Diffusion water dynamics

We finish this section by comparing our results with NMR and incoherent neutron scattering experiments on water dynamics. Self-diffusion constants on the millisecond time scale have been measured by NMR with the pulsed field gradient spin echo (PFGSE) method. Applying this technique to oriented egg phosphatidylcholine bilayers, Wassail [68] demonstrated that the water motion was highly anisotropic, with diffusion in the plane of the bilayers hundreds of times greater than out of the plane. The anisotropy of... 

Payer80 states that the UNSAT-H model was developed to assess the water dynamics of arid sites and, in particular, estimate recharge fluxes for scenarios pertinent to waste disposal facilities. It addresses soil-water infiltration, redistribution, evaporation, plant transpiration, deep drainage, and soil heat flow as one-dimensional processes. The UNSAT-H model simulates water flow using the Richards equation, water vapor diffusion using Fick s law, and sensible heat flow using the Fourier equation. 

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. 

Water dynamics is slowed down by the electric field of the cation, as revealed by diffusion coefficient reduced by a factor of two, compared with pure SPC/E water [132]. A reduction of D of water in ionic solutions is also observed experimentally, with values, determined with the tracer technique, ranging from 1.22 10-5 cm /s for Li+ to 0.52 and 0.53 10 5 cm /s for Fe3+ and Al3+, respectively [206]. 

In order to understand the effect of temperature on the water dynamics and how it leads to the glass transition of the protein, we have performed a study of a model protein-water system. The model is quite similar to the DEM, which deals with the collective dynamics within and outside the hydration layer. However, since we want to calculate the mean square displacement and diffusion coefficients, we are primarily interested in the single particle properties. The single particle dynamics is essentially the motion of a particle in an effective potential described by its neighbors and thus coupled to the collective dynamics. A schematic representation of the d)mamics of a water molecule within the hydration layer can be given by ... 

The above dynamic exchange model applies mainly to the rotational relaxation of interfaeial water molecules. It has also been extended by Bhattaeharyya et al. [6] to treat wave-number-dependent relaxation, whieh includes translational diffusion more realistically. As stated, the model is simple and phenomenological but captures several essential aspects of water dynamics at the biological interface, in addition to being analytically tractable. 

The dynamic exchange model employs a hydrodynamic approach wherein the dynamics of the three species in the surface layer is described by a reaction-diffusion equation and the bulk water dynamics is described by a simple diffusion equation. Therefore, in this approach, the interactions are not considered explicitly... 

In addition, protein motion reduces the retardation of the water dynamics, because the dimension of the water translational space is increased and at the same time the decay of the orientational correlation is accelerated. In spite of this accelerated dynamics, hydration water diffusion remains anomalous for a thermalized protein. 

Mapping a water trajectory to a many-to-one CG level is inherently different than mapping a larger molecule s trajectory, since for water, atoms mapjjed into a single CG bead necessarily exist on different molecules. Furthermore, the water molecules mapped to a common bead are not likely to remain associated throughout the full simulation because of thermal diffusion. A dynamic mapping scheme is therefore... 

The obtained results have also been conhrmed by studies on water dynamics and water interaction in bovine serum albumin (BSA p) suspension performed on 300% swollen albumin NP by analysis of H NMR relaxation curves and self-diffusion measurements that showed the presence of two well-separated relaxation rates (Bellotti et al. 2010). These rates have been accounted for a surface-limited relaxation regime of water molecules inside albumin NP (60 wt% of the total water content) and a diffusion-limited relaxation of water molecules into the meso-cavities between the packed NP (40 wt% of the total water content). At higher water content, the appearance of slowly relaxing components was assigned to free water molecules external to the NP, supernatant on the top of the sample. Therefore, the non-freezing and freezing water fractions up to Wiot=300%, determined by DSC measurements, may be associated to the water molecules inside and between the packed NP, respectively. While, at higher water content free molecules external to the NP were also present (Bellotti et al. 2010). 

The reversible step may be related to the dynamic crossover in protein hydration water at To 345 5K. NMR self-diffusion results [19] indicate that at this temperature a sudden change in hydration water dynamics occurs and the inverse diffusion constant switches from low-temperature super-Arrhenius behavior to high-temperature Arrhenius behavior. Neutron techniques (QENS) have also been used to study protein hydration water at this high-r crossover. Figure 21 shows the atomic MSD of protein hydration water at the low-r crossover measured using MD simulation. These crossovers can also be shown theoretically. Whenever the slope of an Arrhenius plot of the D T) changes, the specific heat has a peak. The well-known Adam-Gibbs equation (AGE) shows this as... 

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