The crystal structure of the C-nucleoside, 1,3-dimethyl-8-beta-D-ribofuranosylxanthine monohydrate

1,3-Dimethyl-8-beta-D-ribofuranosylxanthine monohydrate, C12H16N4O6.H2O, is monoclinic, P21, with two molecules in a unit cell having a = 8.186(1), b = 19.222(3), c = 4.7655(8) A, beta = 103.79(1) degrees, V = 728.2 A, and Dx = 1.506 g.cm-3. The structure was solved by the direct methods, using MoK alpha radiation and refined with anisotropic temperature-factors. The final agreement factors were R = 0.061, Rw = 0.042 for 2254 structure amplitudes having [Fo[ greater than 2 sigma (Fo). The ribose ring has the C-2'-exo-C-3'-endo (2T3) conformation with pseudorotation parameters P = -3.2, tau m = 39.9 degrees, and a gauche-gauche conformation around the ribose C-4'-C-5' bond. The beta-link conformation is anti, with O-4'-C-1'-C-8-N-9 = -162.6 degrees. The pyrimidine and imidazole rings are planar, making an interplanar angle of 1.0(1) degree. The molecular conformation is stabilized by an unsymmetrical, three-center hydrogen bond from N-7-H to O-5'-H as the major component and to O-4'-H as the minor component. The hydrogen bonding includes a cooperative cycle involving the HN-C-C=O moiety, two hydroxyl groups, and a water molecule.