Water structure concept

Over the years, a large number of models of water structure have been developed in an attempt to reconcile all the known physical properties of water and to arrive at a molecular description of water that accounts correctly for its behavior over a large range of thermodynamic conditions. Early models of water structure have been categorized by Fennema (1996) and Ball (2001) into three general types mixture, uniformist, and interstitial. Mixture models are based on the concept of intermolecular hydrogen bonds... 

Cluster Theories. Historically, the most important study of water structure based on the existence of clusters was Stewart s x-ray diffraction work (142). In his theory, clusters ( cybotactic swarms ) were postulated to exist, each containing on the order of 10,000 water molecules. Although this constituted an apparently reasonable theory at the time, this view has now yielded to the concept of clusters of considerably smaller sizes. It is interesting to note that without much critical analysis, Frenkel (57) viewed Stewart s theory of water as essentially correct. In fact, Frenkel apparently expected that further work on liquid structures in general would be along the lines Stewart advocated. Luck has discussed this in some detail (100). Subsequent to Stewart s papers, Nomoto (113) discussed a water model, based on ultrasonic studies, involving clusters of several thousand water molecules. 

A theory of water structure involving clusters somewhat similar to the original Nemethy-Scheraga concept has been advocated recently by Luck (101). In his theory, the number of water molecules in the clusters near the freezing point may exceed 100 and, in fact, may approach 700. 

This concept promises to be as useful for analyzing water structure in macro-molecular crystals as were the Ramachrandran plots for limiting the conformation possibilities in polypeptides. 

The existence of clathrate-like water structure adjacent to the hydrophobic surfaces of macromolecules is an attractive hypothesis. Models have been proposed which have received some support from thermodynamical arguments [808]. However, this concept has proved ineffective as a basis for the interpretation of the structure associated with the many electron density solvent peaks, separated by 2.8 A to 3.0 A, which are frequently observed on the Fourier X-ray maps close to the surface of a protein [809, 810], Recently, however, some local clathrate-like water has been observed in special cases in the high-resolution studies of the small plant protein, crambin [811], in a hydrated deoxydinucleoside-phosphate drug complex [812], in (Phe4Val6) antamanide hydrate [8131 and in an oligodeoxy-nucleotide duplex [814],... 

Diamond was the first to focus on the concept of hydrophobic association and demonstrated that, at variance with the Bjerrum theory, ion-pairing of univalent organic electrolytes in water is possible [12]. He capitalized on the hydrophobic hydration concept [11,12] typical of large organic ions (yide supra) that increase the water structure via the formation of ice-like cages, thereby decreasing the system... 

Thomlinson [78] was the first chromatographer to point out that the classical electrostatic ion-pair concept did not hold for IPRs that were usually bulky hydrophobic ions he also emphasized that in the interfacial region between the mobile and the stationary phases, the dielectric constant of the medium is far lower than that of the aqueous phase. Chaotropes that break the water structure around them and lipophilic ions that produce cages around their alkyl chains, thereby disturbing the ordinary water structnre, are both amenable to hydrophobic ion-pairing since they are both scarcely hydrated. The practical proof of such ion-pairing mode can be found in References 80 and 81 many examples of such pairing modes are reported in the literature [79-86],... 

Swift and Sayne used concepts similar to those of Bockris and Saluja if a molecule stays associated with an ion for more than the time needed for a diffusional jump, it counts as a primary hydration number. This approach yields approximately 4 solvation molecules for and Ca ", and 5 for Ba and Sr", whereas nonspectroscopic methods for these systems yield values that are two to three times larger. Does NMR measure only water arranged in a first, octahedral layer in the first shell near the ion and is it insensitive to the rest of the water structure near an ion ... 

An alternate or complementary explanation for different levels of water retention is offered by Scheuplein (Figure 18). He cites evidence of different structures and properties in water bound at protein interfaces as compared with free water (89). His concept is that of SC fibrils and lipid matrix arranged to form capillary channels that have water bonded completely, partially, or indirectly to protein surfaces depending on their spacing (20, 69). This concept does not readily explain the reversibility of water retention at ambient temperatures (Figure 15), but it may explain the observations of other workers (Figure 18). Baden and Goldsmith have a similar structural concept (71). 

Structural models emerge from the notion of membrane as a heterogenous porous medium characterized by a radius distribution of water-filled pores. This structural concept of a water-filled network embedded in the polymer host has already formed the basis for the discussion of proton conductivity mechanisms in previous sections. Its foundations have been discussed in Sect. 8.2.2.1. Clearly, this concept promotes hydraulic permeation (D Arcy flow [80]) as a vital mechanism of water transport, in addition to diffusion. Since larger water contents result in an increased number of pores used for water transport and in larger mean radii of these pores, corresponding D Arcy coefficients are expected to exhibit strong dependencies on w. 

It is known that the main specific property of pure liquid water as compared to the other solvents consists in its being highly structured. Innumerable models of the water structure, enumeration, and classification of these models are proposed in the literature (see, e.g.,14)). Some of these models such as the one suggested by Samoilov 15) or the one advanced by Nemethy and Scheraga 16) have greatly influenced the concepts of the water structure some of the other models, as Naberukhin 17) wittily puts it, have demonstrated rather inexhaustible imagination of their authors. The basic limitation of all of these models is due to their qualitative character. It means 17) that the main statements and concepts of these models are in requirement of quantitative specification and that their quantification remains basically unsupported by the primary principles of statistical physics. 

In a simplified form the above conclusions 13) can be summed up as follows the structure and/or the state of water in a complex multicomponent aqueous system (which is an oversimplified physico-chemical concept of a biological system) differ from those of pure liquid water assumingly due to an influence of the system components on these water features. This conclusion seems to agree with the above concept of the water structure 17> since an introduction of a certain amount of an additive into water may distort not only local parameters of the hydrogen bonds network but the averaged ones as well due to the space continuity of the network. 

The above examples imply that the modern concepts of the hydrophobic properties of both biological and synthetical solutes are far from adequate. As is usually the case, an advancement of the concepts seems to depend upon the development of new experimental techniques. We believe that the most important question at present is the one of the influence of the chemical composition of an aqueous medium on the hydro-phobic character of a solute. This question is, on the one hand, essential for the progress of the concepts of hydrophobic properties of solutes and of fundamentals of water structure and solution theory and, on the other hand, it seems important for better understanding the factors regulating biological processes and biological potency of chemical agents. 

The key to understanding water structure in solid and liquid form lies in the concept and nature of the hydrogen bonds. In the crystal of ordinary hexagonal ice (Figure 3.6), each molecule forms four hydrogen bonds with its nearest neighbors. 

The degree of breakdown of the water structure, plus the degree of formation of new ion- or solute-water structures, is measured by the heat of hydration (Table 3.2). Heats of hydration are less ambiguous than structural concepts of hydration numbers and ion-, solute- or clay-water interactions. 

The librational fraction is discussed in the context of the concepts of the water structure The hat potential models the defects of the water (ice) structure and rigid polar molecules reorient relatively freely in these defects. In the case of water the lifetime Tor of this fraction (on the order of 10 13 s) is several times greater than that of the H-bond. 

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