  Halogen bonding (X-bonding) has attracted notable attention among noncovalent
interactions. This highly directional attraction between a halogen atom and an
electron donor has been exploited in knowledge-based drug design. A great deal
of information has been gathered about X-bonds in protein-ligand complexes, as
opposed to nucleic acid complexes. Here we provide a thorough analysis of
nucleic acid complexes containing either halogenated building blocks or
halogenated ligands. We analyzed close contacts between halogens and
electron-rich moieties. The phosphate backbone oxygen is clearly the most
common halogen acceptor. We identified 21 X-bonds within known structures of
nucleic acid complexes. A vast majority of the X-bonds is formed by halogenated
nucleobases, such as bromouridine, and feature excellent geometries.
Noncovalent ligands have been found to form only interactions with suboptimal
interaction geometries. Hence, the first X-bonded nucleic acid binder remains
to be discovered.
