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