Hypoxanthine, tautomers

Theoretical study of proton transfer in hypoxanthine tautomers Effects of hydration ... 

The predominant binding site in 9-substituted 6-oxopurines (guanine and hypoxanthine derivatives) is the N7 atom of the base (Figure 6). The prevailing keto tautomer requires proton at N1 even in mildly acidic conditions, which efficiently prevents platination of the N1 site [7]. Under neutral and basic conditions competition of Pt(II) between the N1 and N7 sites has been reported. Attachment of Pt(II) to the N7 atom acidifies the N1H proton and facilitates coordination of additional platinum ions to both N1 and N3 [7]. In N7,N9-blocked 6-oxopurines, the N1 site is the major coordination site [7,24],... 

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 connection with the forthcoming discussion on the structure of the N(3)H tautomer of guanine (Section VIII), it may be interesting to point out that the N(3)H tautomers of hypoxanthine, of the general structure 46 (with the second proton at N-7 or N-9) are predicted to be about 8-14 kcal/mole less stable than the corresponding N(1)H tautomers, the N(3)H-N(7)H tautomer being more stable by about 6 kcal/mole than the N(3)H-N(9)H one. The structure of the 3-methylhypoxanthine is, in fact, presented by Bergmann et al.liZ as 47. 

The problem may and has been considered also in relation to the crystal structure of other purines, although in somewhat less detail.173 Recent findings indicate that although the crystals of guanine, hypoxanthine, and 8-azaguanine contain the N(9)H tautomer of the bases,174 175 the crystal of 6-mercaptopurine monohydrate is made of the N(7)H form.176 177... 

It may perhaps be useful to remember that the dipole moments of the tautomers cannot and should not be considered as indicative of the relative values of such interactions in the first place, because appropriate calculations must be carried out in this case (as we have seen) in the monopole approximation and, second, because even in the dipole approximation, the mutual orientation of the dipoles of the interacting molecules is important. A glance at the data on the dipole moments of the different compounds mentioned here indicates that, in fact, there is no relation between the value of this moment in the different tautomers and the presence of such tautomers in the crystal. Thus, the dipole moments are predicted to be greater for the N(7)H form than for the N(9)H one in purine and adenine, but greater in the N(9)H form than in the N(7)H one in guanine, hypoxanthine, xanthine, 8-azaguanine, 8-azaxanthine, and 6-mercaptopurine. Also, no general relationship seems to exist between the relative values of the dipole moments and the stabilities of the different tautomers. 

Differentiation of tautomers by the use of UV spectra has not been too rewarding. Thus whereas hypoxanthine almost certainly exists as the lactam form (13) as indicated by IR and other spectral studies, its UV absorption spectrum (Amax 249 nm, pH 5.6) is similar to that of 6-methoxypurine (Amax 252 nm, pH 5.2), a derivative of the lactim form of hypoxanthine, and l,7-dimethyl-6-oxo-l(if),6(if)-purine (43) (B-73MI40902), a definite lactam derivative. On the other hand, the UV spectrum of 8-oxo-7(H),8(/7)-purine (44) is closer to that of its 7- and 9-monomethyl and 7,9-dimethyl (oxo or lactam) derivatives than to 8-methoxypurine (45) (B-73MI40902) implying the oxo structure. Even in basic solutions where the lactim structure is expected, the results may be difficult to interpret. Thus the spectrum of the anion of hypoxanthine is similar to that of the neutral species of adenine, (Amax 258 and 260 nm, respectively) whereas this is not true of the 2- and 8-oxopurines when compared with the corresponding aminopurines. 

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>. 

Quantum mechanical calculations of molcular orbitals have been performed on five examples (8-azapurine, -hypoxanthine, -guanine, -adenine, and -xanthine) by two methods (a) a semiempirical approximation, which included contributions from the a electrons of the skeleton, and (b) the CNDO approximation, which included contributions from all the valence electrons of the molecule. The results were tabulated in parallel for each of the three possible positions of the triazole proton. In all 15 entries, the highest occupied and the lowest unoccupied molecular orbitals were calculated and also the dipole moment, the molecular energy, and the UV absorption maxima (the last-named showed only a modest agreement with experimental results). It was concluded that both types of calculation indicated that relative stabilities for the three tautomers (in each of the five sets) should decrease in the order HN-9, HN-7, and HN-8, and that the HN-8 tautomers should be 85 to 125 kJ (20-30 kcal) per mol less stable than the other two. However, it had to be admitted that, in all sets of three isomers examined experimentally, the HN-8 member has never been found inferior in stability. ... 

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)... 

The effect of methylation on aldehyde-oxidase-catalysed oxidation of hypoxanthine has not been so rigourously analysed. However, a similar requirement for the 3-H tautomer is indicated, as 3-methylhypoxanthine (5 lb) is a much more efficient substrate than hypoxanthine for this enzyme [ 10]. 

---