An ab initio post-Hartree-Fock comparative study of 5-azacytosine and cytosine and their dimers with guanine

The post-Hartree-Fock ab initio studies were performed to predict the prope rties of eight tautomers of 5-azacytosine. All geometries were optimized wi thout symmetry restrictions by the gradient procedure at the MP2 level of t heory using the standard 6-31G(d,p) basis set. Single-point calculations ha ve been performed at the MP2/6-311++G(d,p), MP2/6-31 ++G(2df,2pd). MP4(SDTQ )6-31G(d,p), and CCSD(T)/6-31G(d,p) levels for all eight tautomers. The cal culations of the stabilities of the tautomers in a polar medium have been p erformed using explicitly water molecules which form a first solvation shel l, and PCM solvation model. The results are compared to the corresponding d ata for cytosine tautomers. Ab initio calculations predict a different orde r of tautomers in cytosine and 5-azacytosine; however, the relative stabili ties of the two lowest energy tautomers are the same for both bases. The mo lecular geometries of guanine-5-azacytosine and guanine-cytosine base pairs have been optimized using the MP2/6-31G(d,p) and B3LYP/6-31G(d,p) levels o f theory. The interaction energies have been calculated at the MP2/6-31G(d, p), MP2/6-31 1++G(d,p), MP4(SDQ)/6-31G(d,p), and B3LYP/6-31G(d,p) levels an d corrected for the basis set superposition error. The interaction and solv ation energies of base pair complexes with water have been estimated using first solvation shell of water molecules and PCM solvation model. The study has shown the similar geometrical parameters for fragments of both bases a ssociated with the formation of hydrogen bonds with guanine and different m olecular parameters associated with the moieties involved in the interactio ns with cytosine-5-methyltransferase.