CONFORMATIONAL FLEXIBILITY OF DI-AND TETRAHYDROPYRIMIDINE RINGS IN NUCLEIC-ACID BASES - AN AB-INITIO HF 6-31G-ASTERISK-ASTERISK STUDY/

Using ab initio HF/6-31G* method of calculation it is shown that dihy dropyrimidinone and tetrahydropyrimidin-2,4-dione rings in the uracil, thymine, cytosine, isocytosine and guanine molecules are not conforma tionally rigid. A change in the relevant torsion angles of +/- 20 degr ees causes an energy increase of about 1.5 kcal/mol. The main reason f or conformational flexibility is the non-aromatic and anti-aromatic ch aracter of the cyclic x-systems in these molecules. A decrease in the exocyclic double bond polarity and the presence of electron-donating s ubstituents increase the non-rigidity of the pyrimidine rings. (C) 199 8 Elsevier Science B.V. All rights reserved.