Can anion interaction accelerate transformation of cytosine tautomers? Detailed view form QTAIM analysis.

The relative stabilities and noncovalent interactions of the six low-lying energy tautomers of cytosine nucleobase with some biological anions (such as F, Cl, and CN) have been investigated in gas phase by density functional theory (DFT) method in conjunction with 6-311++G (d,p) atomic basis set. Furthermore, to systematically investigate all possible tautomerisms from cytosine induced by proton transfer, we describe a study of structural tautomer interconversion in the gas phase and in a continuum solvent using DFT calculation. We carried out geometrical optimizations with the integral equation formalism of polarizable continuum (IEF-PCM) model to account for the solvent effect, and the results were compared with those in the gas phase. The result of calculation revealed that anions bind mostly in a bidentate manner via hydrogen bond, and stabilization energies of these complexes are larger than those in the case when anions bind in a monodentate manner. The quantum theory of atoms in molecules (QTAIM), natural bonding orbital (NBO) and energy decomposition analysis (EDA) have also been applied to understand the nature of hydrogen bond interactions in these complexes. NBO analysis reveals that the interaction patterns between the anions and the tautomers are σ-type interaction between lone pairs and $$ \sigma_{{_{{{\text{N}}--{\text{H}}}} }}^{*} $$, $$ \sigma_{{_{{{\text{O}}--{\text{H}}}} }}^{*} $$ and $$ \sigma_{{_{{{\text{H}}--{\text{F}}}} }}^{*} $$ antibonding orbitals. Also, according to these theories, the interactions are found to be partially electrostatic and partially covalent. EDA results identify that these bonds have less than 35% covalent character and more than 65% electrostatic, and the covalent character increases in different anions in the order F > CN > Cl. On the other hand, orbital interaction energies of complexes of F anion are more than those of Cl and CN complexes. The lower orbital interaction energies in complexes of Cl and CNanions imply less charge transfer and stronger ionic bond character. Furthermore, relationship between the orbital interaction energy ( E) with hydrogen bonding energy ( E) and the electron density (ρ( r)) with hydrogen bonding energy of F, Cl and CN complexes have also been investigated. [ABSTRACT FROM AUTHOR]