Translational and rotational dynamics of water

It is clear that the motions (translational and rotational) of water molecules near a lipid bilayer membrane are restricted. Nevertheless, they exhibit rich dynamic behavior [3]. Mueh of the information has come recently from computer simulations, which, as mentioned before, allow detailed follow-up of motion of individual water molecules. 

A further classification was made on the basis of HB partner. Thus, the water molecules hydrogen-bonded to phosphate oxygen are termed Op water, those to carbonyl oxygen atoms are termed Oe water, and finally those clathrating choline groups are termed Choline water . 

Molecular dynamics simulation and also spectroscopic evidence (discussed later) suggest that the rotational dynamics of water around a lipid bilayer depends on the 

Water surrounding lipid bilayers its role as a lubricant 

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Faraone A, Liu L, Mou CY, ShuhPC, Copley JRD, Chen SH. (2003) Translational and rotational dynamics of water in mesoporous silica materials MCM-41-S and MCM-48-S. J Chem Phys 119 3963. 

Translational and rotational dynamics of water molecules in the grooves 153... 

Figure 11.2. Translational and rotational dynamics of water molecules determined by computer simulations in complexed form (solid line) wifli protein and DNA as well as in the free component (dashed Une). (a) Mean-square displacements (MSDs) of water moleeules residing in the first hydration shell, water molecules in the major and minor groove regions of the DNA, and water molecules present in the common region of the eomplex are calculated and shown in the figure for both the complexed form and the tree form, (b) The reorientational time correlation fimction, Cp (t), derived for the same water moleeules located as mentioned above both in complexed and fi ee forms of protein and DNA. The comparison with the pure bulk state is also highlighted in both figures. Adapted with permission fiom Nature Struct. Mol Biol, 16 (2009), 1224. Copyright (2009) Nature Publishing Group.

Closely linked to its extraordinary solvent capacities is water s role in transporting dissolved materials throughout the organism. With the exception of air-filled channels like the tracheal systems of insects, most of the transport processes of organisms involve movement of dissolved solutes. Diffusion of solutes within water is rapid, as is the translational and rotational movement of water itself. The extensive networks of hydrogen bonds that form among water molecules and between water and solutes do not impede this dynamic move-... 

MD simulations of [C6mim][PF6] and water mixtures were carried out [98], The authors found water to be closely associated with the anions and that its presence enhances both the translational and rotational dynamics of the IL. The mean square displacement and rotational correlation functions reflected that the diffusive regime was achieved faster when water was present in the IL or the observed decay of correlations was faster when water was present, respectively [98], From this the authors deduced that the experimentally observed decrease in viscosity is a consequence of the faster translational and rotational dynamics caused by the presence of water [98], In agreement with experiments, the authors found that the fluorescence spectra of Coumarin-153 is red-shifted because of the presence of water [98],... 

At room temperature, these HBs can break and re-form quite rapidly, leading to the rearrangement of the network. At the microscopic level, we need to understand the lifetime and bond-breaking mechanism of individual HBs between two neighboring water molecules. In the case of a water molecule, such bond-breaking events lead to the spatial/rotational displacements that in turn lead to the translational and rotational diffusion of water molecules. Therefore, the basic mechanism of the microscopic dynamics in water is expected to be different from a simple liquid made of nearly spherical units, Uke in the case of argon. 

Very recently, detailed further analyses of the translation and reorientation dynamics of SCW have been reported showing consistency with our results in the limit of very low-density supercritical states for water [47], The dynamical behavior of SCW in this study is also compared to that of supercritical benzene showing that the density dependence of the self-diffusion coefficient and rotational dynamics of SCW is smaller than that of supercritical benzene because SCW is capable of maintaining stronger degrees of structural correlations and orientational anisotropy than benzene, which tends to lose intermolecular correlations at a much faster rate upon decreasing density [47],... 

Geiger al remark that "the dynamical data show that translational and rotational motions of solvation-sheeth water molecules are preceptibly slower (by at least 20 percent) than those in pure bulk water" (, p. 263 abstract). Such changes are not dramatic, but they may be crucially important. 

Water hydration and counter-ion solvation of biological macro molecules considerably alter the translational and rotational dynamics ofbiological macromolecules. The effect can be correlated with a number of different solution properties of a given biological macromolecule such as the hydrated volume Vh- The term Vh is given by... 

