Abstract
Charge inversion of DNA is a counterintuitive phenomenon in which the effective charge of DNA switches its sign from negative to positive in the presence of multivalent counterions. The underlying microscopic mechanism is still controversial whether it is driven by a specific chemical affinity or electrostatic ion correlation. It is well known that DNA shows no charge inversion in normal aqueous solution of trivalent counterions though they can induce the conformational compaction of DNA. However, in the same trivalent counterion condition, we demonstrate for the first time the occurrence of DNA charge inversion by decreasing the dielectric constant of solution to make the electrophoretic mobility of DNA increase from a negative value to a positive value. In contrast, the charge inversion of DNA induced by quadrivalent counterions can be canceled out by increasing the dielectric constant of solution. The physical modulation of DNA effective charge in two ways unambiguously demonstrates that charge inversion of DNA is a predominantly electrostatic phenomenon driven by the existence of a strongly correlated liquid (SCL) of counterions at the DNA surface. This conclusion is also supported by the measurement of condensing and unraveling forces of DNA condensates by single molecular MT.
Highlights
Charge inversion (CI) is a counterintuitive phenomenon in which a macroion in solution of multivalent counterions attracts more opposite charges in excess of its own nominal charge so that its effective charge changes its sign
An electron intensified charge-coupled device (ECCD) camera is used for video recording, and the electrophoretic mobility (EM) of fluorescence dyed single DNA can be measured by analyzing the video
CI cannot be explained by conventional mean-field theories of charge screening
Summary
Charge inversion (CI) is a counterintuitive phenomenon in which a macroion in solution of multivalent counterions attracts more opposite charges in excess of its own nominal charge so that its effective charge changes its sign. Lemay group[6] observed the reversal of the polarity of charged surfaces in water upon the addition of trivalent and quadrivalent ions by atomic force microscopy They found that the bulk concentration of multivalent ions at which CI occurs depends almost only on the valence of counterions. The effect of dielectric constant and finite ion size modify ion-mediated forces between DNA molecules and are quantitatively investigated by the Monte Carlo method[12] Both strong correlation theory and numerical simulation predict the CI of DNA upon the high concentration of trivalent or quadrivalent counterions, it has not been observed for DNA in trivalent solution regardless its high concentration. We found that dielectric constants of the solution modulate the condensing and unraveling forces significantly
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