Gas-Phase Interaction of Calcium (Ca(2+)) with Seleno Derivatives of Uracil.

The structures and relative stabilities of the complexes between Ca(2+) and 2-selenouracil, 4-selenouracil, and 2,4-diselenouracil have been investigated through the use of B3LYP/6-311++G(3df,2p)//B3LYP/6-31+G(d,p) density functional theory (DFT) calculations. In those systems where both types of basic centers, a carbonyl or a selenocarbonyl group, are present, Ca(2+) association with the oxygen is favored. For 2,4-diselenouracil the nitrogen atom at position 3 is the most basic site toward Ca(2+) attachment followed by heteroatoms attached to positions 4 and 2. Although the enolic and selenol forms of selenouracils should not be observed in the gas phase, the corresponding Ca(2+) complexes are the most stable ones. More importantly, all the activation barriers associated with the corresponding tautomeric processes are lower than the entrance channel, and therefore not only these complexes should be observed but also they should be the dominant species in the gas phase. Also, Ca(2+) association has a clear catalytic effect on these tautomerization processes, whose activation barriers decrease between 10 and 15 kcal mol(-1).