A DFT investigation on effects of hydration on the tautomeric equilibria of hypoxanthine

Computational investigations of the proton transfer in the isolated, mono, and dihydrated forms of hypoxanthine (HX) and the effect of hydration on the transition state structures corresponding to proton transfer from the keto form to the enol form of the molecule have been performed at the DFT level of theory applying the B3LYP hybrid density functional and four basis sets namely 6-31G(d,p), 6-31++G(d,p), 6-311G(d,p) and 6-311++G(d,p). It is found that in the gas phase, the molecule would exist mainly in the keto prototropic forms. The geometrical parameters of the tautomers are found to converge even with the 6-31G(d,p) basis set. The diffuse functions have an appreciable effect on the relative stability of different tautomers (especially on the hydrated forms) and on the hydrogen bond lengths. The keto-N7H form is predicted to be most stable in all basis sets in the gas phase. Tautomeric equilibria is Found to shift towards the keto-N9H form on hydration, and with the 6-31G(d,p) and 6-311G(d,p) basis sets, the keto-N9H form is predicted to be the most stable while with the 6-31++G(d,p) and 6-311++G(d,p) basis sets, the keto-N7H form is predicted to remain the most stable. Single point calculations at the B3LYP/cc-pVTZ//B3LYP/6-311++G(d,p) level show that the relative energies are already converged at the B3LYP/6-311++G(d,p) level. The transition states corresponding to the proton transfer from the oxo to the hydroxy form for the mono and dihydrated forms were found to have a zwitterionic structure (H3O+ ...HX- for the monohydrated forms and H5O2+ ...HX- for the dihydrated forms). (C) 2000 Elsevier Science B.V. All rights reserved.