ARE THE AMINO-GROUPS IN THE NUCLEIC-ACID BASES COPLANAR WITH THE MOLECULAR RINGS - ABINITIO HF/6-31G-ASTERISK AND MP2/6-31G-ASTERISK STUDIES

The molecular geometries of the amino-tautomers of cytosine, isocytosine, 1-methylcytosine, adenine, and guanine were studied using the ab initio LCAO-MO method at the Hartree-Fock level with the 6-31G* basis set. Also, MP2/6-31G* geometry optimization and vibrational frequency calculations were carried out for planar conformers of cytosine and isocytosine. All planar structures show one imaginary vibration for which the normal mode correspond to pyramidalization of the amino groups, whereas nonplanar conformers possess only real vibrational frequencies. The dependence of the predicted molecular properties upon planarity or lack of planarity of the studied systems as well as biological consequences of a flat potential energy surfaces for the pyramidalization of the NH2 groups are discussed.