Purines prototropic tautomerism

The tautomeric forms can be frozen when a particular hydrogen is replaced by a methyl group. These methyl derivatives show, m most cases, very characteristic UV spectra similar to their parent NH compounds. When the assignment of the Wmethylpurines is unambiguous, the most favored NH tautomer can be determined on the basis of the similarity of the UV spectra. Moreover H, C, and N NMR spectroscopy can now be used to deleimine prototropic tautomerism. Electronic aspects of tautomerism have been discussed. A recent review summarizes a number of data. The main tautomeric forms of various purines and their methods of detection are summarized in Table 2. 

It has been demonstrated that the a- and /3-substituent parameters for the O - and expositions can be used in a reverse process, i.e., to determine whether the substituent resides in the N - or the N -position of the purine ring. Thus, the prototropic tautomerism that occurs in the imidazole portion of the purine ring was investigated by using the chemical shifts of the C-4 and C-5 carbons. Using the chemical shift for either the C-4 or the C-5 atom allowed the percentage of the N -H tautomer to be calculated using Eqs. (1) and (2). 

Ultraviolet absorption spectrophotometry constitutes a very rapid and simple method for the identification and determination of purines, pyrimidines, and their derivatives. Because many of these compounds can undergo keto-enol tautomerism - a pH-dependent phenomenon - and/or prototropic equilibria, caution should be exercised in selecting the pH of the aqueous solution used for spectrophotometric measurements. As a consequence, spectra obtained at arbitrary pH values may correspond to the summation of spectra belonging to various species. Knowledge of the pKa values of purines and pyrimidines is necessary to choose convenient pH values for spectral measurements. 

We have first developed this apparatus to study very fast prototropic transformations in purine and pyrimidine aqueous solutions. For example Uracil with a pH above 9.5 exists in solution as a mixture of tautomeric anions. The interconversion of these two species is always very fast (100 ns < t < 3 ys) and must be studied with the laser T-jump technique. The study of the variation of the relaxation time with the pH and the concentration has allowed us to show that the N(1)H/N(3)H tautomeric interconversion of uracil monoanions proceeds via a dissociative mechanism as it does for cytosine and isocytosine >. ... 

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