The binding energy and the metalation site of lithium cation for the most stable tautomers of nucleic acid bases were determined by using "hybrid" B3LYP density functional computations. For each isomer of the same base the metal affinity values are very different and depend on the coordination mode of lithium. For thymine and uracil, the experimental indication on the metalation site and on the absolute Value of metal affinity agrees with our data. 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. Results in these cases show that there is always only one tautomer whose metal affinity value corresponds to the experimental measurements. This tautomer is not necessarily the most stable one.