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...
Molecular motion of hydrocarbons
under confinement exhibits several peculiarities and has important
implications in industries like gas recovery. A quasielastic neutron
scattering (QENS) study of the dynamics of propane in nanoporous silica
aerogel was carried out to quantify its molecular mobility. The dynamical
properties of propane were studied as a function of temperature, pressure
and presence of CO2. The effects of pressure, i.e., fluid
density and composition, are found to be more pronounced than the
effects of temperature. At low pressures of propane, many propane
molecules are adsorbed onto the pore surfaces and are thus immobile.
As the pressure of propane loading is increased, more molecules become
available to take part in the diffusional dynamics and thus enhance
the diffusivity. At low pressure the propane molecules take part in
a continuous diffusion, while at higher pressures, the diffusion of
propane molecules within the aerogel occurs via the mechanism of jumps.
Presence of CO2 enhances the jump rate of propane molecules,
thereby increasing the diffusion coefficient. This study aims to aid
in understanding the complex processes involved in hydrocarbon migration
in porous quartz-rich rocks and enhanced hydrocarbon recovery.
Quasi-elastic neutron scattering (QENS) and molecular dynamics simulations (MDS) reveal the effects of water on the structure and dynamics of propane confined in 1.5 nm wide cylindrical pores of MCM-41-S.
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