Abstract
It is well known that common trivalent counter ions can induce DNA compaction or condensation but are unable to invert DNA surface charge in a normal aqueous solution. In the present study, we found that trivalent-hydrolysed metal ions (Fe3+, Al3+) are not only capable of inducing DNA condensation, but they also invert the electrophoretic mobility of DNA by electrophoretic light scattering and single molecular techniques. In comparison with neutral trivalent cations, hydrolysed metal ions such as Fe3+ can induce DNA condensation at a much lower concentration of cations, and its corresponding morphology of condensed DNA was directly observed by atomic force microscopy (AFM). The condensing and unravelling forces of DNA condensates were measured by tethering DNA by magnetic tweezers (MT) measurements at various concentration of Fe3+ and Al3+. We found that a coil–globule transition of DNA by hydrolysed metal ions not only was observed in DNA-complex sizes, but also in the curve of electrophoretic mobility of DNA in solution. In contrast, the transition was not observed in the case of neutral trivalent cations such as La3+ and Co3+. We attribute the transition and charge inversion to the ion-specific interaction between hydrolysed metal ions and phosphates of DNA backbone.
Highlights
DNA is an important biological polyelectrolyte with a high density of negative charge, resulting in very strong Coulomb repulsion between the nucleic acid segments
We focus on the effects of the trivalent metal ions Al3+ and Fe3+, since they are related to the effects of pH variations arising from the metal hydrolysis
The measured electrokinetic properties of DNA in varous trivalent cations ion solution are shown in Figure 2, in which the electrophoretic mobility (EM) of DNA is plotted versus the concentrations of counterions
Summary
DNA is an important biological polyelectrolyte with a high density of negative charge, resulting in very strong Coulomb repulsion between the nucleic acid segments. Physicochemical studies of the large structural changes of long DNAs are important in molecular biology but are helpful in developing new techniques such as DNA extraction and gene therapy, in which DNA is compacted and transfected into cells and tissues to treat some genetic-related diseases [2,3,4] Many condensing agents, such as multivalent cations, which are basic proteins, are able to induce. The ionic specificity arises, because, in addition to long-range electrostatic interactions, short-range interactions, which are quantum mechanical in origin and highly specific to the ion and the interfacial charged group, are in play and need to be considered In reality, both mechanisms might be in action simultaneously, more or less. The results have been analyzed theoretically in terms of the classical two-state model and cooperative phase transition model of Zimm–Bragg, indicating that the site specific binding of the counterions is the main reason of the discrete transition
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