Functions of hydration

The rest of this chapter has been concerned with phenomena and effects that are connected with hydration. The first one (salting out) concerns the solubility of noneiectroiytes as affected by the addition of ions to the soiution. Here two effects are 

Electrostriction is the study of the effects of squeezing of ions and moiecuies by the electrical forces that are exerted upon them by the ions we have been deaiing with (Section 2.22). It is only recently that modelers have begun to take into account the shapes formed by these compressed bodies. In fact, they do become lenshke in shape (not spheres) and when this is taken into account, agreement between theory and experiment is improved. 

Hydrophobic effects are on a list of special phenomena. They are closely tied to salting in because one of the reasons for hydrophobic effects (water pushing-out effects, one could say) is that the ions of the solute tend to attract each other or other nonelectrolytes present and push the water between them out. Structure breaking in a solution, some part of which rejects water in the rearrangements formed, also gives hydrophobic effects. 

Polyelectrolytes occur in ion-exchange membranes and thus their study has great material value. They have a central importance in biology and the study of then-electrochemistry, as ions, their natural interactions in solution, etc., are important although we are only able to give a short description of them in a chapta- of restricted length. 

Finally, the question of the structure of biological water is one of far-reaching importance. Some workers in the last few decades have suggested that water in biological systems is special but our answer is that this special structure is so readily explicable that no mystery exists. Biological cells are sized on the micron scale and contain much soiid material. The surface-to-volume ratio inside such cells is very large. Most of the waters in cells are in fact surface waters. In this sense, biologicai water is special but only because it has lost the netted-up properties of bulk water and adopted the individual two-dimensional structure of water at all surfaces. 

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However, nitrogen adsorption reveals considerable differences, as shown in Fig. 5.14, in terms of surface area distribution of a 28-day hydrated C3S specimen [20], or of specific surface area as a function of hydration time... 

Fig. 5.15 The specific surface area of plain and CaCl2 containing pastes as a function of hydration time (Collepardi).

The distribution coefficient method, often called the A -value method, was conceived by Wilcox et al. (1941) and finalized by Carson and Katz (1942). The best methane, ethane, and propane charts are from the latter reference. Updated charts are presented for carbon dioxide (Unruh and Katz, 1949), hydrogen sulfide (Noaker and Katz, 1954), nitrogen (Jhaveri and Robinson, 1965), isobutane (Wu et al., 1976), and n-butane (Poettmann, 1984), as well as for a method that is a function of hydrate structure (Mann et al., 1989). 

Harvey and Hoekstra (1972) determined the dielectric constant and loss for lysozyme powders as a function of hydration level in the frequency range 10 —10 Hz. At water contents less than 0.3 h, they found a dispersion at 170 MHz, which increased somewhat with increasing hydration, and a new dispersion at about 10 Hz that develops at high hydration. These dispersions, detected by time-domain techniques, remain measurable down to the lowest temperature studied, — 60°C. Water mobility in the hydration shell below 0 C is in line with other observations of nonfreezing water. Above 0.3 h, in the stage of the hydration process at which condensation completes the surface monolayer, water motion increased strongly with increased hydration (Fig. 11). 

Fig. 14. Hydration dependence of capacitance [9 C, in picofarads (pF)] of the composite capacitor containing a sample of lysozyme powder of pH 3.11 as a function of hydration level of the protein. The capacitance data are given for three frequencies. The hydration level was decreased from the high-hydration limit of more than 0.35 h to the low-hydration limit of near 0.07 h by passage of a stream of dry air through the apparatus. The evaporation rate E (O grams of water evaporated per minute) decreases to 0 at the low-hydration limit. From Careri et al. (1986).

Fig. 21. Values of the correlation time, r, for TEMPONE noncovalently bound to lysozyme in the variable environment as a function of hydration level. Error bar shows the range of values that gives acceptable simulated spectra. Fraction of TEMPONE in the variable environment is 0.5 0.2 at high hydration. From Rupley etal. (1980).

