Two uracil molecules connected by an excess electron

In this work we considered a new form of the uracil dimer anion (U₂^-). In this system the excess electron is suspended between two uracil molecules and provides sufficient bonding between them to stabilize the dimer in a local minimum on its potential energy surface. Ab initio calculations are employed to describe the system. Anions of the nucleic acid bases of this kind can be formed in two steps. First a dipole-bound anion is formed by a molecule of one of the bases. Then a molecule of the second base attaches to the diffuse dipole-bound electron at the opposite side with respect to the first base. Removal of the excess electron leads to a significant change of the structure of the complex. Its geometry optimization initiated with the anion geometry converges to a conventional hydrogen-bonded dimer with a total energy lower than the energy of the anion. Thus the two uracils connected by a suspended excess electron in the middle are a metastable system.