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The crystal structure of the isosteric phosphonate analog of cyclic AMP has been determined using three-dimensional data measured on a diffractometer. The crystals belong to the orthorhombic system, space group P2i2i2i. The unit cell constants are: a = 10.755 (2), b = 8.869 (2), c = 15.198 (2) Á, Z = 4, Dohsi = 1.578 g cm-3, Z)caiCd = 1.579 g cm-3. The structure was solved by the use of the Patterson method and the application of the tangent formula. Refinement by the method of full-matrix least squares with anisotropic temperature factors for nonhydrogen atoms and isotropic temperature factors for the hydrogen atoms gave a final R of 0.04 for the 1199 observed reflections. The base displays the syn conformation with respect to rotation about the glycosyl bond, x being -125.8°. The constraints of the cyclization freeze the ribose into the conformation, 3T4(C(3')endo-C{A')-exo), which appears to be characteristic of the cyclic nucleotides. For a similar reason the bond distances and bond angles in the ribose ring show marked differences from those of the nucleotides themselves. The phosphonate ring is in the chair conformation and flattened at the phosphorus end. The molecule is a zwitterion

The crystal structure of dihydrouridine hemihydrate has been determined by x-ray diffraction. Crystals of dihydrouridine contain two independent molecules in the asymmetric unit and a molecule of water. The x-ray structure determination has shown that the conformations of both molecules differ in important respects in the saturated base and the ribose. The molecular conformation of dihydrouridine has, for the first time, provided structural evidence that the rare nucleoside can proinote "loop" formation in the sugar-phosphate chain.

The crystal structure of dihydrouridine, a unique rare nucleoside occurring in the dihydrouridine loop of transfer RNAs, has been determined by X-ray diffraction. Crystals of dihydrouridine are orthorhombic, space group P2]2t2i, a = 8.131, b = 11.766, and c = 23.016 A. There are eight molecules of the nucleoside and four molecules of water per unit cell. The structural solution was obtained by a combination of Patterson search