes [30,31,32,33]. Martin-Molina et al [36] proposed a new mechanism for the charge inversion of colloids in electrolyte solutions based on the hydrophobic effect. They observed charge inversion due to the organic monovalent ion Ph4 As+ in colloids, and the effect was attributed to the hydrophobic effect. The phenomenon seems to be similar to the poor solvent effect of neutral condensing agents such as ethanol in DNA solution [37]
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