[Estimation of the enthalpy of B--A transitions in DNA in water-ethanol solutions]

Enthalpy of reaction of guanidinium chloride with sodium DNA salt was determined microcalorimetrically at 25 degrees C in water-ethanol solutions of various concentrations. At low ethanol contents (up to 25 mole%) reaction is exothermic, its enthalpy being only slightly dependent on ethanol concentration. Above 28 41 mole % of ethanol reaction becomes endothermic. As above 41 mole% of ethanol in the absence of guanidinium ions A-type DNA conformation appears in the solution, which under the influence of guanidinium is converted to the B-type conformation, enthalpy of conformation transition must be included in the measured enthalpy of the reaction. Dependence of the measured enthalpy on ethanol concentration in the B--A transition region testifies to a very small value of the enthalpy change during this transition.