Lithium affinity for DNA and RNA nucleobases. The role of theoretical information in the elucidation of the mass spectrometry data

The binding energy and the metalation site of lithium cation for the most s table tautomers of nucleic acid bases were determined by using "hybrid" B3L YP density functional computations. For each isomer of the same base the me tal affinity values are very different and depend on the coordination mode of lithium. For thymine and uracil, the experimental indication on the meta lation site and on the absolute Value of metal affinity agrees with our dat a. For the remaining bases, the presence of the metal, in some case, gives rise to a complex stability order different from that of naked tautomers. R esults in these cases show that there is always only one tautomer whose met al affinity value corresponds to the experimental measurements. This tautom er is not necessarily the most stable one.