Hydration shell, Structure

F. Floris, M. Persico, A. Tani and J. Tomasi, Hydration shell structure of the calcium ion from simulations with ab initio effective pair potentials, Chem. Phys. Lett., 227 (1994) 126-132. 

G. Palinkas, T. Radnai, W. Dietz, G.Y. Szasz and K. Heinzinger, Hydration shell structures in a magnesium chloride solution from x-ray and MD studies, Z. Naturforsch., 37a (1982) 1049-1060. 

A. Tongraar, K. R. Liedl, and B. M. Rode, The hydration shell structure of LP investigated by Bom-Oppenheimer ab initio QM/MM dynamics, Chem. Phys. Lett. 286, 56-64 (1998). 

The results of these calculations imply that none of the ions would be contact adsorbed when no specific interactions between ions and metal are taken into account in the model. The Li+ ion, believed to be nonspecifically adsorbing, would be able to approach the surface more closely than the anions, mostly because of its small size, which allows it to penetrate the surface layers without displacing water molecules. The simulation results thus indirectly demonstrate the importance of specific chemical interactions for realistic models of the electric double layer. Apparently, also some specific features of the hydration shell structure of the ions must be taken into consideration in order to fully understand the adsorption of ions. 

Spohr [190] studied the adsorption of on the Pt(lOO) surface. The free energy barrier towards iodide adsorption that is produced by the layers of adsorbed water is associated with a significant intermediate increase in coordination number, before the hydration number decreases at short ion-metal distances for geometrical reasons. Philpott and Glosli [109] observed in a series of MD studies of ion adsorption on charged electrodes that the Li+ hydration shell structure in the vicinity of a model metal surface does not depend on the halide counterion (F, Cl , Br , I ). In this study, no specific interactions between the metal surface and water molecules or ions were employed. 

The residence times for the primary water shells around Li+ ions increase significantly with solution concentration, while only a moderate increase was observed for Rb+ ions and almost no noticeable change for Cs+ ions. This can also be explained in terms of different hydration shell structures around these ions. Strong hydrophilic hydration is daminating for small ions. 

We next consider vibrational relaxation at the interface between water and an immiscible liquid. Though total density remains relatively constant on transferring the solute from bulk water across the interface, the change in the hydration shell structure produces a change in the vibrational lifetime that follows the same principle discussed above, namely, that an ionic solute is able to keep its hydration shell. This principle manifests itself in the increase in surface roughness on ion transfer across the liquid/liquid interface, as noted earlier. 

Atta-Fynn R, Bylaska EJ, Schenter GK, de Jong WA. Hydration Shell Structure and Dynamics of Curium(III) in Aqueous Solution First Principles and Empirical Studies. J Phys Chem A. 2011 115 4665-4677. 

Various experimental probes on the hydration-shell structure of Cm (aq) reported in literature have yielded a wide range of coordination numbers. To mention a few, EXAFS experiments measured primary hydration numbers of 9 or 7 (based on the truncation of the EXAFS fitting data) in 1M HCIO4 acid [114] and 10 in 0.25 M HCl acid [115]. High energy X-ray scattering (HEXS) experiments yielded a hydration number of 8.8 [114]. Time-resolved laser fluorescence spectroscopy (TREES) found coordination numbers between... 

Table 12.7 lists the structural properties of the hydration shell of UOjfaq) and UO (aq). For the purposes of comparison, past experimental and theoretical data for the first shell of AnO (An = U, Np, Pu) are also reported in Table 12.7. The hydration shell structure of UO (aq) has been described in detail elsewhere [150] so we will focus on the shell structure of UOjfaq). As shown in Figure 12.8, the AIMD simulations indicate that the first shell of UOjfaq) has five water molecules in the equatorial plane, in contrast to the QM/MM prediction of 4.51. The predicted U(V)=0< distance is very close to previous measurements of other actinyl(V) ions (Np(V) and Pu( V)) and are greater than the previous predicted value by 0.07A. Also, our average first-shell U-0 bond distance is slightly longer than the previous simulated value, which is expected since the first shell of the AIMD simulations contains more water ligands. Previous gas-phase structures exhibit slightly longer bonds as expected. Relative to UO (aq), UO Caq) shows a lengthening of 0.08A and 0.1 A for the U=Oai and bonds, respectively, because of reduced electrostatic attraction. Other first-shell properties of UOjfaq) and UO faq), such as the intramolecular water geometry and tilt angles, compare closely.

Atta-Fynn R, Bylaska EJ, Schenter GK and de Jong WA 2011 Hydration shell structure and dynamics of cnrinm(iii) in aqueous solution First principles and empirical studies. The Journal... 

