Probing DNA's Dynamics and Conformational Substates by Enthalpy Relaxation and Its Recovery

The A and B form of NaDNA with hydration level of between 0.15 and 0.64 (g of water)/(g of NaDNA) have been vitrified by cooling at rates of ≈80 K min -1, and their thermal behavior on reheating at 30 K min -1 was studied from ≈120 to 300 K by differential scanning calorimetry. The calorimetric effects of annealing time and temperature are characteristic of a glass, and they are attributed to enthalpy relaxation of conformational substates of B-DNA. Structural relaxation becomes observable in form of endothermic enthalpy recovery for Γ > 3−4 (water molecules per nucleotide), and the heat effects increase with hydration level linearly up to Γ ≈ 12. Molecular motions caused by the presence of water hydrogen bonded to DNA's functional groups become unfrozen from ≈153 to ≈263 K which is attributed to a very broad distribution of relaxation times. The data suggest that structural relaxation effects as reported here can be linked to the conformational flexibility of B-DNA necessary for protein−DNA interaction.