[Hydration and stability of the double helical complex poly(dA)-poly(dT)]

IR-spectrometry and thermopiezogravimetry are used to study the structure formation and the water environment of the double-helix Na-poly(dA)-poly(dT) complex in films in the 0-100% interval of the relative humidity (RH). It has been found that three different structures of the complex are formed in the 0-32, 32-76 and 88-100% intervals of RH. The calculations of resonance frequencies of out-of-ring (C-0) and in-ring (C-N) vibrations in adenine and thymine using perturbation theory show that the second and the third forms are the X-ray beta-B'- and alpha-B'-forms. Distribution of the binding energy of water molecules has been found from IRS data about water sorption and isotherms of hydration at different temperatures. The enthalpy of helix-coil transition has been found from microcalorimetry data. These results allow us to estimate the energy contribution of water molecules (about 75%) and other interactions (about 25%) in the total stabilization energy of the complex structure.