The crystal structure of cytidine has been refined by least-squares calculations to a final R index of 5.6 %. Three-dimensional photographic data were used. The essential features of the original structure are confirmed, but the standard deviations in the bond lengths are reduced to about 0.006 A and in the bond angles to less than 0.5 °. The distances and angles in the cytosine part of the molecule are closely similar to those found in cytosine monohydrate. The two angles at each external bond to pyrimidine appear to differ significantly. The N-glycosidic linkage is found to be 1.497 A. The two bonds to the furanose ring oxygen atom differ in length by 0.03 A. The existence of an intramolecular C-H • • • O interaction at 3.23 /k is confirmed.The crystal structure of cytidine, CgH1305N3, was determined by one of us fifteen years ago from three electron density projections refined to R=0. 17-0.19 (Furberg, 1950). Paul±rig & Corey (1956), not con. sidering this structure analysis reasonably reliable, deduced bond lengths and angles in the cytosine part of cytidine by considering the crystal structures of a number of other pyrimidines, mainly heavy atom derivatives. This work was later extended by Spencer Kinoshita, 1963), and precise information on the ribose residue has been obtained through three-dimensional analysis of calcium thymidylate (Trueblood, Horn & Luzzati, 1961) and adenosine 5-phosphate (Kraut & Jensen, 1963). However, cytidine is still the only unsubstituted nucleoside the structure of which has been solved. It contains no heavy atoms and its aromatic ring is not protonated. We have therefore refined the structure in order to obtain more exact information on this part of the nucleic acids. ExperimentalThe unit-cell dimensions were redetermined by leastsquares calculations based on measurements of 28 lines registered on a Guinier camcra calibrated against KC1. The following values were found" a--13.991 ± 0.002, b = 14.786 _ 0.002, c=5-116 i0.001 _~.The uncertainties indicated are estimated standard deviations. The space group is P212121 and there are four molecules in the unit cell. Integrated equi-inclination Weissenberg photographs hkO-hk2, hOl-h81 and Okl were taken. Of the total of about 1450 unique reflexions within the Cu Ka limit, 1195 were recorded. Of these 165 were too weak to measure. The intensities of each of the four equivalent reflexions were measured on a Hilger photometer and the mean value taken. The weakest reflexions were estimated visually. Crystals with cross-sections approximately 0.1 × 0-1 mm were used and no correction for absorption was applied. The various levels were placed on an approximately common absolute scale by means of calculated structure factors based on the two-dimensionally refined structure. Refinement of the structureThe coordinates of the original structure were first refined by a number of difference syntheses in the a and c projections, including finally the thirteen hydrogen atoms. A common isotropic temperature factor was applied, and the values 0.12 a...

The structure of adenosine 3',5'-cyclic phosphate has been determined by single-crystal x-ray diffraction. The two molecules in the asymmetric unit show different conformation about the glycosidic bond, while other structural details are essentially the same. The furanose rings are puckered with the C(4') atom out of the best four-atom plane. The bond lengths and angles appear to be normal.

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