Document Type : Research Paper
In the present work, the substituent effect on the strength of H-bonds in the guanine – cytosine base pair was studied when the substituents are connected to the guanine base through a phenyl ring. In this study, guanine was substituted in the H8 and H9 positions by electron donating (ED) and electron withdrawing (EW) groups mediated by a phenyl ring in the gas phase. The calculations were performed at B3LYP/6-311++G(d,p) level. The results show that the values of stabilization energies (EHB) of EW groups are more negative than that of ED groups. These results indicate that the complexes containing EW groups are more stable than ED ones. Also, the energies of the intermolecular hydrogen bond have been estimated by the EML and MAEM’s formulas. Most of the changes of intermolecular hydrogen bond are agreement with the nature of substituents. The influence of substituent on the stability of complexes is investigated by atoms in molecules and natural bond orbital analyses. The atoms in molecules theory were also used to get the most exact understanding to the nature of H-bond interactions. There is excellent relationship between the charge distribution and the stabilization energy.