Low-temperature DSC study of the hydration of ss-DNA and ds-DNA and the role of hydrogen-bonded network to the duplex transition thermodynamics

The study of thermodynamic parameters of ice–water phase transition in aqueous solutions of ss-DNA, at different concentration of polynucleotide single chains is reported and the critical concentration for which the enthalpy of transition is zero has been determined. The thermodynamic parameters of ice–water phase transitions, obtained for native ds-DNA and for ss-DNA in the whole concentration region (0.2–2.0 g H2O/g DNA) and the hydration values (bound water quantity) for native (helix), denatured (single strand coils) and unfolded ss-DNA are compared. The critical values of the hydration at which all the water in the DNA–H2O system exists in the bound (unfrozen) state are established, for these conformations, with great precision: Nds-DNAΣ=(0.55±0.01) g H2O/g DNA, Nss-DNAΣ=(0.40±0.01) g H2O/g DNA. According to this calorimetric data the transformation “double stranded helix”→“single stranded chains” is accompanied by dehydration of ds-DNA: ΔN=NdsΣ−NssΣ=(0.15±0.01) g H2O/g DNA. We suggest that the formation of B-DNA duplex from mixing of their complementary single strands is mostly accompanied by the uptake of structural water molecules and so, the overall hydration of a duplex is mainly determined by its conformation and stability.