SODIUM-IONS AND WATER-MOLECULES IN THE STRUCTURE OF POLY(DA)CENTER-DOT-POLY(DT)

Analysis of X-ray diffraction data from a polycrystalline and well ori ented fiber of the sodium salt of poly(dA). poly(dT) shows that this B '-DNA corresponds to a right-handed antiparallel tenfold double-helix of pitch 32.4 Angstrom with C2'-endo furanose rings in both strands. T he helix contrasts itself from B-DNA in terms of a very narrow minor g roove. Difference electron-density maps have revealed that a continuou s spine of water molecules, two per base pair, propagates along this g roove with the same symmetry as the DNA and establishes new links betw een the two strands. In addition to this hydrated DNA helix, the monoc linic unit cell (space group P2(1)) accommodates about 20 sodium ions and 12 water molecules in the vicinity of phosphate groups. These stru ctured guest molecules provide an intricate network of bridges, rangin g in size from a single sodium ion to a multiple sodium-water-water-so dium unit, connecting phosphate groups belonging to adjacent DNA helic es. The crystallographic R value for this structure is 0.23 for a tota l of 102 reflections extending out to 3.2 Angstrom resolution.