We used high-resolution quasielastic neutron scattering spectroscopy to study the single-particle dynamics of water molecules on the surface of hydrated DNA samples. Both H2O and D2O hydrated samples were measured. The contribution of scattering from DNA is subtracted out by taking the difference of the signals between the two samples. The measurement was made at a series of temperatures from 270 down to 185K. The relaxing-cage model was used to analyze the quasielastic spectra. This allowed us to extract a Q-independent average translational relaxation time τT of water molecules as a function of temperature. We observe clear evidence of a fragile-to-strong dynamic crossover (FSC) at TL=222±2K by plotting logτT versus T. The coincidence of the dynamic transition temperature Tc of DNA, signaling the onset of anharmonic molecular motion, and the FSC temperature TL of the hydration water suggests that the change of mobility of the hydration water molecules across TL drives the dynamic transition in DNA.

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