Temperature effects on hydration and form transitions of DNA

The effects of temperature on the A-B transition of DNA in fibers are determined by fiber X-ray diffraction and measurement of variations of the fiber length with the relative humidity (r.h.). It is observed that the transition from the A to the B helical form occurs at higher r.h. when the temperature is raised. At 7 degrees C, this transition is complete for a r.h. of 88% while the A form remains stable at that same r.h. and is even maintained for r.h. values up to 92% when the temperature is 37 degrees C. The interpretation of the present experimental results is given in terms of hydration effects in which the entropic contribution of the solvent molecules is dominant.