Theoretical study of proton transfer in hypoxanthine tautomers: Effects ofhydration

Computational investigations on the proton transfer in the isolated, monohy drated, and dihydrated forms of hypoxanthine have been performed. Ground st ate geometries were optimized at the HF/6-31G(d,p), HF/6-311++G(d,p), and M P2/6-31G(d,p) levels of theory while those of the transition states corresp onding to the proton transfer from the keto to the enol form of the molecul e were characterized at the HF/G-31G(d,p), HF/6-311++G(d,p), MP2/6-31G(d,p) //HF/6-31G(d,p), and MP2/6-311++G(d,p)//HF/6-311++G(d,p) levels of theory. It is found that, in the gas phase, the molecule would exist mainly in the keto prototropic forms. The transition states corresponding to the proton t ransfer from the oxo to the hydroxy form for the mono- and dihydrated forms were found to have a zwitterionic structure; the geometries are more easil y expressed in the form of H3O+... HX- for the monohydrated forms and H5O2... HX- for the dihydrated forms of the molecule.