Hydration of nucleic bases in dilute aqueous solutions. Apparent molar adiabatic and isothermal compressibilities, apparent molar volumes and their temperature slopes at 25 degrees C

The concentration increment of the ultrasound velocity has been measured with an accuracy of +/- 0.03 cm/s in dilute aqueous solutions of a variety of nucleic bases and their derivatives in the concentration range 0.5-1.5 mg/g H2O at temperatures of 15-35 degrees C. A new method for the precise measurement of ultrasound velocity in small volumes of liquids has been used. The values of the apparent molar adiabatic compressibilities plus the corresponding temperature slopes, apparent molar volumes with their temperature slopes, and apparent molar isothermal compressibilities at infinite dilution have been obtained. The regularities describing the signs of these values and their dependence on the chemical structure of the solute have been revealed. It is shown that these regularities can be described as a consequence of partial 'normalization' of some of the properties of water around the bases, namely, weaker structural contribution to compressibility, less negative temperature slope of compressibility and less negative structural contribution to the coefficient of thermal expansion of water.