Tautomeric transformations and reactivity of alloxan

The various tautomers and rotamers of alloxan are investigated in this work. Alloxan has a unique structure: it contains four hydrogen bond acceptor CO and two donor NH sites. This allows it to exhibit various tautomeric forms. The density functional calculations (B3LYP/6-311+G(d,p)) reported in this work indicate that the tetraketo form is the most stable, both in the gas and solution phase, followed by the two monohydroxy forms. We have also studied the mechanism of tautomerization. The charge transfer interactions indicate that the entity that migrates is a proton and not a hydrogen atom. The four-membered ring transition states for the direct processes are highly strained and lead to high activation barriers. However, the involvement of even a single water molecule reduces the barrier to a plausible level. This occurs because of the release of strain due to the formation of six-membered rings, and also due to resonance stabilization. The mechanism also changes to almost a two-step process involving first the extraction of the proton by the nucleophilic attack by water which leads to the formation of the transition state, followed by transfer of one of its own protons by water to the alloxan oxygen. Hydration of alloxan results in stabilization of the molecule, but significantly reduces its electrophilicity. The anions and cations of alloxan are also investigated, as are its spectral characteristics.