In the traditional view hydrophobic interactions are assumed to be driven by the release of water molecules from the hydrophobic hydration shells upon the approach of one nonpolar solute to another. Although the ideas about the structure of the hydrophobic hydration shell have changed, this view is essentially unaltered... 

It is possible to indicate by thermodynamic considerations 24,25,27>, by spectroscopic methods (IR28), Raman29 , NMR30,31 ), by dielectric 32> and viscosimetric measurements 26), that the mobility of water molecules in the hydration shell differs from the mobility in pure water, so justifying the classification of solutes in the water structure breaker and maker, as mentioned above. 

As mentioned above, water structure in reversed micelles deviates considerably from the structure in the bulk-phase. Therefore, the hydration shell of macromolecules entrapped in reversed micellar systems should be changed and thus also their conformation. According to the results of several authors this is indeed the case. 

Protein crystals contain between 25 and 65 vol% water, which is essential for the crystallisation of these biopolymers. A typical value for the water content of protein crystals is 45% according to Matthews et al. l49,150). For this reason it is possible to study the arrangement of water molecules in the hydration-shell by protein-water and water-water interactions near the protein surface, if one can solve the structure of the crystal by X-ray or neutron diffraction to a sufficiently high resolution151 -153). 

The information obtained from X-ray measurements on the arrangement of the water molecules naturally depends very much on the resolution and state of refinement of the crystal structure investigated. For detailed information on the organization of water molecules in the protein hydration shell at the surface and on the bulk water in the crystals a 1,2 to 1,8 A resolution range is necessary 153>. 

Recently Blake et al.153) made such studies in the case of human (HL) and tortoise egg-white (TEWL) lysozyme based on crystallographic refinements at 1,5 and 1,6 A resolution, respectively. By these investigations they attempted to obtain information on the perturbations of water structure in the hydration shell by neighboured protein molecules and by high salt concentrations as well as on the degree of order of the bound water. The authors came to the conclusion that the number of ordered water molecules are 128 in TEWL and 140 in HL, whereas the overall content is made up of 650 and 350 water molecules per lysozyme molecule. 

One of the most thoroughly investigated examples of polymeric biomolecules in regard to the stabilization of ordered structures by hydration are the DNAs. Only shortly after establishing the double-helix model by Watson and Crick 1953 it became clear, that the hydration shell of DNA plays an important role in stabilizing the native conformation. The data obtained by the authors working in this field up until 1977 are reviewed by Hopfinger155>. 

Aqueous solutions are not suitable solvents for esterifications and transesterifications, and these reactions are carried out in organic solvents of low polarity [9-12]. However, enzymes are surrounded by a hydration shell or bound water that is required for the retention of structure and catalytic activity [13]. Polar hydrophilic solvents such as DMF, DMSO, acetone, and alcohols (log P<0, where P is the partition coefficient between octanol and water) are incompatible and lead to rapid denaturation. Common solvents for esterifications and transesterifications include alkanes (hexane/log P=3.5), aromatics (toluene/2.5, benzene/2), haloalkanes (CHCI3/2, CH2CI2/I.4), and ethers (diisopropyl ether/1.9, terf-butylmethyl ether/ 0.94, diethyl ether/0.85). Exceptionally stable enzymes such as Candida antarctica lipase B (CAL-B) have been used in more polar solvents (tetrahydrofuran/0.49, acetonitrile/—0.33). Room-temperature ionic liquids [14—17] and supercritical fluids [18] are also good media for a wide range of biotransformations. 

Molecular dynamics calculations have been performed (35-38). One ab initio calculation (39) is particularly interesting because it avoids the use of pairwise potential energy functions and effectively includes many-body interactions. It was concluded that the structure of the first hydration shell is nearly tetrahedral but is very much influenced by its own solvation. 

Electrodes based on solutions of cyclic polyethers in hydrocarbons show a selective response to alkali metal cations. The cyclic structure and physical dimensions of these compounds enable them to surround and replace the hydration shell of the cations and carry them into the membrane phase. Conduction occurs by diffusion of these charged complexes, which constitute a space charge within the membrane. Electrodes with a high selectivity for potassium over sodium (> 1000 1) have been produced. 

A fourth solvent structural effect refers to the average properties of solvent molecules near the solute. These solvent molecules may have different bond lengths, bond angles, dipole moments, and polarizabilities than do bulk solvent molecules. For example, Wahlqvist [132] found a decrease in the magnitude of the dipole moment of water molecules near a hydrophobic wall from 2.8 D (in their model) to 2.55 D, and van Belle et al. [29] found a drop from 2.8 D to 2.6 D for first-hydration-shell water molecules around a methane molecule. 

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