  The effect of magnesium ion Mg2+ on the dielectric relaxation of semidilute
DNA aqueous solutions has been studied by means of dielectric spectroscopy in
the 100 Hz-100 MHz frequency range. De Gennes-Pfeuty-Dobrynin semidilute
solution correlation length is the pertinent fundamental length scale for
sufficiently low concentration of added salt, describing the collective
properties of Mg-DNA solutions. No relaxation fingerprint of the DNA
denaturation bubbles, leading to exposed hydrophobic core scaling, was detected
at low DNA concentrations, thus indicating an increased stability of the
double-stranded conformation in Mg-DNA solutions as compared to the case of
Na-DNA solutions. Some changes are detected in the behavior of the fundamental
length scale pertaining to the single molecule DNA properties, reflecting
modified electrostatic screening effects of the Odijk-Skolnick-Fixman type. All
results consistently demonstrate that Mg2+ ions interact with DNA in a similar
way as Na1+ ions do, their effect being mostly describable through an enhanced
screening.
