Bound water definition

Another property relevant to the current discussion is the distribution of water in the active site. Specifically, we characterize the population of various water wires connecting the zinc-bound water/hydroxide and His 64 found in the SCC-DFTB/MM simulations. These wires were identified following a definition of hydrogen-bond in terms of both distance (O—O < 3.5 A) and angle (O—H—O > 140°) and care... 

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. 

Some evidence234 for Zn—OH attack in anhydride hydrolysis has been obtained using the complex (65) (Section 61.4.11) but the evidence is not definitive, and other mechanisms could apply. Large rate enhancements occur in the Zn11- and Cu -promoted hydrolysis of the lactam (66) (Section 61.4.10). Rates increase commensurate with the ionization of a metal-bound water molecular and sigmoidal pH-rate profiles are observed. Rate enhancements of 9 x 105 and 1 x 103 occur with (66)—Cu—OH and (66)—Zn—OH compared with the free ligand. A number of other reactions which are believed to proceed via M—OH species, in kinetically labile systems, are considered in Section 61.4.3. 

The present evidence is thus that kinetic effects may account for half or more of permittivity decreases of ionic solutions and this may be an important factor in determing the amplitude of the Y dispersion in conducting biopolymer solutions and lead to revisions in estimated nature and amount of bound water. The effect may also have some bearing on dielectric properties of cell interiors and membranes if these have appreciable conductances. It would seem premature to attempt definitive answers to such questions until the relative importance of static and kinetic effects in presumably simpler ionic solutions has been better established experimentally in comparison with theory which treats them self-consistently. 

A solid compound that contains a definite percentage of bound water. 

Chemically bound water is most reasonably defined as including that present in interlayer spaces, or more firmly bound, but not that present in pores larger than interlayer spaces. As will be seen in Chapter 8, the distinction between interlayer space and micropores is not sharp water adsorbed on surfaces of pores further blurs the definition. From the experimental standpoint, the determination is complicated by the fact that the amount of water retained at a given RH depends on the previous drying history of the sample and on the rate at which water is removed. An approximate estimate is obtained by equilibrating a sample, not previously dried below saturation, with an atmosphere of 11% RH (F12,F13,F14). Saturated aqueous LiCl HjO gives the required RH (partial pressure of water vapour = 2,7 torr at 25°C). To achieve apparent equilibrium in a reasonable time (several days), the sample must be crushed and the system evacuated the salt solution should be stirred, at least intermittently. Young and Hansen (Y5) found the composition of the C-S-H in C3S paste thus... 

As with calcium silicate pastes, the gelatinous nature of the principal reaction product renders any definition of chemically bound water somewhat arbitrary. The three definitions of water content described in Section... 

The amino acid residue that bridges the two-metal sites is shown in italic boldface. When the symbol H2O is given this may represent from one to three metal-bound water molecules. R is the distance between the metal atoms. When Trp is listed as a ligand, the a-amino and a amide carbonyl are the ligands. See footnote of Table 1 for the definitions of other terms. 

With this picture the terms hydration shell and bound water are understood to mean the water at the hydration end point. With this definition several questions should be addressed. 

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."... 

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