Theoretical study of uracil tautomers. 2. Interaction with water

The interaction of 2-hydroxy, 4-hydroxy, and 2,4-dihydroxy tautomers of uracil with a water molecule is studied at the B3LYP/6-31+G(d,p) computational level. Depending on the nature of the tautomers, cyclic or open structures are formed. In most of the cyclic structures, water accepts the OH or NH protons and donates its proton to the O or N atoms of the uracil tautomers, Two anticooperative structures where water acts as a biacceptor are also formed. The intermolecular distances, binding energies, and frequency shifts of the stretching vibrations are rationalized in terms of the deprotonation energies and the proton affinities of the sites involved in complex formation. The relative order of stability of the uracil tautomers, free and complexed with water, is compared. Deprotonation of the O10H bond of the 3,4(2)-U tautomer greatly influences the geometry and the binding energies of the water complexes. Complex formation with water also results in a decrease of the deprotonation energy. The concerted double-proton transfer of the tautomerization process U double left right arrow 1,4(1)-U mediated by a water molecule is also examined. The lowering of the proton-transfer barrier because of excess entropy is discussed.