  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.
