Vicinal water definition

Water in food products can be described as being free or bound. The definition of what consitiutes bound water is far from clear (see Fennema, 1985) but it can be considered as that part of the water in a food which does not freeze at — 40°C and exists in the vicinity of solutes and other non-aqueous constituents, has reduced molecular mobility and other significantly altered properties compared with the bulk water of the same system (Fennema, 1985). The actual amount of bound water varies in different products and the amount measured is often a function of the assay technique. Bound water is not permanently immobilized since interchange of bound water molecules occurs frequently. 

One intriguing aspect of enzymatic catalysis in low water environments (this includes 85% dioxane where the water activity is substantially less than unity) is the effect of reaction pH. In aqueous solutions, the ionogenic functional groups of an enzyme respond to the pH of the solution. Definitive pH optima exist for all enzymes. In organic solvents, however, the lack of water as a bulk solvent makes pH an unmeasurable variable although there will be a small concentration of protons in the vicinity of... 

The most inclnsive definition of hydration shell describes it as consisting of all thermodynamically altered water molecnles in the vicinity of a solnte. From a thermodynamic standpoint, hydration can be viewed as binding of water molecnles to the hydration sites of a solnte. The energetics of this association is modulated by the type of solute-solvent interactions (electrostatic, hydrogen bonding, van der Waals) and by solnte-indnced solvent reorganization. The latter occnrs even in the absence of appreciable solute-solvent interactions becanse the eqnUib-rium distribution of hydrogen-bonded water networks of the bulk becomes disrupted at the solute surface. 

If the sponge is left to dry in the sun, this adsorbed water will evaporate, leaving only a small proportion of water bound chemically to the salts and to the cellulose of the sponge fibers. Like water in sponge, water is held in food by various physical and chemical mechanisms (Table 3.1). It is a convenient oversimplification to distinguish between free and bound water. The definition of bound water in such a classification poses problems. Fennema (1985) reports seven different definitions of bound water. Some of these definitions are based on the freezability of the bound component, and others rely on its availability as a solvent. He prefers a definition in which bound water is that which exists in the vicinity of solutes and other non-aqueous constituents, exhibits reduced molecular activity and other significantly altered properties as compared with bulk water in the same system, and does not freeze at -40"C."... 

Infrared spectroscopy on thin samples of ice about 10-20 nm thick have also shown that the H-bond network on the surface of ice in contact with water vapour is not exactly the same as in the bulk, but more hquid-hke, indistinguishable from that of liquid water (21). This transition layer extends over more than 10 HjO layers at 0 °C but rapidly shrinks (22), becoming hardly a single layer at 10 °C. The structure of this layer on the surface of ice is not precisely known. Thus X-ray diffraction indicates that its density is more in the vicinity of 1.2gcm than around the expected value 1 (23). It at least shows that the arrangements of H2O molecules on the ice surface are definitely different from those in the bulk. 

A small but important class of atmospheric aerosol particles are the ice nuclei. These nuclei promote the freezing of water drops in clouds (see Fletcher, 1962). In this way they play a definite role in the formation of precipitation in mixed clouds containing both water drops and ice crystals. This kind of precipitation formation is due to the fact that the saturation vapour pressure over ice is smaller than over liquid water. In this way ice crystals grow by condensation while drops tend to evaporate. Thus, if human activity emits ice nuclei to the atmosphere the precipitation distribution can be modified. Results of measurements show that in the vicinity of steel works and aluminum foundries the concentration of ice nuclei active at a temperature of — 20 °C is unusually high. It is believed that this is caused by the presence of some metal oxides in the air (Pruppacher, 1973). More recent studies on ice nuclei also showed that lead compounds (e.g. Pbl 2) in exhaust gases of vehicles also have ice nucleating ability. It is believed, however, that anthropogenic ice nuclei cannot play an important role, except in local scale processes (see Pruppacher, 1973). 

As an example of the type of information that can be obtained, the composition and anisotropy of the hydration shells in the vicinity of the mercury surface is analyzed. Figure 27 shows the average coordination number of the ions as a function of position (the mercury surface is located at z = 0 A). Here, the coordination number is defined as the number of water molecules the ion-oxygen distance of which is smaller than 2.5 A, 3.2A, 3.9 A, and 4.2A for Li+, F , Cl , and I, respectively. This definition is based on the positions of the first minima of the ion-oxygen pair correlation functions. 

The above mentioned observation prompted some authors to further analyze both theoretically and experimentally the parameters governing the concentration of the subtrates in the vicinity of the enzyme. Whereas the water-soluble substrates (malonyl-CoA, NADPH-NADH) did not exhibit unexpected properties, it was observed that not only the acyl-CoA partitions between water and the biological membranes (as had been known for years), but that, unexpectedly, the partition coefficient seemed to vary as a function of the surface density (4). More surprising was the observation that the partition of a given amount of acyl-CoA between water and a given amount of membranes was independent of the water volume, i.e. was in marked contrast to the definition itself of a partition coefficient which involves the ratios of concentrations of a given amphiphilic molecule in membrane and in water (5). 

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