Guanine 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,... 

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. 

Books sometimes disagree on the preferred tautomer of guanine—6.f versus 6.g. A parallel can be found in 2-pyridone, which also has two likely tautomeric forms, 6.h and 6.i. To determine which form of 2-pyridone predominates, the ultraviolet spectra of two isomers, l-methyl-2-pyridone (6.j) and 2-methoxypyridine (6.k), were obtained with the observed Amax wavelengths shown. The Amax of 2-pyridone is at 229 nm. Does 2-pyridone likely favor tautomer 6.h or 6.i Which tautomer of guanine is likely favored, 6.f or 6.g (Elguero, J., Marzin, C., Katritzky, A. R., Linda, P. The Tautomer-ization of Heterocycles. Adv. Heterocycl. Chem., Suppl. 1 New York Academic Press, 1976.)... 

PPP calculations have been performed148 for a number of other tautomeric forms of guanine (imine and lactim). No discussion of the calculated results has, however, been given. 

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.

Cytosine, uracil, and guanine have tautomeric forms with aromatic hydroxyl groups. Draw these tautomeric forms. 

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. 

It is important that we draw certain of the purine and pyrimidine bases in their preferred tautomeric forms. The correct pairings are given early in the chapter. What alternative pairings would be possible with these (minor) tautomers of thymine and guanine Suggest reasons (referring to Chapter 43 if necessary) why the major tautomers are preferred. 

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...

The tautomeric forms I had copied out of Davidson s book were, in Jerry s opinion, incorrectly assigned. My immediate retort that several other texts also pictured guanine and thymine in the enol form cut no ice with Jerry. Happily he let out that for years organic chemists had been arbitrarily favoring particular tautomeric forms over their alternatives on only the flimsiest of grounds (3). 

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