Prototropic tautomerism and microsolvation in antitumor drug imexon: a DFT study

Density functional theory method, at the B3LYP/6-311+G(d, p) level has been used to explore the geometries, relative energies, and electronic properties of all hypothetically possible prototropic tautomers of imexon. The specific interactions of the tautomeric forms of imexon with one and two solvating water molecules have been investigated. The relative stability order of the complexes remains unchanged upon interaction with one water molecule. The addition of a second water molecule, however, stabilizes the oxo-amino form more than the oxo-imino structure. The bulk water environment has been simulated by a combination of microhydration and the conductor-like polarizable continuum model. The energy profile corresponding to the prototopic tautomerisms connecting oxo-imino form with oxo-amino, hydroxyl-amino, and one rare tautomer has been studied. We found that the tautomerism activation barriers are high enough as to conclude that only the oxo-imino tautomer should be found in the gas phase. Our results present clear evidence that microhydration with one and two solvating water molecules considerably lower these barriers by a concerted multiple proton transfer mechanism.