Sunghwan Kim - Effects of microsolvation on the adenine-uracil base pair and its radical anion: adenine-uracil mono- and dihydrates

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  Publication Details (including relevant citation   information):

  S. Kim and H.F.   Schaefer;

  Journal of Physical Chemistry A,   2007111, 10381.



  Microhydration effects upon the adenine-uracil (AU) base pair and   its radical anion have been investigated by explicitly   considering various structures of their mono- and dihydrates at   the B3LYP/DZP++ level of theory. For the neutral AU base pair, 5   structures were found for the monohydrate and 14 structures for   the dihydrate. In the lowest-energy structures of the neutral   mono- and dihydrates, one and two water molecules bind to the AU   base pair through a cyclic hydrogen bond via the N9-H   and N3 atoms of the adenine moiety, while the   lowest-lying anionic mono- and dihydrates have a water molecule   which is involved in noncyclic hydrogen bonding via the   O4 atom of the uracil unit. Both the vertical   detachment energy (VDE) and adiabatic electron affinity (AEA) of   the AU base pair are predicted to increase upon hydration. While   the VDE and AEA of the unhydrated AU pair are 0.96 and 0.40 eV,   respectively, the corresponding predictions for the lowest-lying   anionic dihydrates are 1.36 and 0.75 eV, respectively. Because   uracil has a greater electron affinity than adenine, an excess   electron attached to the AU base pair occupies the π* orbital of   the uracil moiety. When the uracil moiety participates in   hydrogen bonding as a hydrogen bond acceptor (e.g., the   N6-H6a⋯O4 hydrogen bond between   the adenine and uracil bases and the   Ow-Hw⋯N and Ow-Hw⋯O   hydrogen bonds between the AU pair and the water molecules), the   transfer of the negative charge density from the uracil moiety to   either the adenine or water molecules efficiently stabilizes the   system. In addition, anionic structures which have   C-H⋯Ow contacts are energetically more favorable than   those with N-H⋯Ow hydrogen bonds, because the   C-H⋯Ow contacts do not allow the unfavorable electron   density donation from the water to the uracil moiety. This   derealization effect makes the energetic ordering for the anionic   hydrates very different from that for the corresponding neutrals.


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