Bond energies and attachments sites of sodium and potassium cations to ONAand RNA nucleic acid bases in the gas phase

Gas-phase metal affinities of DNA and RNA bases for the Na+ and K+ ions wer e determined at density functional level employing the hybrid B3LYP exchang e correlation potential in connection with the 6-311+G(2df,2p) basis set. A ll the molecular complexes, obtained by the interaction between several low -lying tautomers of nucleic acid bases and the alkali ions on the different binding sites, were considered. Structural features of the sodium and pota ssium complexes were found to be similar except in some uracil and thymine compounds in which the tendency of potassium ion toward monocoordination ap peared evident. B3LYP bond energies for both metal ions were in agreement w ith the available experimental results in the cases of uracil and thymine f or which the most stable complex was obtained starting from the most stable tautomer of the free nucleic acid base. For adenine, although the interact ion of the ions with the most stable free tautomer generated the least stab le molecular complex, the best agreement with experiment was found in just this case. For the remaining cytosine and guanine bases, our calculations i ndicated that the metal ion affinity value closest to experiment should be determined taking into account the role played by the different tautomers o f the free bases with similar energy and all the possible complexes obtaine d by them.