Role of Electrostatic Interactions on Supramolecular Organization in Calf-Thymus DNA Solutions under Flow

L. Bravo-Anaya , Denis Roux (1) , J. Soltero Martínez , Francisco Carvajal Ramos , Frédéric Pignon (1) , Oonagh Mannix , Marguerite Rinaudo (2)

1 LR - Laboratoire de rhéologie
2 CERMAV - Centre de Recherches sur les Macromolécules Végétales

Previous investigations were conducted on two concentrations of DNA solution: 4 18 mg/mL, for which it has been shown that no supramolecular organization is induced under flow at 19 low shear rates; and 10 mg/mL, in which a liquid crystalline-type texture is formed under flow at 20 low shear rates, attesting to an orientation of pre-organized chains. Rheological experiments are 21 discussed and their results supported by small-angle X-ray scattering (SAXS) and flow birefringence 22 visualization experiments. Scattering from polyelectrolytes has a characteristic signal, which is here 23 observed in SAXS, showing a strong correlation peak between charged chains in water, for both 24 concentrations. This peak is weaker in the presence of 0.01 M NaCl and suppressed in salt excess at 25 0.1 M NaCl. No plateau in the σ(í µí»¾´)µí»¾´) plot was observed in analysis of rheological experiments on low 26 DNA concentration (4 mg/mL). As typically observed in polyelectrolyte systems both the dynamic 27 moduli and shear viscosity were higher in water as electrostatic forces dominate, than in the 28 presence of salt, especially at low shear rates. The rheological results for concentrations of 0.01 M 29 NaCl are lower than in water as expected due to partial screening of electrostatic repulsions. 30 Rheological data for concentrations of 0.1 M NaCl are unexpected. Electrostatic forces are partially 31 screened in the low salt concentration, leading to a drop in the rheological values. For high salt 32 concentration there are no longer interchain repulsions and so steric interactions dominate within 33 the entangled network leading to the subsequent increase in rheological parameters. Regardless of 34 the solvent, at high shear rates the solutions are birefringent. In the 10 mg/mL case, under flow, 35 textures are formed at relatively low shear rate before all the chains align going to a pseudo-nematic 36 liquid crystalline phase at high shear rate. The electrostatic repulsion between semi-rigid chains 37 induces a correlation between the chains leading to an electrostatic pseudo-gel in water and loosely 38 in 0.01M NaCl at low stress applied. To our knowledge, this is the first time that such behavior is 39 observed. In 0.1M NaCl, DNA behavior resembles the corresponding neutral polymer as expected 40 for polyelectrolyte in salt excess, exhibiting a yield stress. When texture appears in water and in 41 0.01M NaCl, a critical transition is observed in rheological curves, where the viscosity decreases 42 sharply at a given critical shear stress corresponding to a plateau in the σ(í µí»¾´)µí»¾´) plot also observed in 43 creep transient experiment. 44

Mots clés: electrostatic; rheology; birefringence; SAXS; external ionic concentration; DNA

Domaines: Matière Molle [cond-mat.soft]; Mécanique des matériaux [physics.class-ph]; Matériaux et structures en mécanique [physics.class-ph]; Mécanique des fluides [physics.class-ph]

L. Bravo-Anaya, Denis Roux, J. Soltero Martínez, Francisco Carvajal Ramos, Frédéric Pignon, et al.. Role of Electrostatic Interactions on Supramolecular Organization in Calf-Thymus DNA Solutions under Flow. Polymers, 2018, 10 (11), pp.1204. ⟨10.3390/polym10111204⟩. ⟨hal-01989793⟩