Intramolecular proton transfer in the tautomers of C8 oxidative adenine: ADFT study

A density functional theory study has been performed in order to explore th e influences of oxygen at the C8 position on the intramolecular proton tran sfer between the 6-oxo form and 6-hydroxy form of C8-oxidative adenine. The 8-oxo form with the oxygen at the C8 position has been found to be greatly energetically favored by the purines. The form of the N atom at C6 is deci sive in adenine and in its CS oxidative derivatives. In the isolated condit ion, the relative stability order for the C8-oxidative adenines follows 8-o xa-A(am) > 8-hydroxy-A(am) > 8-oxo-A(im) > 8-hydroxy-A(im). Inclusion of th e bulk electrostatic interaction or introducing a hydration water in the sy stem changes the relative stabilities of the C8-oxidative adenines to 8-oxo -A(am) > 8-oxo-A(im) 8-hydroxy-A(am) > 8-hydroxy-A(im). The form of the oxy gen at the Cs position determines the stability in the hydrated condition. The presence of the oxygen at the C8 position in adenine increases the taut omer rate from the imino to the amino form of adenine. Therefore, oxidation at the C8 position in adenine reduces the possibility of rare tautomers. H owever, polar solvents are expected to slow the tautomer rate. The tunnelin g effect might be the predominant factor in the tautomer process in the ade nine derivatives at room temperature. Since the influence of the oxidation on the quantum corrected tautomerism rate constants has been found to be sm all, the oxygen at the C8 position of the oxidative adenines will not affec t the tautomer rate significantly.