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 approximate to 80 K min(-1), and their thermal behavior on reheating at 30 K min(-1) was studied from approximate to 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 Gamma > 3-4 (water molecules per nucleotide), and the heat effects increase with hydration level linearly up to Gamma approximate to 12. Molecular motions caused by the presence of water hydrogen bonded to DNA's functional groups become unfrozen from approximate to 153 to approximate 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.