Purine tautomer imino

The tautomerisation of the purine bases adenine and guanine and of the pyrimidine bases thymine, cytosine, and uracil has important implications in molecular biology, and the occurrence of rare tautomeric forms of these bases has been suggested as a possible cause of spontaneous mutagenesis (Lowdin, 1965 Pullman and Pullman, 1971 Kwiatowski and Pullman, 1975). Three of the most likely tautomers for cytosine are shown in [87]—[89], together with the less likely imino forms [90] and [91] (Scanlan and Hillier,... 

Some of the impetus for studying tautomeric equilibria in heterocycles arises because of the postulate that point mutations in genetic material may be introduced when a given base exists in a tautomeric form during replication [279, 305-307], Cytosine, in particular, has imino and hydroxy tautomers that are within 3 kcal/mol of the global minimum illustrated above (because of the very large number of possible tautomers for the purines and pyrimidines, only the lowest energy tautomers are presented). This analysis has been made based on a... 

Replacement of the proton at N1 of G by a metal ion has qualitatively similar consequences as substitution of the imino proton of T and U. The Pt(II) binding to this site increases the basicity of the purine ring, but it is not clear how the residual electron density is distributed. The N1 platinated guanine bases titrate with a pA a of 5 (176, 177), and from the high pD dependence of the H8 resonances in the H NMR spectra it is concluded that the most likely protonation site is N7 (cf. Fig. 16). In its protonated state, the nucleobase represents again a metal-stabilized rare tautomer. 

Figure 13-1. The eight most stable structures of guanine in the order or decreasing stability, with the corresponding terminology. A, Im, O and H stand for amino, imino, oxo and hydroxy (see text). The atom indexation of the purine frame is given on the first tautomer. The H atoms are not drawn...

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