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 K down to 185 K. 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 fragileto-strong dynamic crossover (FSC) at TL = 222 ± 2 K by plotting log τT vs. 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. . It was also found, from neutron and Xray scattering, or from Mössbauer spectroscopy, that the measured mean-squared atomic displacement x 2 of the bio-molecules exhibits a sharp rise in the same temperature range [1,2,3,4,5]. This sharp increase in x 2 was taken as a sign for a dynamic transition (or sometimes called glass-transition) in the bio-molecules occurring within this temperature range. In most of these papers, the authors suggest that the transition is due to a strong rise of anharmonicity of the molecular motions above this transition temperature [1]. Later on, it was demonstrated that the dynamic transition can be suppressed in dry bio-molecules [2], or in bio-molecules dissolved in trehalose [5]. Moreover, it can be shifted to a higher temperature for proteins dissolved in glycerol [4]. Thus the dynamic transition can be controlled by changing the surrounding solvent of the bio-molecules. On the other hand, it was found some time ago from Raman scattering that supercooled bulk water has a dynamic crossover transition at 220 K [6], similar to that predicted by Mode-Coupling theory [7]. Approximate coincidence of these two characteristic temperatures, one for the slowing down of bio-chemical activities and the sharp rise in x 2 in bio-molecules and the other for the dynamic crossover in water, suggests a relation between the dynamic transition of bio-molecules and that of their * Author to whom correspondence should be addressed. Electronic mail: sowhsin@mit.edu hydration water [8].Another striking experimental fact is that this dynamic transition temperature, as revealed by change of slope in x 2 vs. temperature plot, occurs at a universal temperature range from 250 to 200 K in all bio-molecules examined so far. This list includes globular proteins, DNAs, and t-RNAs. This feature points to the plausibility that the dynamical transitions are not the intrinsic properties of the bio-molecules themselves but are imposed by the hydration water on their surfaces.However, x 2 (mostly coming from hydroge...

High-resolution quasielastic neutron scattering spectroscopy was used to measure H2O and D2O hydrated RNA samples. The contribution of scattering from RNA was subtracted out by taking the difference of the signals between the two samples. The measurements were made at a series of temperatures from 270 K down to 180 K. The relaxing-cage model was used to analyze the difference quasielastic spectra. We observed clear evidence of a fragile-to-strong dynamic crossover (FSC) at TL=220 K in RNA hydration water. We further show that the mean-square displacements of the hydrogen atoms in both RNA and its hydration water exhibit a sharp change in slope at approximately the same temperature 220 K. This latter fact suggests that the dynamic transition in RNA is triggered by the abrupt change of mobility of the hydration water at its FSC temperature.

A super-Arrhenius-to-Arrhenius dynamic crossover phenomenon has been observed in the translational alpha-relaxation time and in the inverse of the self-diffusion constant both experimentally and by simulations for lysozyme hydration water in the temperature range of TL = 223 +/- 2 K. MD simulations are based on a realistic hydrated powder model, which uses the TIP4P-Ew rigid molecular model for the hydration water. The convergence of neutron scattering, nuclear magnetic resonance and molecular dynamics simulations supports the interpretation that this crossover is a result of the gradual evolution of the structure of hydration water from a high-density liquid to a low-density liquid form upon crossing of the Widom line above the possible liquid-liquid critical point of water.