Charge Inversion Of DNA Research Articles

DNA-Lysozyme Nanoarchitectonics: Quantitative Investigation on Charge Inversion and Compaction.

The interaction between DNA and proteins is fundamentally important not only for basic research in biology, but also for potential applications in nanotechnology. In the present study, the complexes formed by λ DNA and lysozyme in a dilute aqueous solution have been investigated using magnetic tweezers (MT), dynamic light scattering (DLS), and atomic force microscopy (AFM). We found that lysozyme induced DNA charge inversion by measuring its electrophoretic mobility by DLS. Lysozyme is very effective at neutralizing the positive charge of DNA, and its critical charge ration to induce charge inversion in solution is only 2.26. We infer that the high efficiency of charge neutralization is due to the highly positively charged (+8 e) and compact structure of lysozyme. When increasing the concentration of lysozymes from 6 ng·µL−1 to 70 ng·µL−1, DNA mobility (at fixed concentration of 2 ng·µL−1) increases from −2.8 to 1.5 (in unit of 10−4 cm2·V−1·S), implying that the effective charge of DNA switches its sign from negative to positive in the process. The corresponding condensing force increased from 0 pN to its maximal value of about 10.7 pN at concentrations of lysozyme at 25 ng·µL−1, then decreases gradually to 3.8 pN at 200 ng·µL−1. The maximal condensing force occurs at the complete DNA charge neutralization point. The corresponding morphology of DNA–lysozyme complex changes from loosely extensible chains to compact globule, and finally to less compact flower-like structure due to the change of attached lysozyme particles as observed by AFM.

Demonstration of pH-controlled DNA-surfactant manipulation for biomolecules.

The understanding of DNA–surfactant interactions is important for fundamental physical biology and developing biomedical applications. In the present study, we demonstrated a DNA–surfactant nano-machine model by modulating the compaction of DNA in dodecyldimethylamine oxide (DDAO) solutions. By controlling DDAO concentration and pH of solution, we are able to adjust the compacting force of DNA so as to pull biomolecular subunits connected to it. The pulling force of the machine depends on DDAO concentration and pH of solution, ranging from near zero to about 4.6 pN for 10 mM DDAO concentration at pH = 4. The response time of the machine is about 3 minutes for contracting and 2 minutes for releasing in 5 mM DDAO solution. We found that DDAO has no significant influence on DNA under basic conditions, but compacts DNA under acidic conditions, which is enhanced with decreasing pH of solution. Meanwhile, we found the accompanying charge inversion of DNA in the process of DNA compaction by DDAO.

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.

Single Molecular Demonstration of Modulating Charge Inversion of DNA.

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.

The effect of pH on charge inversion and condensation of DNA.

Charge inversion and condensation of DNA in solutions of trivalent and quadrivalent counterions are significantly influenced by the pH value of the solution. We systematically investigated the condensation and charge compensation of DNA by spermidine, hexammine cobalt(iii) (cohex, [Co(NH3)6](3+)) and spermine in solutions of a wide range of pH values from 3 to 9.3 by dynamic light scattering, magnetic tweezers, and atomic force microscopy. In trivalent counterion solution, we found that there is a critical concentration (0.75 mM for cohex and 0.5 mM for spermidine), under which the electrophoresis mobility of DNA initially increases, reaches a maximum, and finally decreases when the pH value is decreased. In contrast, above the critical concentration, the electrophoretic mobility of DNA increases monotonously with decreasing pH value of the solution. The corresponding condensing force has the same dependence on the pH value. However, for the case of quadrivalent counterions, the electrophoretic mobility of DNA is monotonously promoted by lowering the pH value of the solution at any concentration of counterions in which charge inversion of DNA may occur. In atomic force microscopy images and force spectroscopy of magnetic tweezers, we found that maximal charge neutralization and condensation force correspond to the most compact DNA condensation. We propose a mechanism of promoting DNA charge neutralization: small and highly mobile hydrogen ions tend to attach to the DNA-counterion complex to further neutralize its remaining charge, which is related to the surface area of the complex. Therefore, this further neutralization is prominent when the complex is toroidal which corresponds to the case of mild ion concentration while it is less prominent for more compact globules or rod complexes at high counterion concentration.

The suppression and promotion of DNA charge inversion by mixing counterions.

