Hydration-dehydration

Mathematical Model of the Nucleic Acids Conformational Transitions with Hysteresis over Hydration-Dehydration Cycle... 

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. 

The reaction is reversible and its stereochemical requirements are so pronounced that neither the cis isomer of fumaric acid (maleic acid) nor the R enantiomer of malic acid can serve as a substrate for the fumarase catalyzed hydration-dehydration equilibrium... 

Acid—Base Catalysis. Inexpensive mineral acids, eg, H2SO4, and bases, eg, KOH, in aqueous solution are widely appHed as catalysts in industrial organic synthesis. Catalytic reactions include esterifications, hydrations, dehydrations, and condensations. Much of the technology is old and well estabhshed, and the chemistry is well understood. Reactions that are cataly2ed by acids are also typically cataly2ed by bases. In some instances, the kinetics of the reaction has a form such as the following (9) ... 

Reaction Mechanism and Kinetics. The equiHbria involved ia the hydration—dehydration of ethylene first proposed (117) can be expressed as follows ... 

The method, as so far developed, is limited by the condition that the hydration-dehydration reaction must proceed at a rate that is slow compared with the time needed to obtain a polarogram. In principle, the method is capable of much wider application to covalent-hydration studies if use is made of oscillographic polarographic techniques or of chronopotentiometry. These refinements are currently being investigated. 

Asai, H. (1961). Study of the hydration-dehydration in polyelectrolyte solutions by the ultrasonic technique. Journal of the Physical Society of Japan, 16, 761-6. 

Several processes often occur in lipids, including oxidation, hydration, dehydration, decarboxylation, esterification, aromatization, hydrolysis, hydrogenation and polymerization. In fact, the chemistry of these materials can be affected, for example, by heat (anthropogenic transformations), humidity, pH, and microbial attacks. 

A thorough understanding of the hydration profile for a solid forming a crystal hydrate is important for several reasons. First, since an anhydrate and hydrate(s) are distinct thermodynamic species, they will have different physical-chemical properties (e.g., solubility) that may affect bioavailability. Second, a desired hydrate species can be formed and used (and retained) simply by controlling the desired, established environmental conditions. Third, since significant quantities of water can be sorbed/liberated as a hydrate becomes hydrated/dehydrated, the physical-chemical properties of the immediate system (including other nearby solids) can be markedly affected. 

---