Bound and Free Water Molecules

To get a better insight into the solvation dynamics of coumarin vithin CD a multishell continuum model and molecular hydrodynamic theory have been used 

Using fs resolution, two residence times of water at the surface of two proteins have been reported (Fig. 7.6) [21]. The natural probe tryptophan amino acid was used to follow the dynamics of water at the protein surface. For comparison, the behavior in bulk water was also studied. The experimental result together with the theoretical simulation-dynamical equilibrium in the hydration shell, show the direct relationship between the residence time of water molecules at the surface of proteins and the observed slow component in solvation dynamics. For the two biological systems studied, a bimodal decay for the hydration correlation function, with two primary relaxation times was observed an ultrafast time, typically 1 ps or less, and a longer one typically 15-40 ps (Fig. 7.7) [21]. Both times are related to the residence period of water at the protein surface, and their values depend on the binding energy. Measurement of the OH librational band corresponding to intermolecular motion in nanoscopic pools of water and methanol 

7 Intra- and Intermolecular Proton Transfer and Related Processes 

To get more insight into the effect of confinement on the binding between HPMO and the host, time-resolved anisotropy measurements have been carried out [58]. The result (Fig. 7.12) shows a remarkable difference in the anisotropy decays, especially for the HSA protein case. While in dioxane, the rotational time constant (45 ps) is close to the expected one using hydrodynamic theory [58], this time increases with the rigidity of the host (97 ps for a micelle, 154 ps for yS-CD 

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In the second item above, the presence of bound and free water molecules was noted. Both bound ions and ionic surfactant groups are hydrated to about the same extent in the micelle as would be observed for the independent ions. The dehydration of these ionic species is an endothermic process, and this would contribute significantly to the AH of micellization if ion dehydration occurred. In the next section we discuss the thermodynamics of micellization, but it can be noted for now that there is no evidence of a dehydration contribution to the AH of micelle formation. The extent of micellar hydration can be estimated from viscosity... 

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. 

Fig. 5 (a) Power spectrum of oxygen atoms of bound and free water molecules, (b) Power spectrum of hydrogen atoms of different kinds of water molecules. 

According to the DEM, the dynamics in the hydration layer has multiple timescales. The fast one is bulk-like and the slow ones are at least an order of magnitude slower and depend on the transition rates between bound and free water molecules. 

Figure 6.8 Sketch of proposed molecular mechanism of protein-surfactant interaction for CITREM + sodium caseinate (0.5 % w/v in aqueous medium (pH = 7.2, ionic strength = 0.05 M) at 293 K. Picture (I) shows the water molecules bound with polar groups of the protein and surfactant, as w ell as w ater molecules structured as a result of hydrophobic hydration around the hydrocarbon chain of the surfactant. (For clarity, the free w ater molecules are not shown.) Picture (H) demonstrates the release of bound and structured water molecules resulting Rom the predominantly hydrophobic interactions between protein and surfactant. Reproduced Rom Semenova et al. (2006) with permission.

Another descriptor of the mobility of water molecules in contact with the clay layers is the water self-diffusion coefficient. A fine recent review summarizes the theoretical and practical aspects of measurement by spin-echo nmr methods of this parameter (36) The plot of the decrease in the water self-diffusion coefficient as a function of C, the amount of suspended clay, for the same samples, is again a straight line going through the origin. By resorting once more to a similar analysis in terms of a two-state model (bound and "free water), one comes up (25) with a self-diffusion coefficient, for those water molecules pinched in-between counterions and the clay surface, of 1.6 10 15 m2.s 1,... 

The first type of relaxation processes reflects characteristics inherent to the dynamics of single droplet components. The collective motions of the surfactant molecule head groups at the interface with the water phase can also contribute to relaxations of this type. This type can also be related to various components of the system containing active dipole groups, such as cosurfactant, bound, and free water. The bound water is located near the interface, while free water, located more than a few molecule diameters away from the interface, is hardly influenced by the polar or ion groups. For ionic microemulsions, the relaxation contributions of this type are expected to be related to the various processes associated with the movement of ions and/ or surfactant counterions relative to the droplets and their organized clusters and interfaces [113,146]. 

A schematic representation of the hydration shell around a protein. The shapes in the real systems are seldom perfectly spherical. The shell is not rigid or static. The big filled circle ( ) represents the oxygen atom and the small filled circle ( ) represents the hydrogen atom of a water molecule. The dotted lines are the hydrogen bonds and dashed arrows represent the transition between the bound and free water. The solid arrows represent the diffusion in and outside the hydration layer. 

Figure 7.6 (A) An illustration of the dynamic equilibrium of water molecules at the hydration layer of a protein, with bound (1), quasi-free (2) and free water molecules (3).

According to analysis by differential scanning calorimeter (DSC), water molecules in the membranes can be classified into three types, non-freezing, freezing bound and free water.40 Also, the weight ratios of freezable water and... 

Next, we must consider the bound and free water in the water pool. The free water appears at W, = 10 in the core of the water pool in both AOT and HTAC reversed micelles. On the other hand, in the Ci2Eg reversed micelles, water molecules are hydrated to the oxygen of the EO chain and almost all the water molecules are bound up to Wo = 30. As Wo decreases, the photomerization proceeds favorably. It is suggested that bound water is more important than free water for photomerization. As Wq decreases, the size of the water pool also decreases to result in suppression of the mobility of the water. Therefore, the generated active oxygens come to be stable in the region of bound water, especially at low Wo-... 

In the implementation of the model, it is further assumed that as the bound water molecule is immobilized by the protein surface, it cannot rotate or translate. Thus, it must become free to move. The bound to free (and free to bound) transition is described as a chemical reaction. The free water molecules, on the other hand, are assumed to behave as molecules in bulk water, although their rotation and translation diffusion are generally modified due to their interaction with the protein. This surface layer of bound and free water is coupled to the bulk water outside the layer. Although this is a key feature, it is ignored in many other models. In addition, we allow the possibility of the bound water having a preferred orientation due to its interaction with the protein. 

There is a problem of bound water both in hfeless nature and in biological systems at various levels of the stractural hierarchy. In this regard we can talk about the existence of the phenomenon of bound water with different from hulk water strac-tures and properties [17-19]. Let us consider a model of water associated with biopolymer. The basis of this model is the fact that water molecule can be represented as a distorted tetrahedron because an ideal tetrahedron bond angle is 109.28, and free water molecule bond angle is 104.5. The model assumes that the water may produce a continuous three-dimensional netwoik of tetrahedral particles. 

In an operating PEMFC, the MRI technique can be appHed to plot the concentration distribution of water molecules with various chemical environments including the bound water molecules by the sulfonic anions and free water molecules in hydrophilic water clusters in NAFION. Since MRI is a nondestructive technique, it enables the obtaining of unique in situ information about the distribution of water molecules in an operating PEMFC. 

The state of water in fully hydrated sulfonated poly(ether ether ketone)-silica hybrid proton exchange membranes were characterized in terms of the exchange rate between bound and free water, the water dynamics in each phase, and the relative water populations by H ODESSA and transverse magnetization relaxation NMR. The exchange rate, the amount of bound water, and the reorientation of free water molecules increase in the presence of silica particles. ... 

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