NMR Studies of the Tautomerism in Pseudoisocytidine

The structure of pseudoisocytidine may have two isomers. We would like to designate them as K1 and K3 for N 1 H and N 3 H, respectively. The authenticity of these two isomers was judged by 1 H NMR. The chemical shift value of N 1 H in K1 is found more upfield than N 3 H in K3, whereas the chemical shifts of rest protons remain the same. Theoretical calculations show that K1 is less stable than K3 by ca. 9 Kcal/mol in gas phase while a methyl group replaces the furanose moiety. This energy reduces as low as 2 Kcal/mol in solution depending on the polarity of the solvent. Thus, the equilibrium of two tautomers occurs most likely in solution. The 1 H and 13 C NMR studies have been carried out in the pH 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 corresponding 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 establishes the proton exchange phenomenon. The exchange rate, catalyzed by both [H + ] and [OH − ], depends on the pH.