NMR studies of the tautomerism in pseudoisocytidine

The structure of pseudoisocytidine may have two isomers. We would like to d esignate them as K1 and K3 for N1H and N3H, respectively. The authenticity of these two isomers was judged by H-1 NMR. The chemical shift value of N1H in K1 is found more upfield than N3H in K3, whereas the chemical shifts of rest protons remain the same. Theoretical calculations show that K1 is les s stable than K3 by ca. 9 Kcal/mol in gas phase while a methyl group replac es the furanose moiety. This energy reduces as low as 2 Kcal/mol in solutio n depending on the polarity of the solvent. Thus, the equilibrium of two ta utomers occurs most likely in solution. The H-1 and C-13 NMR studies have b een carried out in the pK range of 1 to 12. The pKa's of deprotonation of N -1 and N-3 sites are found to be 9.36 and 9.42, for K1 and K3 respectively. On the other hand, the pKa's of protonation of the same sites correspondin g to these two isomers are 3.79 and 3.69, respectively. A critical analysis of line broadening of C-2 in K1 and K3 in the pH range of 5 to 7 establish es the proton exchange phenomenon. The exchange rate, catalyzed by both [H] and [OH-], depends on the pH.