Prototropic tautomers of purine

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). 

---