Dehydration versus hydration

The hydration shell is formed with the increasing of the water content of the sample and the NA transforms from the unordered to A- and then to B form, in the case of DNA and DNA-like polynucleotides and salt concentrations similar to in vivo conditions. The reverse process, dehydration of NA, results in the reverse conformational transitions but they take place at the values of relative humidity (r.h.) less than the forward direction [12]. Thus, there is a conformational hysteresis over the hydration-dehydration loop. The adsorption isotherms of the NAs, i.e. the plots of the number of the adsorbed water molecules versus the r.h. of the sample at constant temperature, also demonstrate the hysteresis phenomena [13]. The hysteresis is i( producible and its value does not decrease for at least a week. 

In practice, one proceeds as follows. The value of bh >s determined for the reaction with a series of acids of similar structure, that is, for carboxylic acids or ammonium ions, etc. Limiting the data to a single catalyst type improves the fit. since the inclusion of data for a second ype of acid catalyst might define a close but not identical line. This means that Ga may be somewhat different for each catalyst type. A plot of log(kBH/p) versus log(A BH(7//i) is then constructed. This procedure most often results in a straight line, within the usual —10-15 percent precision found for LFERs. One straightforward example is provided by the acid-catalyzed dehydration of acetaldehyde hydrate,... 

The dielectric constant curve versus temperature was also investigated for K4[Fe CN)6]-3H20 by Bristoti et al. (80) in the temperature range -80-150°C. Four peaks were observed in the curve from — 80—25°C, due to the presence of the water of hydration. A single peak with a maximum at 105°C was due to a paraelectric order-disorder transition. It is in this temperature region that the dehydration reaction... 

Salting-out is not well understood. One popular explanation for this effect relies on the relative hydration of the protein versus bulk electrolyte. In this model, the electrolyte is assumed to bind bulk water as water of hydration near the ion s surface. Likewise, the protein needs to be hydrated with water. As the bulk electrolyte and the protein compete for bulk water to hydrate their respective surfaces, the protein become partially dehydrated and prefers to fill such exposed surface (dehydrated surface) with other protein molecules thus, facilitating crystal contacts. Thus, the solubility of the protein is reduced as the electrolyte is added to the protein solution. The effectiveness of ions (cations and anions) to cause phase separation (or lower solubility) has been documented by... 

FIGURE 7.7 Influence of cell hydration on the distribution of unfrozen water versus changes in Gibbs free energy of this water (in chloroform-rf). (Adapted from J. Colloid Interface Sci., 283, Turov, V.V., Gun ko, V.M., Bogatyrev, V.M. et al.. Structured water in partially dehydrated yeast cells and at partially hydrophobi-zed fumed silica surface, 329-343, 2005, Copyright 2005, with permission from Elsevier.)... 

FIGURE 3.2 Br0nsted plot for the dehydration of acetone hydrate in acetone, catalyzed by acid. The logarithm of the catalytic rate constant is plotted versus the negative of the dissociation constant of the acid. Points are shown for 32 carboxylic acids and 15 phenols. The point labeled 47 represents 2,4-dinitrophenol. Acids of other types (e.g., oximes) fell much further off the line (see Section 3.6.3). Source Bell and Higginson (1949) by permission of the Royal Society of London. 

---