Effects of Positive and Negative Ionization on Prototropy in Pyrimidine Bases: An Unusual Case of Isocytosine.

Intramolecular proton-transfers (prototropic conversions) have been studied for the guanine building block isocytosine (iC), and effects of positive ionization, called one-electron oxidation (iC - e → iC), and negative ionization, called one-electron reduction (iC + e → iC), on tautomeric conversions when proceeding from neutral to ionized isocytosine have been discussed. Although radical cations and radical anions are very short-lived species, the ionization effects could be investigated by quantum-chemical methods. Such kind of studies gives some information about the labile protons and the most basic positions in the neutral and radical forms of the tautomeric system. For investigations, the complete isomeric mixture of isocytosine has been considered and calculations performed in two extreme environments, apolar {DFT(B3LYP)/6-311+G(d,p)} and polar {PCM(water)//DFT(B3LYP)/6-311+G(d,p)}. For selected isomers, the G4 theory has also been applied. There are no good relations for energetic parameters of neutral and ionized forms. Ionization energies depend on localization of labile protons. Tautomeric equilibria for neutral and ionized isocytosine, favored sites of protonation and deprotonation, and favored structures of protonated and deprotonated forms strongly depend on environment. Acidity of iC is close to that of the iC conjugate acid, and basicity of iC is close to that of the iC conjugate base. This increase of acid-base properties of charged radicals explains the proton-transfer in ionized pairs of nucleobases. When compared to other pyrimidine bases such as uracil (U) and cytosine (C), which exhibit analogous tautomeric equilibria between nine prototropic tautomers as isocytosine, the tautomeric preferences for iC, iC, iC, U, U, U, C, C, and C are completely different. The differences suggest that acid-base properties of functional groups, their stabilities, and ionization energies play a principal role in proton-transfers for pyrimidine bases and influence compositions of tautomeric mixtures.