Hydration in Chemistry and Biology

An understanding of covalent hydration is essential for all who work with heteroaromatic compounds containing doubly bonded nitrogen atoms. As chemists become more aware of the circumstances in which hydration occurs, and the means for detecting it, many new examples will probably be discovered and many puzzling discrepancies solved. Many of the values for ionization constants and ultraviolet spectra which are in the literature refer to partly hydrated equilibrium mixtures and should be replaced by values for the pure substances. 

At the present time, the greatest importance of covalent hydration in biology seems to lie in the direction of understanding the action of enzymes. In this connection, the enzyme known as xanthine oxidase has been extensively investigated.This enzyme catalyzes the oxidation of aldehydes to acids, purines to hydroxypurines, and pteridines to hydroxypteridines. The only structural feature which these three substituents have in common is a secondary alcoholic group present in the covalently hydrated forms. Therefore it was logical to conceive of this group as the point of attack by the enzyme. 

A hypothesis for the oxidation of purines in the presence of this enzyme has been elaborated by Bergmann and his colleagues. It postulates that the purine, often in one of its less prevalent tautomeric forms, is adsorbed on the protein, or the riboflavin coenzyme, of the enzyme then hydration occurs under the influence of the electronic field of the enz5rme, and this must involve a group that is not sterically blocked by the enzyme but which is accessible to the electron-transport pathway of the riboflavin moiety. Finally, the secondary alcohol is assumed to be dehydrogenated in this pathway to give a doubly 

The situation in the pteridine series is somewhat more complex. Pteridine, 2-, 4-, and 7-hydroxypteridine, and some of the dihydroxy-pteridines are oxidized, stepwise and quantitatively, in the presence of xanthine oxidase to a single substance, 2,4,7-trihydroxypteridine. Notably, 6-hydroxypteridine, which readily forms a covalent hydrate, is not attacked. 

Bergmann has suggested that oxidation is ruled out at positions (where hydration occurs readily) which are not accessible to the enzyme after the pteridine is adsorbed on it. Alternatively, the destruction of co-planarity by hydration may prevent adsorption of the pteridine on the enzyme. The case of xanthopterin (2-amino-4,6-dihydroxypteridine) may be relevant. The neutral species of this substance exists as an equilibrium mixture of approximately equal parts of the anhydrous and 7,8-hydrated forms (in neutral aqueous solution at 20°). Xanthine oxidase cataljrzes the oxidation of the anhydrous form in the 7-position but leaves the hydrated form unaffected and about two hours is required to re-establish the former equilibrium. 

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B. E. Conway, Ionic Hydration in Chemistry and Biology, pp. 444-465, Elsevier, New... 

Intramonomer bands other than are also sensitive to H-bonds, although in a much less spectacular way. This sensitivity proves to be most useful in chemistry and biology, and, as will be seen in the case of the H2O molecule, allows precise measurements of the number of H-bonds from which much structural and dynamic information can be obtained. It will prove useful to follow the development of the H-bond network of a macromolecule during hydration (Ch. 10). In physics and chemistry, the hypersensitive band ensures the... 

More recently, assisted by photochemistry, spectroscopy in its varied forms, chromatography, computers, and applied electronics, radiation chemistry is assaulting many of the problems associated with the properties of transient species at an unprecedented rate. Commonplace, already, is the study of intermediates lasting only milli- and micro-seconds. A rapidly developing subdivision is on the horizon—that of nanosecond and picosecond chemistry. Knowledge of the nature and rates of these reactions has been of inestimable aid in untangling reaction mechanisms in chemistry and biology. For example, the discovery of the hydrated electron and the determination of its rate constants has aided the interpretation of reactions in aqueous media. Recent studies on solvated and... 

We turn now to the kinetics o/Fe(II) oxidation. The ability of Fe(II) and Fe(III) to undergo reversible oxidation and reduction plays an important role in the chemistry and biology af natural waters and water de-ironing. Dissolved iron is removed in the form of hydrated Fe203 from natural waters by oxidation with dissolved oxygen and subsequent hydrolysis and flocculation. The oxidation can be represented by the following equation ... 

Biochemistry and chemistry takes place mostly in solution or in the presence of large quantities of solvent, as in enzymes. As the necessary super-computing becomes available, molecular dynamics must surely be the method of choice for modeling structure and for interpreting biological interactions. Several attempts have been made to test the capability of molecular dynamics to predict the known water structure in crystalline hydrates. In one of these, three amino acid hydrates were used serine monohydrate, arginine dihydrate and homoproline monohydrate. The first two analyses were by neutron diffraction, and in the latter X-ray analysis was chosen because there were four molecules and four waters in the asymmetric unit. The results were partially successful, but the final comments of the authors were "this may imply that methods used currently to extract potential function parameters are insufficient to allow us to handle the molecular-level subtleties that are found in aqueous solutions" (39). 

The irradiation of water is immediately followed by a period of fast chemistry, whose short-time kinetics reflects the competition between the relaxation of the nonhomogeneous spatial distributions of the radiation-induced reactants and their reactions. A variety of gamma and energetic electron experiments are available in the literature. Stochastic simulation methods have been used to model the observed short-time radiation chemical kinetics of water and the radiation chemistry of aqueous solutions of scavengers for the hydrated electron and the hydroxyl radical to provide fundamental information for use in the elucidation of more complex, complicated chemical, and biological systems found in real-world scenarios. 

Berndt M, Kwiatkowski JS (1986) Hydration of biomolecules. In N4ray-Szab6 G (ed) Theoretical chemistry of biological systems. Studies in physical and theoretical chemistry, vol. 41. Elsevier, Amsterdam, pp 349-422... 

Hydration Forces Water and Biomolecules Water and Protein Folding Computational Chemistry in Biology Computation and Modeling of Protein Folding... 

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