7 research outputs found
Dielectric relaxation of DNA aqueous solutions
We report on a detailed characterization of complex dielectric response of
Na-DNA aqueous solutions by means of low-frequency dielectric spectroscopy (40
Hz - 110 MHz). Results reveal two broad relaxation modes of strength
20<\Delta\epsilon_LF<100 and 5<\Delta\epsilon_HF<20, centered at 0.5
kHz<\nu_LF<70 kHz and 0.1 MHz<\nu_HF<15 MHz. The characteristic length scale of
the LF process, 50<L_LF<750nm, scales with DNA concentration as
c_DNA^{-0.29\pm0.04} and is independent of the ionic strength in the low added
salt regime. Conversely, the measured length scale of the LF process does not
vary with DNA concentration but depends on the ionic strength of the added salt
as I_s^{-1} in the high added salt regime. On the other hand, the
characteristic length scale of the HF process, 3<L_HF<50 nm, varyes with DNA
concentration as c_DNA^{-0.5} for intermediate and large DNA concentrations. At
low DNA concentrations and in the low added salt limit the characteristic
length scale of the HF process scales as c_DNA^{-0.33}. We put these results in
perspective regarding the integrity of the double stranded form of DNA at low
salt conditions as well as regarding the role of different types of counterions
in different regimes of dielectric dispersion. We argue that the free DNA
counterions are primarily active in the HF relaxation, while the condensed
counterions play a role only in the LF relaxation. We also suggest theoretical
interpretations for all these length scales in the whole regime of DNA and salt
concentrations and discuss their ramifications and limitations.Comment: 15 pages, 9 figure
Short-fragment Na-DNA dilute aqueous solutions: Fundamental length scales and screening
Dielectric spectroscopy is used to investigate fundamental length
scales of 146 bp short-fragment (nucleosomal) dilute Na-DNA solutions.
Two relaxation modes are detected: the high- and the low-frequency mode.
Dependence of the corresponding length scales on the DNA and on the (uni-valent)
salt concentration is studied in detail, being different from the case of long,
genomic DNA, investigated before. In low-added-salt regime, the length scale of
the high-frequency mode scales as the average separation between DNAs, though it
is smaller in absolute magnitude, whereas the length scale of the low-frequency
mode is equal to the contour length of DNA. These fundamental length scales in
low-added-salt regime do not depend on whether DNA is in a double-stranded or
single-stranded form. On the other hand, with increasing added salt, the
characteristic length scale of the low-frequency mode diminishes at low DNA
concentrations probably due to dynamical formation of denaturation bubbles
and/or fraying in the vicinity of DNA denaturation threshold
Manning free counterion fraction for a rodlike polyion: Aqueous solutions of short DNA fragments in presence of very low added salt
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