transoid and cisoid Conformations in Silver-Mediated Cytosine Base Pairs: Hydrogen Bonding Dictates Argentophilic Interactions in the Solid State.

We have synthesized and structurally characterized examples of the well-known silver-mediated DNA base pair in its simplest possible form, as [AgI-bis-(N3-cytosine)]+. The compounds show differences such as variable coordination geometry, conformation with cisoid and transoid arrangements, and, in one case, intramolecular base pairing. Collectively, these compounds represent three of five permutations of linear/bent coordination geometry and cisoid/transoid arrangement of the cytosine ligands and contain the global minimum conformation as determined by DFT calculations: bent-transoid. Furthermore, these compounds show no argentophilic intercomplex interactions in the solid state, in marked contrast to alkylated and nucleoside analogues, and so do not form the supramolecular 1D "metallo-DNA" duplex observed in those cases but instead form hydrogen-bonded sheets. This marked difference may be attributed, in part, to the additional hydrogen bond donor site at N1 of the nucleobase that ubiquitously participates in intermolecular interactions.