Journal of Thermal Analysis and Calorimetry, Vol. 66 (2001) 133–144 THE ROLE OF BOUND WATER ON THE ENERGETICS OF DNA DUPLEX MELTING G. M. Mrevlishvili 1 , A. P. S. M. C. Carvalho 1 , M. A. V. Ribeiro da Silva 1 , T. D. Mdzinarashvili 2 , G. Z. Razmadze 2 and T. Z. Tarielashvili 2 1 Centro de Investigação em Química, CIQ(UP), Departamento de Química, Faculdade de Ciências do Porto, Rua do Campo Alegre, 687, P-4169-007 Porto, Portugal 2 Department of Physics, Tbilisi State University, 380028, Republic of Georgia Abstract 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 transi- tions for native and denaturated DNA, at different low water contents. We suggest that the main con- tribution 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 contribu- tion (70%) of the enthalpy of transition of all type of duplexes. For DNA in the condensed state the ratio α=∆C p /∆S ~2 is smaller than for DNA in diluted aqueous solutions (α2245 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. Keywords: DNA, hydration, low-temperature DSC, melting enthalpy of DNA Introduction The results of the determination of apparent heat capacities of naturally occurring DNA in both native (helix) and denatured (coils) states in dilute aqueous solutions [1], and the analysis of a large amount of available experimental data (obtained also for polymeric nucleic acid duplexes by using the new generation of differential scanning calorimetry, (DSC) [2, 3]), suggests that the melting of duplexes is accompanied by positive changes in the heat capacity (∆C p ) ~(170–400) J K –1 mol –1 ([1–3] and references therein). This has significantly changed the understanding of the origins of duplex stability [3]. This posi- tive change in the heat capacity implies a large temperature dependence of the enthalpy and entropy of DNA duplex melting [1b]. The significant contribution to the thermody- namic parameters of helix-coil transition (∆H, ∆S, ∆C p , ∆G) for all duplexes is the energy accompanying of changes in hydration of duplexes [1, 4–5]; the entropic cost of transfer- ring a single water molecule from the liquid to a site of double helical DNA is significant 58±6JK –1 mol –1 and differs from the entropic cost for polynucleotide chains in the state of statistical coils ~15±2JK –1 mol –1 [5]. The structure of water in grooves of the helix and 1418–2874/2001/ $ 5.00 © 2001 Akadémiai Kiadó, Budapest Akadémiai Kiadó, Budapest Kluwer Academic Publishers, Dordrecht