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 angstrom resolution in space group P2(1) 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.