MO studies on the prototropic tautomerism and protonation of 2-thiopurine

Abstract

Heats of formation and relative energies have been calculated for the nine tautomeric forms of 2-thiopurine (2TP) in the gas phase using the AM 1 method. For the four more stable room temperature tautomers the proton affinities have also been calculated and the preferred sites of protonation determined. Using the AM1-SM2 solvation model it was found that in aqueous solution the thione forms are more stabilized than the thiol ones. As a result of the aqueous solvation the equilibrium is displaced toward the thione forms and in particular to the more polarizable 2TP(1,7) species.