The role of bound water on the energetics of DNA duplex melting

A combination of common and low-temperature differential scanning calorimetry (DSC) techniques was used to detect the thermodynamic parameters of heat denaturation and of ice-water phase transitions for native and denaturated DNA, at different low water contents. We suggest that the main contribution to the enthalpy of the process of the heat denaturation of DNA duplex (35±5 kJ/mol bp) is the enthalpy of disruption of the ordered water structure in the hydration shell of the double helix (26±1 kJ/mol bp). It is possible that this part of the energy composes the non-specific general contribution (70%) of the enthalpy of transition of all type of duplexes. For DNA in the condensed state the ratioα=ΔCp/ΔS ~2 is smaller than for DNA in diluted aqueous solutions (α≅2–4). This means that there are other sources for the large heat capacity change in diluted solutions of DNA – for example the hydrophobic effects and unstacking(unfolding) of single polynucleotide chains.