Purines covalent hydration

For reasons discussed in Section VI, a survey of the purine series (29) is being made in this Department, but so far no example (including 2-hydroxy- and 8-trifluoromethyl-2-hydroxy-purine) of covalent hydration has come to light. An examination of ionization constants disclosed no apparent anomalies, although the interpretation is made more difficult by the ease of anion formation in the 9-position, which often competes with that from other anionic substituents. The only abnormal spectrum seems to be that of the anion of 2-mercaptopurine which is being further examined. 

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. 

Coordinated ligands, reactions of, 58, 123 Covalent hydration in heteroaromatic compounds, 4, 1, 43 in nitrogen heterocycles, 20, 117 Current views on some physicochemical aspects of purines, 24, 215 Cyclazines and related N-bridged annulenes, 22, 321... 

Purine Studies. Part IV. A Search for Covalent Hydration in 8-Substituted Purines. 

The ionization of 8-azapurines, which has not previously been reviewed, is complicated in many cases by covalent hydration, a phenomenon discovered by the present author in the pteridine and quinazoline series in 1952, and twice reviewed in this series. - This occurrence of covalent hydration, which does not exist to any detectable degree in the purine series, - indicates the increased n deficiency conferred by the additional doubly bonded 8-nitrogen atom. 

The oxidation of purines by xanthine oxidase is thought to require prior covalent hydration of these substances, although only a small percentage may be hydrated at equilibrium (Bergmann etal., 1960). 

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