چكيده
The interaction fo cations and anions with nucleobase plays an important role in many biological processes. In present report, we have examined the effect of interaction cations and anions (such as Be++, Mg++ , Ca++, F- and Cl-) on structures, binding energies (BE), metal ion affinities (MIA) and electron affinity(AA) of the seven tautomers of cytosine and thiocytosine by density functional theory (DFT) calculations at B3LYP/6-311++G(2d,2p) level. Furthermore we have investigated the role of cations and anions in proton-transfer process. Result of calculation revealed that cations and anions bind mostly in a bidentate manner, and stabilization energies of these complexes are larger than those in the case that cations and anions bind in a monodentate manner. The calculated values of metal ion affinity (MIA) and binding energy for nucleobase-caion complexes were found to be in the order of Be++ >Mg++ >Ca++, also the calculated values of anion affinity (AA) and binding energy for nucleobase-anion complexes were found to be in the order F-> Cl-. On the other hand, thiocytosine-X( X= Be++, Mg++ , Ca++, F- and Cl-) complexes were found to be more stability than cytosine-X ones. The quantum theory of atoms in molecules (QTAIM) and natural bonding orbital (NBO) have also been applied to understand the nature of interactions in these complexes. NBO analysis reveals that the interaction patterns between the cations and the tautomers are LP (X) (X=O, N, C) LP (cation) interaction also the interaction patterns between anions and the tautomers are LP (anion) LP(X-H) (X=F-, Cl-) interaction. Also, according to the quantum theory of atoms in molecules, the interactions are found to be partially electrostatic and partially covalent
