MOLECULAR AND ELECTROSTATIC PROPERTIES OF THE N-METHYLATED NUCLEIC-ACID BASES BY DENSITY-FUNCTIONAL THEORY

Complete geometry optimizations using a density functional theory (DFT ) with the combined Becke3 and LYP functional potentials (BS-LYP) and the conventional ab initio Hartree-Fock (HF) method with the 6-31G(d,p ) basis set were carried out for the fundamental tautomeric forms of n ucleic acid bases (cytosine, thymine, guanine and adenine) and their d erivatives methylated at the N1 (pyrimidines) or N9 (purines) position s. At the HF/6-31G(d,p) geometries, the dipole moments, electronic den sities and molecular electrostatic potentials (MEPs) were computed usi ng the HF/6-31G(d,p), MP2(fc)/6-31G(d,p), DFT(B3-LYP)/6-31G(d,p), DFTB 3-LYP)/6-31 + + G(d,p) methods and DFT with inclusion of Becke nonloca l, gradient-corrected exchange energy terms (DFT(NLE) method) with the numerical DNP basis set. The same properties were also computed using the DFT(B3-LYP)/6-31G(d,p) method for the corresponding optimized geo metries of the molecules. The charges that reproduce the MEP maps from the ab initio (I-IF, MP2) and DFT calculations were fitted and compar ed. The ground state molecular parameters (rotational constants, dipol e moments) of the methylated bases are compared with the molecular par ameters calculated at the same level for the nonmethylated DNA bases a nd with available experimental data. The results show that the DFT cal culations reproduce well the MP2 results for the MEPs, the ESP charges and the dipole moments of the DNA bases and their N-methylated deriva tives.