Water-structure perturbants

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. 

Several investigators have noted the direct reaction of peroxynitrite with many buffer anions at neutral to alkaline pH (Hughes and Nicklin, 1970 Keith and Powell, 1969). We have found that many common buffer anions, such as formate, will accelerate the decomposition of peroxynitrite (Fig. 28). The mechanism is unknown, but may involve perturbations of water structure by anions. 

This water structure illustrates the level of complexity of these periodic four-connected nets of water molecules that can occur when perturbed by molecular species that have both hydrophilic and hydrophobic character. If the condition of high-resolution periodicity throughout the water structure is relaxed, as in protein crystals, even greater complexity is possible. 

Mancinelli R, Botti A, Bruni F, Ricci MA, Soper AK. Perturbation of water structure due to monovalent ions in solution. Phys. Chem. Chem. Phys. 2007 9 2959-2967. 

In the field of biology, the effects of hydration on equilibrium protein structure and dynamics are fundamental to the relationship between structure and biological function [21-27]. In particular, the assessment of perturbation of liquid water structure and dynamics by hydrophilic and hydrophobic molecular surfaces is fundamental to the quantitative understanding of the stability and enzymatic activity of globular proteins and functions of membranes. Examples of structures that impose spatial restriction on water molecules include polymer gels, micelles, vesicles, and microemulsions. In the last three cases since the hydrophobic effect is the primary cause for the self-organization of these structures, obviously the configuration of water molecules near the hydrophilic-hydrophobic interfaces is of considerable relevance. 

The amazingly very low solubility of P-CD in water (about 10 times lower than the solubility of a- and y-CD) is explained by its relatively rigid structure and resulting from this high ability to crystallise [3] or by the xmfavourable 7-fold symmetry of P-CD leading to a perturbation of the water structure [4]. 

The method suggested in the present paper provides the correlation volume. The thickness of the layer of water thus calculated, which is affected by a solute molecule, indicates how deeply a single solute molecule has perturbed the structure of the vicinal water molecules. 

It is clear that these questions can be answered if information about the local structure and intermolecular interactions in the layers of the perturbed water can be obtained. 

An important step in understanding the local structure around a nonpolar solute in water was made by Jorgensen et al. Using Monte Carlo simulations based on an intermolecular potential, which contained Lennard-Jones and Coulomb contributions, they determined the number of water molecules in the first hydration layer (located between the first maximum and the first minimum of the radial distribution function) around a nonpolar solute in water. This number (20.3 for methane, 23 for ethane, etc.) was surprisingly large compared with the coordination numbers in cold water and ice (4.4 and 4, respectively). These results provided evidence that major changes occur in the water structure around a nonpolar solute and that the perturbed structure is similar to that of the water—methane clathrates, ... 

Li pulsed gradient spin-echo (PGSE) measurements on LiPPh2 in thf or Et20 solutions show that the compound is a monomer in the former, but a dimer in the latter solution.9 Proton NMR chemical shifts have been used to examine perturbations in water structure in LiOH, KF or KC1 solutions.10... 

When the factors affecting each of the above steps of the solution process are considered, it seems clear that the two latter steps should depend on the size and the effective surface area (or volume) of the solute molecule, and on the magnitude of the molecular solute-water and water-water interaction energies. The water structure-perturbing effects of various additives have been discussed above. It is therefore evident that the solubility of a compound in water and in an aqueous solution of a salt or some other solute may differ. This should be particulary taken into account while studying the water solubility of readily solube compounds 41) as the saturated aqueous solution of such a compound should be regarded as the aqueous medium, the structure of water in which has been modified by the dissolved compound (even assuming the absence of the solute-solute interactions). 

Not so far answered experimentally is the nature of the transition between the Interfaclal film region and bulk water. That is, if bulk water is resting on a polymer surface, there will be a thin region of structurally perturbed water substance and a transition from this region to bulk water structure. The situation is Illustrated schematically in Figure 13. It seems reasonable to assume that the thickness of this transition region is about that of the adsorbed film at P°, or about Xo sugges-... 

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