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The native DNA duplex may be viewed as a cooperatively-ordered H bonded structure, including well localized Watson–Crick base pairs and H-bounded networks of hydration parts of the DNA–solvent interface in the grooves of the helix. In this paper, we present the effect of different metal ions (Li + , Mg 2+ and Cu 2+ ) on the denaturational heat capacity increment (Δ C p ) for calf thymus DNA. Since the contribution from the ordered hydration water fraction disruption energy to the total enthalpy and heat capacity increment values of double helix melting is significant, it must be possible to detect the effect of metal ions on the structural ordering/disordering of the H-bounded network in the hydration shell of the DNA duplex. Experimental results suggest that Li + , which is preferentially adsorbed in the minor groove of B-DNA and should contribute significantly to the stabilization of B-form, has a pronounced influence on the value of Δ C p . For the system Mg 2+ –DNA, the values of Δ C p are also significant, which can be explained by the formation of inner hydration sphere complexes and immobilization of structural water by the grooves of the duplex, stabilizing the helix. The effect of Cu 2+ ions is much more pronounced. The satellite peaks of calf thymus DNA become lower as the concentration of Cu 2+ increases and for concentrations of Cu 2+ higher than 0.010 M, they disappear and Δ C p =0. This is indicative of the known preference of Cu 2+ for purins and GC rich sites of DNA, binding to the N7 of guanine. As a result, disruption of the base stacking and hydrogen bounded water networks in the grooves of the double helix takes place. |
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