Hypoxanthine tautomerization

Hypoxanthine is oxidized at carbon 2 by both molybdenum hydroxylases, although xanthine oxidase is much more effective as a catalyst in this reaction [ 10]. A methyl substituent in this position prevents oxidation by either enzyme. Introduction of A-methyl substituents into the hypoxanthine nucleus produces dramatic effects on enzymic oxidation rates and also gives some insight into the productive modes of binding to each enzyme. Thus, it has been proposed that hypoxanthine tautomerizes in the xanthine oxidase-substrate complex to the 3-NH-form with a simultaneous shift of the NH-group in the imidazole ring from position 9 to 7 [ 198,200]. In support of this hypothesis, when tautomerism in the imidazole ring is prevented by substitution at N-7 or N-9, such compounds are almost refractory to oxidation (see Table 3.9)... 

The amino groups are replaced with oxygen. Although here a biochemical reaction, the same can be achieved under acid-catalysed hydrolytic conditions, and resembles the nucleophilic substitution on pyrimidines (see Section 11.6.1). The first-formed hydroxy derivative would then tautomerize to the carbonyl structure. In the case of guanine, the product is xanthine, whereas adenine leads to hypoxanthine. The latter compound is also converted into xanthine by an oxidizing enzyme, xanthine oxidase. This enzyme also oxidizes xanthine at C-8, giving uric acid. 

The nucleoside formed from hypoxanthine and ribose is known as inosine (Ino or I) and the corresponding nucleotide as inosinic acid. Further substitution at C-2 of -H by -OH and tautomerization yields xanthine (Xan). Its nucleoside is xanthosine (Xao, X). A similar hydroxylation at C-7 converts xanthine to uric acid, an important human urinary excretion product derived from nucleic acid bases. 

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

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... 

Table 14-2. Computed barrier height (kcal/mol) for guanine and hypoxanthine corresponding to the keto-enol tautomerism in the ground and the lowest singlet tt-jt excited state obtained at the B3LYP/6-311++G(d,p) and TD-B3LYP/6-311++G(d,p)//CIS/X levels, respectively in the gas phase and in aqueous solution [163, 165]a...

Fig. 3. (A) Lactim-imino and (B) lactam-amino tautomeric forms of the purine base hypoxanthine.

As is the case for pyrimidines, purines also exist in oxo, thio, and amino tautomeric forms. The lactam-amino and lactim-imino forms of the purine base hypoxanthine are shown in Fig. 3. 

There are various forms of tautomerism which operate in the different purine species. (1) Prototropy which involves attachment of the proton to any one of the four ring nitrogen atoms (Scheme 5). Corresponding CH tautomers, for example (52), seem to be of little significance. (2) Amine-imine tautomerism which operates in the aminopurines such as adenine (Scheme 6). (3) Lactam-lactim tautomerism as in the hydroxypurines such as hypoxanthine (Scheme 7) and the related thioxo-thiol tautomerism (53) and (54) in the biologically imporfant mercaptopurines (Scheme 8). The subject has recently been discussed in some detail <76AHC(Si)502>. 

Purines show typical absorption spectra which are useful for the identification and the determination of their structure tautomeric equilibria can also be studied. By comparison of the spectra of natural and synthetic derivatives of purines, the N9 glycosylic linkage was established for nucleosides. The relatively simple spectra of purines show two main bands. Purine shows maxima around 220 nm and at 263 nm in neutral solution. Since the imidazole ring has no characteristic UV absorption, the spectra of purines and pyrimidines show similarities. Similar observations have been made for 7-deazapurines, 8-azapurines etc. In the series of adenine hypoxanthine, and xanthine (at pH 6) only one maximum is observed guanine, isoguanine, and purinc-2,6-diamine show two maxima. 

Lactam-lactim interconversion and the tautomerism between the thione and thiol forms, which could occur in the pyrimidine portion of hypoxanthine and 6-mercapto-purine, respectively, have been investigated by using the chemical shift of the C-6 carbon. The relative position of the C-6 resonance made it possible to calculate the amount of N -H tautomer by using Eq. (3)... 

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