Azaguanine: A theoretical study of its tautomerism and protonation in the gas phase and aqueous solution

The tautomerism and protonation of 8-azaguanine (azaG) have been studied by means of ab initio methods, both in the gas phase and in aqueous solution. An elimination procedure to choose the most stable tautomeric forms, based on AM1 and HF/6-31G* energies, has been applied. Tautomers azaG(1,9), azaG (1,7) and azaG(9,15) have been selected and their energies calculated at MP 2/6-311++G**//HF/6-31G* level. Self-consistent reaction field IPCM, based o n polarizable continuum model (PCM), has been applied to study the solvent effects. The stability order in the gas phase is azaG(1,7) = azag(1,9) > az aG(9,15), whereas in solution the order becomes azaG(1;7) > azaG(1,9) > aza G(9,15), the latter being just 0.76 kcaI/mol over azaG(1,7). The calculatio ns of proton affinities allowed to unambiguously determine the preferred si tes of protonation of these species. (C) 1998 Academic Press.