Adenine tautomeric forms

FIGURE 11.4 The common purine bases—adenine and guanine—in die tautomeric forms predominant at pH 7. 

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,... 

Most coenzymes have aromatic heterocycles as major constituents. While enzymes possess purely protein structures, coenzymes incorporate non-amino acid moieties, most of them aromatic nitrogen het-erocycles. Coenzymes are essential for the redox biochemical transformations, e.g., nicotinamide adenine dinucleotide (NAD, 13) and flavin adenine dinucleotide (FAD, 14) (Scheme 5). Both are hydrogen transporters through their tautomeric forms that allow hydrogen uptake at the termini of the quinon-oid chain. Thiamine pyrophosphate (15) is a coenzyme that assists the decarboxylation of pyruvic acid, a very important biologic reaction (Scheme 6). 

The presence of the cation protonated on N-1 cannot account for the fluorescence of aqueous acidic adenine solutions (pH = 2), since the 1-methyl derivative does not fluoresce under the same conditions (Borresen, 1967). It has therefore been suggested that other tautomeric forms of the cation are also present, the fluorescent tautomer probably being protonated on the amino-group with another proton on N-7. Quantum mechanical calculations (Veillard and Pullman, 1963) indicate similar proton affinity for N-1 and N-3, and a lesser one for N-7. There are numerous calculations in the literature on the electronic structure of adenine (see Boyd, 1972, and references quoted therein) and a recent one on N-7-H and N-9-H tautomers protonated on N-1 (Jordan and Sostman, 1972). The N-9-H form is preferred according to hoth MINDO and CNDO/2 calculations. 

The bases are monocyclic pyrimidines (see Box 11.5) or bicyclic purines (see Section 11.9.1), and all are aromatic. The two purine bases are adenine (A) and guanine (G), and the three pyrimidines are cytosine (C), thymine (T) and uracil (U). Uracil is found only in RNA, and thymine is found only in DNA. The other three bases are common to both DNA and RNA. The heterocyclic bases are capable of existing in more than one tautomeric form (see Sections 11.6.2 and 11.9.1). The forms shown here are found to predominate in nucleic acids. Thus, the oxygen substituents are in keto form, and the nitrogen substituents exist as amino groups. 

A number of physicochemical properties which have been investigated for a series of simple derivatives of the different tautomeric forms of adenine, prominent among which are dipole moments and ultraviolet absorption spectra, throw additional light on the problem. Table VIII indicates the results of a theoretical evaluation of the dipole moments for the different tautomeric forms (12-19) of adenine as well as the experimental values of the moments in methyl and benzyl derivatives of these forms with the exception of the derivatives of the N(1)H form for which, unfortunately, no experimental data could be obtained because of lack of sufficiently soluble 1-substituted derivatives. 

Table 26. Electronic (En), repulsion (Er), and molecular (Em) energies (in ev) for the hydrogen-bonded pair adenine-thyminea, in the tautomeric form I...

Figure 10.2 Tautomeric forms of uric acid. Although uric acid does not occur in nucleic acids (it is a degradation product of adenine and guanine), the tautomeric structures observed here are typical of all purine bases of this type. At pH 7, the keto forms predominate.

Mistakes in base incorporation can be made this is largely a result of the transient existence of tautomeric forms of the bases (Chap. 7). If at the instant of insertion of a new nucleotide by DNA polymerase the base in the template shifts to its rare tautomeric form, which has altered base-pairing specificity, an incorrect nucleotide may be added to the chain e.g., one containing guanine instead of adenine opposite the enol form of thymine. 

Fig. 15.4. Keto/enol and amino/imino tautomeric forms of uracil, cytosine, adenine, guanine. The keto and amino forms predominate, the enol and imino forms are only present in equilibrium at 0.01 Vo or less...

Tautomeric enol and imino forms of bases occur only rarely, and can lead to mutations. It should be emphasized that in none of the above described mismatch base pairs is there any evidence for the existence of rare tautomeric forms. For the A-C pair, protonation at (adenine)N(l) appears more probable than the imino form (Fig. 20.6). However, conclusive evidence is still lacking because hydrogen atoms cannot be located at the attainable resolution of about 2 A. Moreover, in none of the crystal structures of the nucleosides and nucleotides or of the bases themselves is there any evidence of the enol-imino tautomers (Part II, Chaps. 15, 16, 17). 

Figure 27.41. Base Pair with Mutagenic Tautomer. The bases of DNA can exist in rare tautomeric forms. The imino tautomer of adenine can pair with cytosine, eventually leading to a transition from A-T to G-C.

Among the species produced upon radiolysis of water, hydroxyl radical ( OH) is the most reactive. Indeed, its reaction rate with the four bases and related nucleosides is diffusion-controlled. The main reactive sites of hydroxyl radicals on nucleobases are the double-bonds of the heterocycles. Accordingly, addition of OH at the C8 position of adenine and guanine yields the corresponding reducing 8-hydroxy-7,8-dihydropurin-7-yl radical (Fig. 2). Oxidation of this intermediate leads to the formation of related 8-hydroxypurines that are in dynamic equilibrium with their more stable 8-oxo-7,8-dihydropurine tautomeric form. Competitive reduction of the latter purine radical gives rise to imidazole ring opened compounds the... 

Table 11. Calculation of the percentages of the N7-H tautomeric forms of purine and adenine from C-4 and C-5 chemical shifts ...

N NMR spectroscopy was used to study the tautomerism of adenine derivatives enriched by the N-15 isotope." These enriched samples were prepared as probes for further biochemical studies. While tautomeric mixtures of N -H and N -H were reported for adenine and 2-MeS-adenine based on the " N NMR spectra, an N -H species was suggested for the solution of S-Br-adenine. These observations were supported by quantum chemical calculation of the individual tautomeric forms for isolated molecules and for the inclusion of solvent effects (both continuum and discrete models). 

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