Conformational flexibility of pyrimidine ring in adenine and related compounds

The structural non-rigidity of aromatic pyrimidine rings in adenine and sel ected related compounds has been investigated by quantum chemical non-empir ical methods of calculation at the MP2 and DFT levels of theory. The result s of the calculations demonstrate that the pyrimidine ring possesses a nota ble degree of conformational flexibility despite its aromatic character, Th e transition of the heterocycle from a planar equilibrium geometry to a non -planar structure with an endocyclic torsion angle of +/-20 degrees results in an energy increase of less than 2.8 kcal/mol. An analysis of the popula tion of ground and excited vibrational levels for the lowest out-of-plane v ibration of the ring indicates that in adenine 45% of the molecules have a non-planar pyrimidine ring with relevant torsion angle up to +/-17 degrees at any moment of time. (C) 2000 Elsevier Science B.V. All rights reserved.