Water and ions in a high resolution structure of B-DNA

A detailed picture of hydration and counterion location in the B-DNA duplex d(GCGAATTCG) is presented. Detailed data have been obtained by single crys tal x-ray diffraction at atomic resolution (0.89 Angstrom) in the presence of Mg2+. The latter is the highest resolution ever obtained for a B-DNA oli gonucleotide. Minor groove hydration is compared with that found in the Na and Ca2+ crystal forms of the related dodecamer d(CGCGAAT-TCGCG). High res olution data (1.45 Angstrom) of the Ca2+ form obtained in our laboratory ar e used for that purpose. The central GAATTC has a very stable hydration spi ne identical in all cases, independent of duplex length and crystallization conditions (counterions, space group). However, the organization of the wa ter molecules (tertiary and quaternary layers) associated with the central spine vary in each case.