Assisted intramolecular proton transfer in (uracil)2Ca2+ complexes
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The structure and relative stability of the complexes between uracil dimers and Ca2+, as well as the proton transfer (PT) processes within these dimers, have been investigated by the density functional theory methods. Although in uracil dimers PT occurs as an almost synchronous double PT processes that connect the diketo dimer with a keto-enol dimer, the process within the most stable (uracil)2Ca2+ complexes is much more complicated, and the product of the reaction looks like the result of an intramolecular PT from one of the NH groups of one monomer to one of the carbonyl groups of the same monomer. An analysis of the force profile along the reaction coordinate shows that the intimate mechanism implies three elementary steps, two intermolecular PTs, and an in-plane displacement of one monomer with respect to the other. The result of this so-called assisted intramolecular proton transfer is the formation of a dimer in which only one monomer is a keto-enol derivative, the other monomer being apparently unchanged, although it suffers significant structural rearrangements along the reaction coordinate. Quite importantly, this dimer is significantly stabilized upon Ca2+ association; therefore, while the most stable uracil dimers correspond systematically to associations involving only the diketo forms, in (uracil)2Ca2+ complexes the most stable structures correspond to those in which one of the monomers is a keto-enol uracil isomer.
KeywordsUracil dimers Proton transfer Ca2+ complexes DFT calculations Reaction force analysis
This work has been partially supported by the DGI Project No. CTQ2009-13129-C01, by the Project MADRISOLAR2, Ref.: S2009PPQ/1533 of the Comunidad Autónoma de Madrid, by Consolider on Molecular Nanoscience CSC2007-00010, and by the COST Action CM0702. A generous allocation of computing time at the CCC of the UAM is also acknowledged.
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