Intermolecular interactions and water structure in a condensed phase B-DNAcrystal

By controlled dehydration, the unit cells of dodecamer DNA-drug crystals ha ve been shrunk from 68 000 (normal state) to 60 000 (partially dehydrated i ntermediate state) to 51 000 Angstrom(3) (fully dehydrated state), beyond w hich no further solvent loss occurs. The total solvent content in the norma l crystals is similar to 40% by volume, reducing to similar to 20% in the f ully dehydrated phase. The 25% reduction in cell volume induced a dramatic enhancement in the resolution of the X-ray diffraction data (from 2.6 to be yond 1.5 Angstrom). We have determined the structures of the normal, partia lly dehydrated and fully dehydrated crystals, Details of the ligand binding have been presented in the preceding article. The present paper describes the unique features of the structure of the fully dehydrated phase, This st ructure was refined with 9015 unique observed reflections to R = 14.9%, mak ing it one of the most reliable models of a-form DNA available. The crystal s exist as infinite polymeric networks, in which neighbouring dodecamer dup lexes are crosslinked through phosphate oxygens via direct bonding to magne sium cations, The DNA is packed so tightly that there is essentially only a single layer of solvent between adjacent molecules. The details of the cry stal packing, magnesium bridging, DNA hydration and DNA conformation are de scribed and compared with other experimental evidence related to DNA conden sation.