Manning free counterion fraction for a rodlike polyion: Aqueous solutions of short DNA fragments in presence of very low added salt

Abstract

We quantified the Manning free (uncondensed) counterions fraction θ for dilute aqueous solutions of rodlike polyions: 150 bp DNA fragments, in the presence of a very low concentration of monovalent salt c(salt)<0.05 mM. Conductivity measurements of these solutions for DNA base pair concentration range 0.015≤c≤8 mM were complemented by fluorescence correlation spectroscopy (FCS) measurements of the DNA polyion diffusion coefficient D(p)(c). We observed a crossover in the normalized conductivity σ(c)/c that nearly halved across the c=0.05-1 mM range, while D(p)(c) remained rather constant, as we established by FCS. Analyzing these data we extracted θ(c)=0.30-0.45, and taking the Manning asymmetry field effect on polyelectrolyte conductivity into account we got θ(c)=0.40-0.60. We relate the θ(c) variation to gradual DNA denaturation occurring, in the very low salt environment, with the decrease in DNA concentration itself. The extremes of the experimental θ(c) range occur toward the highest, above 1 mM, and the lowest, below 0.05 mM, DNA concentrations and correspond to the theoretical θ values for dsDNA and ssDNA, respectively. Therefore, we confirmed Manning condensation and conductivity models to be valuable in description of dilute solutions of rodlike polyions.

Generalized polymer effective charge measurement by capillary isotachophoresis

“…Different methods have been investigated for polymer effective charge determination including conductivity measurements [14], osmotic pressure [15], scattering techniques [16] and electrophoretic techniques [17][18][19][20][21][22][23][24][25]. Conductivity and osmotic pressure measurements are generally performed in the absence of additional salts (or at very low ionic strength), while neutron scattering techniques requires specific and restricted-access source of radiations.…”

Effect of Ionic Ordering in Conductivity Experiments of DNA Aqueous Solutions

“…As known the electric current in DNA water solutions is caused by the motion of counterions and DNA macromolecules [17][18][19][20][21][22][23]. In case of DNA solutions without added salt (salt free solution) the conductivity increases as the concentration of DNA increases because of counterion dynamics in the ion-hydrate shell of macromolecule [17,18].…”

Crowding-induced Cooperativity in DNA Surface Hybridization

“…While helix state dsDNA is modeled as rigid rod, which can rotate freely on its anchored point. Due to counterion condensation effect, dsDNA is assumed to take 0.75 e − charge per nucleotide pair, while ssDNA takes 0.5 e − per nucleotide, according to recent experimental results52. Cations ( Na + ) and anions ( Cl − ) are explicitly included and are assumed to have a hydrated radius of 0.35 nm, while solvents only enter the theory implicitly through the dielectric constant ε = 7853.…”