Hydration critical level

Riboflavin also may adsorbed on growing lactose crystals and alter the crystalline habit. Since it is naturally present in the whey from which lactose hydrate is made and is present in all dairy foods, its influence on lactose crystallization may be of special interest. Adsorption is dependent upon concentration of riboflavin in solution, on degree of lactose supersaturation and on temperature (Leviton 1943, 1944 Michaels and Van Krevald 1966). No adsorption occurs below a certain minimum (critical) concentration of riboflavin (2.5 uglml), but adsorption increases linearly with riboflavin concentration above this critical level. Increasing the temperature of crystallization results in reduced riboflavin adsorption. Adsorption is favored at lower supersat-... 

After the mix has set, the hydration of the less reactive fractions of the lime continues (as does the slower hydration of cement). This results in a progressive stiffening of the mix as well as a progressive rise in temperature. The latter increases the water vapour pressure within the hydrogen bubbles and if that pressure exceeds a critical level, it causes cracking within the cake, and loss of strength. 

The activity of a soluble DNA polymerase in wheat embryos starts to increase during the first six hours after imbibition commences and continues to do so for at least the 1st day [91]. DNA replication occurs after radicle expansion, some 15 h after the initial hydration of the embryo [90]. The polymerase appears to be synthesized de novo during germination, for if protein synthesis is inhibited during the first nine hours then subsequent DNA synthesis is inhibited. Inhibition of protein synthesis at times after nine hours is progressively less effective in preventing DNA synthesis, presumably indicating that critical levels of the polymerase are synthesized prior to the ninth hour. 

Anhydrous sodium tripolyphosphate is slow to hydrate in contact with the atmosphere under normal ambient conditions and generally remains free-flowing. If the relative humidity is below a critical relative humidity, which is different for both anhydrous forms of STP and dependent on temperature, hydration does not take place. For prolonged storage at room temperature, relative humidities above ca 60% in the air result in water absorption. For shorter periods, high levels of humidity can be tolerated. However, even at higher humidities, the amount of water absorbed is small. The heats evolved from vapor hydration of STP-I and -II have been estimated at 343 and 334 kj /mol (82.0 and 79.9 kcal/mol), respectively (25). 

The mechanism of the inhibitive action of LiOH proposed by Stark et al. [7] is attributed to the formation of lithium silicate that dissolves at the surface of the aggregate without causing swelling [7], In the presence of KOH and NaOH the gel product incorporates Li ions and the amount of Li in this gel increases with its concentration. The threshold level of Na Li is 1 0.67 to 1 1 molar ratio at which expansion due to alkali-silica reaction is reduced to safe levels. Some workers [22] have found that when LiOH is added to mortar much more lithium is taken up by the cement hydration products than Na or K. This would indicate that small amounts of lithium are not very effective. It can therefore be concluded that a critical amount of lithium is needed to overcome the combined concentrations of KOH and NaOH to eliminate the expansive effect and that the product formed with Li is non-expansive. 

The activation energy of absorption I is reported in Table I for Na-F86.5 at different hydration levels. It increases with increasing water content. Because of the rapid shift of the critical frequency with water content relevant activation energies for the other samples cannot be given. 

G. Bell, A. E. M. Janssen, and P. J. Halling, Water activity fails to predict critical hydration level for enzyme activity in polar organic solvents interconversion of water concentrations and activities, Enzyme Microb. Technol. 1997, 20, 47 477. 

Ocular damaging and irritant agents can be identified and evaluated by the Draize rabbit test [114]. However, more recently this test has been criticized on the basis of ethical considerations and unreliable prognosis of human response. Alternative methods such as the evaluation of toxicity on ocular cell cultures have been recommended and are being indicated as promising prognostic tools [115-120]. Direct confocal microscopic analysis [121], hydration level of isolated corneas [122], and various other tests on isolated corneas or animal eyes have also been proposed for evaluation of ocular toxic effects. 

