Abstract
Previous investigations were conducted on two concentrations of DNA solution: 4 mg/mL, for which it has been shown that no supramolecular organization is induced under flow at low shear rates; and 10 mg/mL, in which a liquid crystalline-type texture is formed under flow at low shear rates, attesting to an orientation of pre-organized chains. Rheological experiments are discussed and their results supported by small-angle X-ray scattering (SAXS) and flow birefringence visualization experiments. Scattering from polyelectrolytes has a characteristic signal, which is here observed in SAXS, showing a strong correlation peak between charged chains in water, for both concentrations. This peak is weaker in the presence of 0.01 M NaCl and suppressed in salt excess at 0.1 M NaCl. No plateau in the σ() plot was observed in analysis of rheological experiments on low DNA concentration (4 mg/mL). As typically observed in polyelectrolyte systems both the dynamic moduli and shear viscosity were higher in water as electrostatic forces dominate, than in the presence of salt, especially at low shear rates. The rheological results for concentrations of 0.01 M NaCl are lower than in water as expected due to partial screening of electrostatic repulsions. Rheological data for concentrations of 0.1 M NaCl are unexpected. Electrostatic forces are partially screened in the low salt concentration, leading to a drop in the rheological values. For high salt concentration there are no longer interchain repulsions and so steric interactions dominate within the entangled network leading to the subsequent increase in rheological parameters. Regardless of the solvent, at high shear rates the solutions are birefringent. In the 10 mg/mL case, under flow, textures are formed at relatively low shear rate before all the chains align going to a pseudonematic liquid crystalline phase at high shear rate. The electrostatic repulsion between semi-rigid chains induces a correlation between the chains leading to an electrostatic pseudo-gel in water and loosely in 0.01 M NaCl at low stress applied. To the best of our knowledge, this is the first time that such behavior is observed. In 0.1 M NaCl, DNA behavior resembles the corresponding neutral polymer as expected for polyelectrolyte in salt excess, exhibiting a yield stress. When texture appears in water and in 0.01 M NaCl, a critical transition is observed in rheological curves, where the viscosity decreases sharply at a given critical shear stress corresponding to a plateau in the σ() plot also observed in creep transient experiment.
Highlights
DNA is a semi-rigid polyelectrolyte with a high persistence length (Lp~50 nm) [1,2,3].Understanding electrostatic interactions between stiff chain segments in DNA and polyelectrolyte solutions is important, both fundamentally and for biological applications [4]
These interactions depend on the charge density of the polyelectrolyte favoring parallel alignment of the chains, and on the external salt concentration screening the electrostatic repulsions [5]
The aim of this paper is to identify the appearance of different flow regions and, the conditions at which the onset of a stress plateau appears in flow experiments in relation to the electrostatic repulsions
Summary
DNA is a semi-rigid polyelectrolyte with a high persistence length (Lp~50 nm) [1,2,3]. Understanding electrostatic interactions between stiff chain segments in DNA and polyelectrolyte solutions is important, both fundamentally and for biological applications [4]. These interactions depend on the charge density of the polyelectrolyte favoring parallel alignment of the chains, and on the external salt concentration screening the electrostatic repulsions [5]. DNA may be considered a good model for the study of dynamics and macroscopic properties of semi-rigid macromolecules in solution. Of particular interest is the transformation from steady-state flow to so-called bulk shear inhomogeneities
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