Structure of a d(TGGGGT) quadruplex crystallized in the presence of Li+ ions

A parallel 5′‐d(TGGGGT)‐3′ quadruplex was formed in Na+ solution and crystallized using lithium sulfate as the main precipitating agent. The X‐ray structure was determined to 1.5 Å resolution in space group P21 by molecular replacement. The asymmetric unit consists of a characteristic motif of two quadruplexes stacked at their 5′ ends. All nucleotides are clearly defined in the density and could be positioned. A single bound Li+ ion is observed at the surface of the column formed by the two joined molecules. Thus, this small alkali metal ion appears to be unsuitable as a replacement for the Na+ ion in the central channel of G‐quartets, unlike K+ or Tl+ ions. A well conserved constellation of water molecules is observed in the grooves of the dimeric structure.