Adequate description of both the equilibrium geometry and vibrational spectra is critically important for the free energy calculations. Table 21.1 presents a comparison of equilibrium geometries of sulfuric acid monohydrate obtained at different levels of theory with experimental data of Leopold with co-authors [118]. As maybe seen from Table 21.1, both ab initio and DFT methods reproduce the hydrate structure with sufficient accuracy. The PW91PW91 and MP2 in combination 6-311-1—l-G(3df,3pd) basis set provide the best overall agreement with experimental data. 

Likhtenshtein and colleagues (Belonogova et al., 1978, 1979 Likhten-shtein, 1976) carried out a series of measurements on the hydration dependence of the mobility of spin labels covalently bound to several proteins. The results were correlated with Mossbauer spectroscopic data obtained in parallel experiments. Spin-labeled human serum albumin and a-chymotrypsin showed a critical hydration level for onset of motion at relative humidity 0.8, equivalent to 0.2 h. The temperature dependence of the spin label spectrum showed a critical temperature of 230 K, below which motion was frozen. Serum albumin labeled at surface sites... 

Steinhoff et al. (1989) measured the temperature and hydration dependence of the ESR spectra of hemoglobin spin-labeled at cysteine )8-93. They observed the critical temperature near 200 K, as described above, and the sensitivity of the spectrum to hydration level. Spectrum simulations suggested that there were two types of motion in the dry protein, a fast vibration of the label within a limited motion cone upon the addition of water, a hydration-dependent motion assigned to the fluctuations of the protein, of correlation time 10 sec in samples of high hydration and at 300 K. The temperature dependence of the motional properties of a spin probe (TEMPONE), diffused into hydrated single crystals, closely paralleled the motional properties of the label. This was taken to be evidence for coupling between the dynamical properties of the protein and the adjacent solvent. 

Khurgin et al. (1977) measured the chymotrypsin-catalyzed breakdown of the amide substrate A -succinyl-L-phenylalanine-/>-nitroaniline at low hydration levels. For this substrate the acylation process is rate limiting. Figure 28 shows the extent of reaction for 1 1 enzyme-substrate mixtures, of nominal pH 7.5, reacted for 5—7 days. The intent of the experiments was to define the critical water concentration at which activity could first be detected. This was determined as the intercept of the linear region of the response with the abscissa. For chymotrypsin with no added buffer, the critical hydration level was at relative humidity 0.48, which corresponds to 0.12 A (Luescher-Mattli and Ruegg, 1982a). The reaction grows explosively (Fig. 28) above this hydration level. Addition of 0.57 g of sodium acetate per g of chymotrypsin reduced the critical hydration level by about half. This may reflect the hydration of the the salt, rather than a specific effect on the enzyme. 

Temperature and hydration level are linked in determining the dynamics of protein and solvent. The dry protein shows, for all temperatures, only the restricted motion found below the critical temperature for hydrated samples. A fully hydrated sample shows strong temperature dependence for the dynamic properties of both protein and hydration water, for temperatures above the critical temperature. Partially hydrated samples behave complexly. Goldanskii and Krupyanskii (1989) gave a particularly good discussion of the linkage between the effects of temperature and hydration. 

For other enzymes activity has been detected in some cases at 0.1 h or, rarely, at a lower hydration level. Most of the enzymes studied show onset of activity between 0.1 and 0.2 h, but there is not enough information to arrive at a consensus value. There appears to be no single hydration level that is critical for enzyme catalysis. Perhaps it is to be expected that different mechanisms should be associated with different roles of solvent. 

Protonic percolation may be the event that imposes a lower limit on the hydration level for onset of enzyme activity, for those enzymes dependent on general catalysis. The hydration level at which chymotrypsin first displays activity, 0.12 h, is in agreement with this suggestion. The chymotrypsin mechanism includes general catalysis, but not significant substrate rearrangement in the rate-determining step. As noted, other enzymes show a critical hydration level between 0.1 and 0.2 h. 

We start from the experimental observation that lysozyme powders exhibit a single critical hydration level for the onset of enzyme activity and the onset of surface motions, displayed in the dynamics of a spin... 

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