Hydroxypurines structure

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. 

The lactam-lactim, tautomerism, of hydroxypurines illustrated, for example, for hypoxanthine by the structures 8 and 9. 

If the study of the relative stability of the lactam and lactim forms of hydroxypurines thus escapes a rigorous quantitative treatment, the situation is more agreeable with respect to some other electronic properties of the two forms, which are much less sensitive to the details of the structure and, in particular, to the distance of the... 

O-H bond. Among such properties a prominent one is the ultraviolet absorption spectrum and the theory may therefore be used for the examination of some of the spectroscopic shifts which accompany the lactam-lactim tautomerization. Much caution must, however, be exercised in this respect. Thus, in a recent paper Kwiatkowski135,137 performed Pariser-Parr-Pople-type calculations on the electronic structure of hydroxypurines, essentially to interpret their ultraviolet spectra. In these calculations he assumed that these compounds exist predominantly in their lactim form, and the results of his calculations, at least for 6- and 8-hydroxypurine, did not seem to contradict this assumption. It is only in the case of the 2-hydroxy isomer that a particularly striking disagreement between theory and experiment led him to admit that this last compound may exist in the lactam form. Calculations carried out for this form gave, in fact, a more satisfactory agreement with experiment.138 As we have seen, unambiguous infrared spectroscopy evidence clearly show s that all three isomers exist essentially in the lactam form. This shows that ultraviolet absorption may provide only very uncertain evidence about the lactam-lactim tautomerism in hydroxypurines and related compounds. 

Whatever be the difficulties in dealing satisfactorily with the problem of the lactam-lactim tautomerism in hydroxypurines, the predominance of the lactam tautomer granted, there remains the problem of the detailed structure of the most probable lactam form for each isomer. The problem is essentially that of the site of location of the imidazole proton. From that point of view forms 34-38 have to be considered for 2-hydroxypurine, forms 39—42 for 6-hydroxypurine (hypoxanthine), and forms 43-45 for 8-hydroxypurine. There are, in addition, some betaine tautomeric forms but these are probably of low stability and will not be considered further. Before describing the results of theoretical calculations, it may be useful to indicate that from the experimental point of view we may, in this respect, turn again for significant evidence to infrared spectroscopy... 

In all three isomers the most stable tautomeric form involves one proton at N-7, the second one being at N-3 in 2-hydroxypurine, at N-l in 6-hydroxypurine, and at N-9 in 8-hydroxypurine. The preferential attachment of a proton at N-7 of these isomers seems thus a general feature of their structure. Although this situation agrees with the probable preeminence of the N(7)H tautomers in solution, it should not be considered as prejudging the nature of the tautomer present in the crystal of these substances, a problem which will be discussed in detail in Section XI. 

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