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UV spectroscopy and conductometry of Mg-DNA aqueous solutions

Dynamics and structure of aqueous DNA solutions with univalent counterions , as studied by dielectric spectroscopy, are well known [1]. Now, we are interested in properties of DNA aqueous solutions with polyvalent counterions in order to study behaviour in the vicinity of the attractive (correlation) regime of electrostatic interactions. The first step is to study DNA with divalent counterions. So, we prepared Mg-DNA by dialyzing Na-DNA (Sigma D1626) against solution. DNA degree of polymerization is in range 2-20 kbp. Corresponding contour length is since DNA monomer size is . Mg-DNA concentration in the pure water and added salt solutions we determined using the UV spectrophotometry in order to compare it with gravimetrically determined concentrations . We find that they agree well, that is, is concentration-independent. This result suggests preservation of double stranded (ds) nature of Mg-DNA even in pure water. We measured dc electrical conductivity of pure water Mg-DNA solutions at in the concentration range . We found that normalized Mg-DNA conductivity is concentration-independent in contrast to the Na-DNA which shows a two fold increase going from high to the low DNA concentration. Concentration independent , as well as , indicate better stability of DNA with divalent counterions in contrast to DNA with univalent counterions. This finding complies with UV-spectrophotometry results by Record [2]. Our second result is that of pure water Mg-DNA solution is about two times smaller than the conductivity of Na-DNA pure water solutions above 0.5 mg/mL where both keep native dsDNA conformation. Polyelectrolyte conductivity is understood to be composed of two contributions. The first one is due to the free counterions while the second one is due to the polyions: , where is DNA molar basepair concentration, and are counterion and polyion molar conductivities, while is free counterion fraction, inverse of Manning parameter [3]. Since Mg valency is two times Na valency, the Manning parameter of Mg-DNA is also two times larger. This means that is two times smaller for Mg-DNA, which complies well with our mon results. We remind here that and note that the above necessitates for both Mg-DNA and Na-DNA to be similar. To have similar , Mg-DNA would, due to a lower effective charge, also have to assume a more compact conformation than Na-DNA. 1. S Tomić, S. Dolanski Babić, T. Vuletić, S. Krča, D. Ivanković, L. Griparić, R. Podgornik, Phys. Rev. E 75, 021905 (2007). 2. M. T. Record, Biopolymers 14, 2137 (1975). 3. G. S. Manning, J. Chem. Phys. 51, 924 (1969). ; ibid 51, 934 (1969).

dynamics of polyelectrolytes; DNA; polyvalent counterions; UV spectrophotometry; conductometry

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