Interactions of hydrated metal ions with nucleotides: the crystal structure of barium adenosine 5'-monophosphate heptahydrate.

The crystal and molecular structure of barium adenosine 5'-monophosphate heptahydrate was determined from x-ray diffraction data. Crystals of barium adenosine 5'-monophosphate heptahydrate are monoclinic, space group C2, with a = 32.559(3), b = 6.969(3), c = 9.597(1) A, and beta = 100.31(1) degrees. Intensity data were collected with an automated diffractometer. The structure was solved by the heavy-atom method and refined by least-squares to R = 0.034. This structure provides an example of an outer-sphere metal-nucleotide complex, in which a completely hydrated metal ion interacts with the nucleotide only through water bridges. The barium ion is coordinated to eight water molecules, which form a slightly distorted square antiprism. Seven of the eight water molecules from the barium hydration shell are hydrogen bonded to phosphate groups; three of these water molecules are also hydrogen bonded to other suitable acceptor sites on the base and ribose moieties. The conformation about the glycosidic bond is anti, with chiCN = 69 degrees, and, as in most nucleotide structures, the conformation about the C(4')-C(5') bond is gauche-gauche. However, the ribose displays an unusual conformation (best described as C(4')-exo) not previously observed in crystal structures of nucleosides or nucleotides, other than 3',5'-cyclic nucleotides. It is possible that this unusual conformation is a consequence of the metal-water-nucleotide bridging interactions.