DNA is one of most important biological polyelectrolytes,which is negatively charged in physiological condition.Most of Its charge is neutralized by attracting cations in solution.In some conditions,the effective charge of DNA switches its sign from negative to positive,implying charge inversion of DNA.The underlying microscopic mechanism of the counterintuitive phenomenon is still not fully understood although specific chemical affinity and electrostatic ion correlation are considered as two possible driving forces.In this review,we present some recent experimental progress in the modulation and control of DNA charge by single molecular techniques.It has been shown that DNA charge inversion can be modulated bidirectionly by decreasing or increasing the dielectric constant of solution to make the electrophoretic mobility of DNA increase from a negative value to a positive value.In this meanwhile,charge inversion and condensation of DNA in solution of trivalent and quadrivalent counterions are significantly influenced by pH value of the solution.When mixing quadrivalent counterion with mono-,di-and tri-valent counterions in solution,suppression and promotion of DNA charge inversion can be observed.In addition,hydrophobic effect can play an important role in DNA charge inversiton and compaction.We show that the organic monovalent ions of tetraphenyl chloride arsenic (PhnAs+) can induce DNA compaction and even invert its electrophoretic mobility.Thus,hydrophobic effect can be the main driving force of DNA charge inversion and compaction by the organic monovalent ion.
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