Structures, stabilities and tautomerizations of uracil and diphosphouracil tautomers

A DFT method (B3LYP) and two ab initio methods (MP2 and CCSD(T)) are used to study the stability order and tautomerization processes of all possible uracil and diphosphouracil tautomers. The obtained order of uracil tautomers stability is different from the previous computational investigations. Reliable predictions on the stability order and geometrical structures for the diphosphouracil tautomers are presented for the first time. The dienol tautomers of diphosphouracil are shown to be much more stable than the enol-keto and diketo forms, whereas for uracil the diketo form is more favored from the energetical point of view. The comparison of the structures and stability order of the tautomers of the two analogues, allows us to suggest that the interaction between the O-H bond and the delocalized π system is crucial for the stabilization of the dienol form of diphosphouracil tautomers. On the other hand, for uracil tautomers only the interaction between the O-H bond with the lone pair of the neighboring nitrogen atom plays a major role. Other differences of intramolecular interactions of the two analogues are attributed to the unique bonding property of the phosphorous atom. The energy barriers of eight rotamerization processes and nine proton transfer processes of both analogues are reported. For both systems rotamerization processes are proved to be far more facile than proton transfer processes, since energy barriers for rotamerizations are only several kcal/mol, while energy barriers for proton transfers are up to 50 kcal/mol. The rotamerizations of diphosphouracil rotamers are easier than that of the uracil rotamers, while the proton transfer barriers of uracil tautomers are lower than that of the diphosphouracil tautomers. All these differences are the results of different molecular properties of the two analogues, and also an evidence different intramolecular interactions existed in the two analogues.