Permyakov and Burstein (1977) measured the steady-state fluorescence of tryptophan in several proteins as a function of hydration. They suggested that hydration increases the flexibility of the protein. 

Azurin has a single buried tryptophan. Fluorescence anisotropy has been measured as a function of hydration level for azurin incorporated in a polymer film (Careri and Gratton, 1986). In the wet film the value of the anisotropy is close to that for azurin in solution at high temperature and low viscosity. At low hydrations and in the dry film, motion of the tryptophan chromophore is frozen. 

Incoherent quasielastic neutron scattering measured as a function of hydration for powders of deuterated phycocyanin has been used to probe water motions (Middendorf et al., 1984). The simplest model accounting for the data was jump diffusion of water molecules between localized-sorption sites and the development of clusters of surface water at higher hydration (half-coverage of the surface, 0.15 h). This model is consistent with the picture developed from sorption thermodynamics. 

Baer, Hiltner, and colleagues (see Hiltner, 1979, and references cited therein) have used dynamic mechanical analysis to examine the hydration of collagen, elastin, and several polypeptides. A torsional pendulum constructed of the sample was examined for low-frequency (i.e., IHz) mechanical loss as a function of hydration and temperature. A common feature is a dispersion that is absent in the dry protein and appears at... 

Differential scanning calorimetry investigations of the SC can involve direct analysis either of the isolated membrane or of the extracted components, often with these materials being examined as a function of hydration or enhancer treatment. The material under investigation is sealed in the sample compartment, while the reference cell is usually left blank (unless the sample is suspended in a solvent, in which case, the reference cell will contain an equivalent mass of solvent). As a sample is heated, its internal energy in-... 

Figure 5 shows the thermally induced (10-60°C) changes in the CHj stretching frequency of intact human SC plotted as a function of hydration and reheating. Corresponding data for the extracted lipid samples are depicted in Fig. 6. For SC, the frequency increased with temperature and showed a small, but definite, inflection point between 35 and 45°C. Under dry conditions, the midpoint was estimated to be 45°C, decreasing to a constant value of 35°C as the hydration level was increased. This behavior closely mimicked the calorimetric results described, suggesting that the 35°C inflection observed by IR corresponded to the same endothermic process measured by DSC. However, no evidence of a thermal transition at 35°C was observed by IR in the...

Swarts SG, Sevilla MD, Becker D, Tokar CJ, Wheeler KT. (1992) Radiation-induced DNA damage as a function of hydration. I. Release of unaltered bases. Radiat Res 129 333-344. 

Among hydrophobic model systems, one experimental investigation of particular interest concerns the structure of water contained in a carbon powder [20]. The structure of water has been determined by both x-ray and neutron diffraction, as a function of hydration, and from room temperature down to 77 K. In agreement with previous work [13-17,38,42,43], this study gave support to the existence of a region near the interface where the properties of water are markedly different from those of the bulk liquid. From x-ray measurements, which yield information about the oxygen-oxygen distribution function, it appears that, at the... 

Figure 4.41. Comparison of the hydration reactivity of the cement minerals C3S, P-C2S and ordinary Portland cement (OPC) determined as a function of hydration time by integrating their Si spectra. From Barnes et al. (1985), by permission of copyright owner.

Functions of hydration, 203 Furth model, in molten salt theory, 638 Fused oxides, and the structure of liquid water, 726... 

Protein dynamics, as a function of hydration, 191 Raman spectroscopy, and electrolytic solutions. 

Solvent causes a net softening of the local atomic potentials, particularly for exposed sidechain atoms, with a commensurate increase in fluctuation amplitudes. Recent inelastic neutron scattering studies of lysozyme as a function of hydration are in accord with this conclusion.2551 In addition, there are significant localized conformational changes, particularly for sidechains and exposed loop regions, and these, coupled with the increased fluctuations observed in the solvent simulation, may be of functional importance. These points are amplified in Chapt. X, where a thermodynamic analysis of the effect of solvent on the conformational equilibria of a dipeptide model is given. 

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