Conformational flexibility of di- and tetrahydropyrimidine rings in nucleic acid bases. An ab initio [formula omitted] study

Abstract

Using ab initio HF 6–31 G∗∗ method of calculation it is shown that dihydropyrimidinone and tetrahydropyrimidin-2,4-dione rings in the uracil, thymine, cytosine, isocytosine and guanine molecules are not conformationally rigid. A change in the relevant torsion angles of ± 20° causes an energy increase of about 1.5 kcal/mol. The main reason for conformational flexibility is the non-aromatic and anti-aromatic character of the cyclic π-systems in these molecules. A decrease in the exocyclic double bond polarity and the presence of electron-donating substituents increase the non-rigidity of the pyrimidine rings.