Differential scanning calorimetric and x-ray study of the binding of the water of primary hydration to calf-thymus DNA

Differential scanning calorimetry has been used to study the thermal properties of hydrated films of calf-thymus Na-, K- and CsDNA between 20 and 320 degrees C. A broad endothermic transition near 75 degrees C and a sharp exothermic transition near 240 degrees C are observed. The broad transition is due to the dehydration of the DNA, while the exothermic transition is due to pyrolysis of the sample. The peak temperatures of both transitions increase as the scan rate is increased. Based on a Kissinger analysis, the net activation energy for the desorption of the primary water of hydration is about 0.6 eV while that for the pyrolysis is about 1.9 eV. X-ray diffraction patterns suggest that heating the DNA films to 180 degrees C once does not, but thrice does, destroy their structural ordering.