Non-bound water

Water occurs in glass-ionomer and related cements in at least two different states (Wilson McLean, 1988 Prosser Wilson, 1979). These states have been classified as evaporable and non-evaporable, depending on whether the water can be removed by vacuum desiccation over silica gel or whether it remains firmly bound in the cement when subjected to such treatment (Wilson Crisp, 1975). The alternative descriptions loosely bound and tightly bound have also been applied to these different states of water combination. In the glass-poly(acrylic acid) system the evaporable water is up to 5 % by weight of the total cement, while the bound water is 18-28 % (Prosser Wilson, 1979). This amount of tightly bound water is equivalent to five or six molecules of water for each acid group and associated metal cation. Hence at least ten molecules of water are involved in the hydration of each coordinated metal ion at a carboxylate site. 

Scanning electron microscopy shows the cement to consist of zinc oxide particles embedded in an amorphous matrix (Smith, 1982a). As with the zinc phosphate cement, a separate globular water phase exists since the cement becomes uniformly porous on dehydration. Porosity diminishes as the water content is decreased. Wilson, Paddon Crisp (1979) distinguish between two types of water in dental cements non-evaporable (tightly bound) and evaporable (loosely bound). They found, in the example they examined, that the ratio of tightly bound to loosely bound water was 0-22 1-0, the lowest for all dental cements. They considered that loosely bound water acted as a plasticizer and weakened the cement. 

Wilson, Paddon Crisp (1979) have shown that the water present in the cement can be divided, somewhat arbitrarily, into boimd water of hydration (non-evaporable) and loosely held (evaporable) water. The amount of tightly bound water increases as the cement ages and in one example reached 42 % of the total water. 

One last class of mononuclear non-haem iron enzyme that we have not yet considered, consists of the microbial superoxide dismutases with Fe(III) at their active site. The crystal structure of the E. coli enzyme shows a coordination geometry reminiscent of protocatechuate 3,4-dioxygenase, with four endogenous protein ligands, three His and one Asp residue, and one bound water molecule (Carlioz et ah, 1988). 

If the aqua complex is very inert, as for example in the case of [Rh(H20)6] " or [Ir(H20)6], the experiment is best performed by dissolving the complex enriched in 0-water, [M(H2 0)6] ", in non-enriched water (2,26). Preparing the initial condition in this way leads to a relatively intense signal for bound water. The exchange rate constant can then be measured by observing the decrease in the bound water signal with time. The decrease in the mole fraction of labeled water coordinated to the metal, x, is described by Eq. (7) ... 

Figure 6 shows the calculated tc values plotted vs. inverse absolute temperature. In the temperature range below —15° C, it is seen that the rc values are not dependent on Wc but dependent on temperature. The rc value increases from ca, 3 x 10-8 sec at —15°C to ca. 3 x 10 7 sec at — 60°C. This shows that the bound water in the system is in the state between viscous liquid and non-rigid solid in this temperature range. As seen from the figure, the In rc vs. temperature-1 (A-1) plots are apparently linear. The temperature dependence of rc may be expressed with considerable accuracy by the Arrhenius equation in the form (12)... 

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. 

NMRD studies of Gd3+ complexes of DOTPME and DOTPMB indicate q< 1 suggesting that the inner coordination sphere of these complexes is obstructed due to the steric encumbrance of the alkoxy substituents [ 104]. In a multinuclear NMR study of Ln3+ complexes (Ln = La, Gd, Dy, Tm and Yb) with a fluorinated ethyl ester analog of DOTP (F-DOTPME), the 19F NMR spectra reveal up to 16 resonances, which demonstrate that these complexes exist in aqueous solution as a mixture of stereoisomers [105]. [Gd(F-DOTPME)] afforded a water proton relaxivity typical of non-hydrated complexes. 170 NMR of the Dy 1 complex confirmed the lack of a bound water molecule. 

Lanthanide complexes of mono- and tetra-amide /1-cyclodextrin derivatives of DOTA have been characterized [140]. The proton NMR spectra of the Eu3+ complexes in methanol-d, show that, while the tetra-amide complex occurs in solution exclusively as a C4-symmetry SAP structure, the mono-amide complex, with less than C4-symmetry, occurs predominantly as two SAP isomers (A/XXXX and Al8885), with the presence of a small amount of the twisted SAP isomer. Luminescence and relaxivity measurements confirm that the Eu3+, Tb3+ and Gd3+ complexes of the eight-coordinate mono-amide ligand possess one bound water molecule, while the tetra-amide complexes have q = 0. The relaxivity of the /LCD mono-amide Gd3+ complex is enhanced when non-covalently bound to a second Gd3+ complex bearing two phenyl moieties (MS-325, AngioMARK , EPIX/Mallinckrodt). 

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