In the preset study, we report the suppression and promotion of DNA charge inversion by mixing a quadrivalent counterion (spermine) with mono-, di- and trivalent counterions by dynamic light scattering (DLS) and single molecule electrophoresis (SME) methods. We find that the electrophoretic mobility of DNA in spermine solution decreases in the presence of monovalent sodium ions and divalent magnesium ions. It means that the charge neutralization of DNA by the quadrivalent counterion is suppressed when adding extra mono- or divalent counterions. More specifically, at a high concentration of spermine, the positive mobility can switch back to a negative value by adding mono- and divalent counterions. Thus, charge neutralization and inversion of DNA by quadrivalent counterions is suppressed in the mono- and divalent ion solution. However, the scenario changes dramatically when we add trivalent ions into the solution of DNA and spermine. In this case, the charge neutralization and inversion of DNA is promoted rather than suppressed by mixing with trivalent ions. The negative electrophoretic mobility can be promoted to a positive value, which corresponds to the charge inversion, by trivalent counterions. Thus trivalent and quadrivalent counterions work cooperatively in DNA charge neutralization and inversion. This promotion also occurs when highly positively charged chitosan is introduced into the solution. We explain the observation by the counterion complexation that is related to DNA condensation, which is supported by the images of atomic force microscopy (AFM).

Charge inversion accompanies DNA condensation by multivalent ions

The condensation of stiff, highly charged DNA molecules into compact structures by condensing agents ranging from multivalent ions1 to small cationic proteins2,3 is of major biological and therapeutic importance4,5, yet the underlying microscopic mechanism remains poorly understood1,6,7,8,9. It has been proposed7,10 that DNA condensation is a purely electrostatic phenomenon driven by the existence of a strongly correlated liquid (SCL) of counterions at the DNA surface. The same theoretical argument predicts that multivalent counterions overcompensate the DNA charge when present at high concentration11, in turn destabilizing the condensates12. Here, we demonstrate the occurrence of DNA charge inversion by multivalent ions through measurements of the electrophoretic mobility of condensed DNA. By observing the multivalent-ion-induced condensation of a single DNA molecule using magnetic tweezers, we further show that charge inversion influences condensation by modulating the barrier for condensate nucleation in a manner consistent with the SCL mechanism.

Lattice-gas model of DNA charge inversion by a positively charged polyelectrolyte

The model of DNA charge inversion by Nguyen and Shklovskii [T. T. Nguyen and B. I. Shklovskii, Phys. Rev. Lett. 89, 018101 (2002)] is extended. A single double-helix strand of DNA is represented by a lattice of negative charges at the positions of the protruding oxygens of the phosphates along the DNA backbone, and the adsorbed polyelectrolyte molecules are represented by charged dimers. A lattice-gas model is used in which dimers adsorbing either parallel or perpendicular to the lattice are treated as separate species, and the model allows for vacancies between adsorbed species. The mean field theory used is formulated as a saddle-point approximation of the exact functional integral representation of the grand canonical partition function, opening the way for the inclusion of the effects of charge fluctuation corrections.

Inversion of DNA charge by a positive polymer via fractionalization of the polymer charge

Charge inversion of a DNA double helix by an oppositely charged flexible polyelectrolyte (PE) is widely used for gene delivery. It is considered here in terms of discrete charges of DNA. We concentrate on the worst scenario case when in the neutral state of the DNA-PE complex, each of the DNA charges is locally compensated by a PE charge and show that charge inversion exists even in this case. When an additional PE molecule is adsorbed by DNA, its charge gets fractionalized into monomer charges of defects (tails and arches) on the background of the perfectly neutralized DNA. These charges spread all over the DNA eliminating the self-energy of PE. Fractionalization leads to a substantial charge inversion of DNA.

Model of Inversion of DNA Charge by a Positive Polymer: Fractionalization of the Polymer Charge

We model one strand of DNA by a one-dimensional lattice (ODL) of negative charges and consider the problem of inversion of its charge by a positive polyelectrolyte (PE). In the neutral state of the ODL-PE complex, each of the ODL charges is locally compensated by a PE charge. When an additional PE molecule is adsorbed by ODL, its charge gets fractionalized into monomer charges of defects (tails and arches) on the background of the perfectly neutralized ODL. Defects spread all over the ODL, eliminating the self-energy of PE. For DNA this fractionalization mechanism leads to a substantial inversion of charge, a phenomenon which is widely used for gene delivery.

Inversion of DNA charge by a positive polymer via fractionalization of the polymer charge

Charge inversion of a DNA double helix by an oppositely charged flexible polyelectrolyte (PE) is widely used for gene delivery. It is considered here taking into account the discreteness of the charges of DNA. We concentrate on the worst scenario case when in the neutral state of the DNA-PE complex, each of the DNA charges is locally compensated by a PE charge and show that charge inversion exists even in this case. When an additional PE molecule is adsorbed by DNA, its charge gets fractionalized into monomer charges of defects (tails and arches) on the background of the perfectly neutralized DNA. These charges spread all over the DNA eliminating the self-energy of PE. Fractionalization leads to a substantial charge inversion of DNA. We show that fractionalization mechanism of charge inversion works also for non-linear polymers such as dendrimers. Remarkably, such fractionalization happens for adsorption of both PE or dendrimers on a two-dimensional charged lattice as well. Relation of fractionalization to other mechanisms of charge inversion is discussed.