Rose and Benjamin studied the water dipole and the water H-H vector reorientation dynamics at the water/Pt( 100) interface and the results are reproduced in Fig. 4. As in the case of the translational diffusion, the effect of the surface is to significantly slow down the adsorbed water layer. We note that the effect is very short range, and that the rotational motion of water molecules in the second layer is already very close to the one in bulk water. 

The nature of the interfacial structure and dynamics between inorganic solids and liquids is of particular interest because of the influence it exerts on the stabilisation properties of industrially important mineral based systems. One of the most common minerals to have been exploited by the paper and ceramics industry is the clay structure of kaolinite. The behaviour of water-kaolinite systems is important since interlayer water acts as a solvent for intercalated species. Henceforth, an understanding of the factors at the atomic level that control the orientation, translation and rotation of water molecules at the mineral surface has implications for processes such as the preparation of pigment dispersions used in paper coatings. 

In a theoretical model, we considered the dynamics of bound water molecules and when they become free by translational and rotational motions. Two coupled reaction-diffusion equations were solved. The two rate constants, kbf and kjb, were introduced to describe the transition from bound (to the surface) to free (from the surface) and the reverse, respectively. We also took into account the effect of the bulk water re-entry into the layer—a feedback mechanism—and the role of orientational order and surface inhomogeneity on the observed decay characteristics. With this in mind, the expressions for the change in density with time were written defining the feedback as follows ... 

This bimodal dynamics of hydration water is intriguing. A model based on dynamic equilibrium between quasi-bound and free water molecules on the surface of biomolecules (or self-assembly) predicts that the orientational relaxation at a macromolecular surface should indeed be biexponential, with a fast time component (few ps) nearly equal to that of the free water while the long time component is equal to the inverse of the rate of bound to free transition [4], In order to gain an in depth understanding of hydration dynamics, we have carried out detailed atomistic molecular dynamics (MD) simulation studies of water dynamics at the surface of an anionic micelle of cesium perfluorooctanoate (CsPFO), a cationic micelle of cetyl trimethy-lainmonium bromide (CTAB), and also at the surface of a small protein (enterotoxin) using classical, non-polarizable force fields. In particular we have studied the hydrogen bond lifetime dynamics, rotational and dielectric relaxation, translational diffusion and vibrational dynamics of the surface water molecules. In this article we discuss the water dynamics at the surface of CsPFO and of enterotoxin. 

In the hydrate lattice structure, the water molecules are largely restricted from translation or rotation, but they do vibrate anharmonically about a fixed position. This anharmonicity provides a mechanism for the scattering of phonons (which normally transmit energy) providing a lower thermal conductivity. Tse et al. (1983, 1984) and Tse and Klein (1987) used molecular dynamics to show that frequencies of the guest molecule translational and rotational energies are similar to those of the low-frequency lattice (acoustic) modes. Tse and White (1988) indicate that a resonant coupling explains the low thermal conductivity. 

The second explanation for the solvent isotope effect arises from the dynamic medium effect . At 25 °C the rotational and translational diffusion of DjO molecules in D20 is some 20% slower than H20 molecules in H20 (Albery, 1975a) the viscosity of D20 is also 20% greater than H20. Hence any reaction which is diffusion controlled will be 20% slower in D20 than in H20. This effect would certainly apply to transition state D in Fig. 3 where in the transition state the leaving group is diffusing away. A similar effect may also apply to the classical SN1 and SN2 transition states, if the rotational diffusion of water molecules to form the solvation shell is part of the motion along the reaction co-ordinate in the transition state. Robertson (Laughton and Robertson, 1959 Heppolette and Robertson, 1961) has indeed correlated solvent isotope effects for both SN1 and SN2 reactions with the relative fluidities of H20 and D20. However, while the correlation shows that this is a possible explanation, it may also be that the temperature variation of the solvent isotope effect and of the relative fluidities just happen to be very similar (see below). 

While these models match experimental data reasonably well at lower fields, recent experiments at higher magnetic fields of 3.4 and 9.2 T show enhancement values that are much higher than predicted with the currently employed theory.41,72,79 At these higher fields, the timescale of molecular interactions that give rise to Overhauser DNP effects is much shorter (sub-picoseconds to picoseconds) and thus should be more sensitive to the rotational diffusion dynamics of water, closely related to the atomistic details of the radical and solvent, instead of translational diffusion dynamics. These atomistic details are not accurately represented in the FFHS or rotational models (Equations (13) and (15)), implying that further work needs to be done to develop more accurate models. 

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