| Abstract
| - Within the context of metal biotoxicity, electrospray ionization mass spectrometry experiments (ESIMS) haverecently been performed by us on the pyrimidine nucleobases (B) uracil and thymine complexed with lead(II) [Int. J. Mass. Spectrom.2005, 243, 279]. Among the ions detected, [Pb(B)−H]+ complexes, where thebase has been deprotonated, have been identified as producing intense signals. In the same study, quantumcalculations based on density functional theory (DFT) have assessed the complexation sites and energies of[Pb(B)−H]+ ions. The present DFT investigations aim at giving an understanding on the energetics andmechanisms associated with uracil's loss of a proton. We specifically assess and quantify the role of leadbinding in this process. For that purpose, intra- and intermolecular proton transfers have been considered.We have found that uracil (U) 1,3-tautomerization can be exergonic when uracil is complexed with Pb2+, inopposition to the situation without lead. The corresponding intramolecular processes were nonetheless foundto occur at geological time scales. In contrast, the addition of a second body to [Pb(U)]2+ complexes, namelyOH- or H2O (as found in the initial water droplet of ESIMS experiments), gives exergonic and fast uracil1,3-proton transfers. Finally, we have shown that intermolecular proton transfers in uracil−H2O, uracil−OH-, or uracil−uracil complexes are able to explain the experimentally detected [Pb(U)−H]+ ions.
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