The effect of CH sub(3), F and NO sub(2) substituents on the individual hydrogen bond energies in the adenine-thymine and guanine-cytosine base pairs

The substituent effects on the geometrical parameters and the individual hydrogen bond (HB) energies of base pairs such as X-adenine-thymine (X-A-T), X-thymine-adenine (X-T-A), X-guanine-cytosine (X-G-C), and X-cytosine-guanine (X-C-G) have been studied by the quantum mechanical calculations at the B3LYP and MP2 levels with the 6-311++G(d,p) basis set. The electron withdrawing (EW) substituents (F and NO sub(2)) increase the total binding energy (E) of X-G-C derivatives and the electron donating (ED) substituent (CH sub(3)) decreases it when they are introduced in the 8 and 9 positions of G. The effects of substituents are reversed when they are located in the 1, 5, and 6 positions of C, with exception of CH sub(3) in the 1 position and F in the 5 position, which in both cases the E value decreases negligibly small. With minor exceptions (X=8-CH sub(3), 8-F, and 9-NO sub(2)), both ED and EW substituents increase slightly the E values of X-A-T derivatives. The individual HB energies (E sub(HB)s) have been estimated using electron densities that calculated at the hydrogen bond critical points (HBCPs) by the atoms in molecules (AIM) method. Most of changes of individual HBs are in consistent with the ED/EW nature of substituents and the role of atoms entered H-bonding. The remarkable change is observed for NO sub(2) substituted derivative in each case.