Theoretical Study of Proton Transfer in Hypoxanthine Tautomers: Effects of Hydration

Abstract

Computational investigations on the proton transfer in the isolated, monohydrated, and dihydrated forms of hypoxanthine have been performed. Ground state geometries were optimized at the HF/6-31G(d,p), HF/6311++G(d,p), and MP2/6-31G(d,p) levels of theory while those of the transition states corresponding to the proton transfer from the keto to the enol form of the molecule were characterized at the HF/6-31G(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 transfer from the oxo to the hydroxy form for the monoand dihydrated forms were found to have a zwitterionic structure; the geometries are more easily expressed in the form of H3O + ‚‚‚HX - for the monohydrated forms and H5O2 + ‚‚‚HX - for the dihydrated forms of the molecule.