[Role of water in the thermal instability of DNA].

We studied structural changes of DNA heated for long times at temperatures between 20 to 100 degrees C at different relative humidities (range 0--100%). DNA structure was analyzed by the following criteria: solubility changes, optical melting, ultraviolet spectroscopy, potentiometry and equilibrium dialysis. It has been found that irreversible changes in DNA secondary structure take place at around 37 degrees C, temperature optimal for the function in many living systems. These irreversible changes may be interpreted as a consequence of formation of interchain contacts (non-covalent "cross-links"). We demonstrate that the whole range of relative humidities may be divided onto three regions. For each of these regions DNA has somewhat different molecular structure and peculiarities are observed in the stabilizing and destabilizing action of water on DNA three-dimensional structure. Irreversible changes of DNA secondary structure as a function of bound water are represented by a bell-shaped curve. Similarity between this curve and a curve for the survival microorganisms is discussed.