Danijel Grgičin, Sanja Dolanski Babić, Tomislav Ivek, Silvia Tomić, Rudi Podgornik
The effect of magnesium ion Mg2+ on the dielectric relaxation of semidilute DNA aqueous solutions has been studied by means of dielectric spectroscopy. Two dielectric relaxations in the 100 Hz - 100 MHz frequency range, originating in the motion of DNA counterions, were probed as a function of DNA and Mg2+ ion concentration in added MgCl2 salt. The high-frequency mode in the MHz range, stemming from the structural organization of the DNA network, reveals de Gennes-Pfeuty-Dobrynin correlation length as the pertinent fundamental length scale for sufficiently low concentration of added salt. No relaxation fingerprint of 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 as compared to the case of DNA solutions with univalent counterions. The presence of Mg2+ does not change qualitatively the low frequency mode in the kHz range correlated with single DNA conformational properties. It does, however, introduce some changes in the effective size of the DNA molecule and in the 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.
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http://arxiv.org/abs/1307.0715
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