Rotational mobility of water

Water molecules are constantly in motion, even in ice. In fact, the translational and rotational mobility of water directly determines its availability. Water mobility can be measured by a number of physical methods, including NMR, dielectric relaxation, ESR, and thermal analysis (Chinachoti, 1993). The mobility of water molecules in biological systems may play an important role in a biochemical reaction s equilibrium and kinetics, formation and preservation of chemical gradients and osmotic pressure, and macromolecular conformation. In food systems, the mobility of water may influence the engineering processes — such as freezing, drying, and concentrating chemical and microbial activities, and textural attributes (Ruan and Chen, 1998). 

Water has an essential role in living systems and is ultimately involved in the structure and function of biological polymers such as proteins. However, in this contribution we sh tll focus primarily not on what the water does for the blopolymer but rather on the effects that the biopolymer has on the water that Interacts with it. Of Interest are alterations in the structural, energetic, and dynamic properties of the water molecules. Studies of the rotational mobility of water molecules at protein surfaces have been interpreted by dividing the solvent molecules into three groups U). The most rapidly reorienting group has a characteristic rotational reorientation time (t ) of not more than about... 

The structure of water next to the metal is strongly perturbed only over two layers, and the range of other inhomogeneities and anisotropies appears to be not more than four layers at most. Detailed structural information has been derived. It has been demonstrated that the translational and rotational mobility of water and ions near a metal surface is reduced compared to the bulk. 

A first example is represented by the Mn(III)/Mn(II) redox switch. The complexes of Mn(II) and Mn(III) with the water-soluble tetraphenylsulpho-nate porphyrin (TPPS, Chart 13) display significantly different ri values at low magnetic field strength (lower than 1 MHz), but very similar values at the fields currently used in the clinical practice (> 10 MHz) (141). However, the longer electronic relaxation rates of the Mn(II) complex makes its relaxivity dependent on the rotational mobility of the chelate. In fact, upon interacting with a poly-p-cyclodextrin, a 4-fold enhancement of the relaxivity of [Mn(H)-TPPS(H20)2] at 20 MHz has been detected, whereas little effect has been observed for the Mn(III)-complex. The ability of the Mn(II)/Mn(III)... 

Nmr methods have unrivalled potential to explore interfaces, as this account has striven to show. We have been able to determine the mobility of hydrated sodium cations at the interface of the Ecca Gum BP montmorillonite, as 8.2 ns. We have been able to measure the translational mobility of water molecules at the interface, their diffusion coefficient is 1.6 10 15 m2.s. We have been able to determine also the rotational mobility of these water adsorbate molecules, it is associated to a reorientational correlation time of 1.6 ns. Furthermore, we could show the switch in preferred reorientation with the nature of the interlayer counterions, these water molecules at the interface tumbling about either the hydrogen bond to the anionic surface or around the electrostatic bond to the metallic cation they bear on their back. And we have been able to achieve the orientation of the Ecca Gum BP tactoids in the strong magnetic field of the nmr spectometer. 

Grivtsov, Zhuravlev, and co-workers (Institute of Physical Chemistry, the U.S.S.R. Academy of Sciences, Moscow) (385, 386) resorted to numerical modeling of molecular dynamics in investigating problems of water adsorption by the hydroxylated surface of the face (0001) in / -tridymite. / -tridymite was chosen as a model form of silica because such a crystalline modification is close in density to that of amorphous silica. The boundary layer in Si02 was considered when each surface Si atom held one OH group. The rotational mobility of the hydroxyl groups is an important factor in the adsorption of water. 

MD simulations and experiments clearly show that the single particle motion of water molecules next to a protein surface is different than in the bulk. Here, single particle refers to measures of the average behavior of individual water molecules, as opposed to coherent behavior of collections of water molecules, which will be discussed in more detail below. The perturbation of the translational and rotational mobility of protein hydration water (defined using the 4 A distance criterion) is depicted in Fignre 16.1a and b, respectively. We will discuss the data for the native (N) state first, and snbsequently compare the native and MG states. In bulk water, after an initial rapid ( 2ps) rise corresponding to ballistic motion. 

The mobility of water molecules, particularly their rotational characteristics in liquid water at the interfaces, is influenced by the number of the hydrogen bonds per molecule. The proton exchange between these clusters can be represented by scheme described in Section 10.1 (Equations 10.24 through 10.27). The concentrated aqueous suspensions of nanosilicas A-50 and A-300 are characterized by different temperature dependences of transverse relaxation time T2 (Figure 1.99). 

Relaxation phenomena (TSDC), molecular mobility (NMR, TPDMS), and chemical reactions (TPDMS of associative desorption of water) are observed for adsorbed water/LiChrolut EN adsorbent over a wide temperature range. These phenomena are characterized by very different activation energies from 10 kJ/mol (rotational mobility of hydroxyls in WAW molecules), 20-40 kJ/mol (rotational mobility of the molecules in SAW), 40-80 kJ/mol (rotational and translational mobility of the water molecules in pores of different sizes), and 60-200 kJ/mol (molecular and associative desorption of water) (Figure 5.34). As a whole, all the distribution functions of activation energy fiJS) obtained using different methods are well concordant. This is caused by the nature of activated processes whereas all the processes are caused by the molecular mobility of water dependent on the topological and chemical characteristics of confined space in nano- and mesopores in LiChrolut EN adsorbent. 

The apparent molal volumes and compressibilities of galactose, glucose, maltose, sucrose, and dextran have been calculated fiom measurements of the density and ultrasound velocity of their aqueous solutions at 25 C. C- and H-n.m.r. lattice relaxation times T, have been used to provide information on the temperature dependence of the rotational mobility of both the sugar and the water molecules in concentrated aqueous solutions of sucrose and trehalose. ... 

More recently Yasuda and Sharma have demonstrated that polymeric solids having little or no water content also exhibit variability of surface properties due to short-range rotational mobility of the surface polymer moleculesThe same has been found to be true for plasma-treated surfaces and for surface of hydrophj ic polymers grafted onto a hydrophobic substrate polymer by using the sophisticated ESCA technique. 

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