Manning free counterions fraction for a rod-like polyion - short DNA fragments in very low salt

TL;DR

This study measures the fraction of free counterions around dilute rod-like DNA fragments in very low salt conditions, revealing how DNA denaturation affects counterion condensation and validating Manning models.

Contribution

It provides experimental quantification of free counterion fractions for DNA in ultra-low salt, linking counterion behavior to DNA denaturation and confirming theoretical models.

Findings

Counterion fraction varies from 0.30 to 0.60 with concentration.

Conductivity decreases significantly across the low concentration range.

DNA denaturation correlates with changes in counterion condensation.

Abstract

We quantified the Manning free (uncondensed) counterions fraction $\theta$ for dilute solutions of rod-like polyions - 150bp DNA fragments, in very low salt $<0.05$mM. Conductivity measurements of aqueous DNA solutions in the concentration range $0.015\leq c \leq 8$~mM (bp) 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 $\sigma(c)/c$ which nearly halved across $c=0.05-1$ mM range, while $D_p(c)$ remained rather constant, as we established by FCS. Analyzing these data we extracted $\theta(c)=0.30 - 0.45$, and taking the Manning asymmetry field effect on polyelectrolyte conductivity into account we got $\theta(c)=0.40-0.60$. We relate the $\theta(c)$ variation to gradual DNA denaturation occuring, in the very low salt environment, with the decrease in DNA concentration itself. The extremes of the experimental $\theta(c)$ range occur towards the highest, above 1 mM and the lowest, below 0.05 mM, DNA concentrations, and correspond to the theoretical $\theta$ values for dsDNA and ssDNA, respectively. Therefore, we confirmed Manning condensation and conductivity models to be valuable in description of dilute solutions of rod-like polyions.