INVESTIGATION OF THE CRYSTAL STRUCTURES OF NUCLEOSIDES CONTAINING SULFUR

The crystal structures of three nucleosides containing sulfur have been determined by X-ray analysis: 4\u27-thiouridine, 5-fluoro-4\u27-thio-(alpha)-2\u27-deoxyuridine and 2,2\u27-anhydro-3\u27-O-acetyl-2\u27-thio-1-(beta)-D-arabinofuranosylcytosine hydrochloride. The study of the crystal structures of 4\u27-thiouridine and 5-fluoro-4\u27-thio-(alpha)-2\u27-deoxyuridine showed that the substitution of the ring oxygen by sulfur alters substantially the stereochemical features of furanoside nucleosides. The g(\u27-) orientation of O(5\u27) is favored more and the g(\u27+) less. The puckering amplitude adopts higher values. The preference of deoxyribonucleosides for N-type puckers appears to increase. The changes in the geometry are localized in the sugar. The angle at the ring heteroatom decreases to about 95(DEGREES) with concomitant enlargements in the internal C-C-C angles. Also, the angle formed by the ring heteroatom, atom C(1\u27) and the glycosyl nitrogen increases by 3(DEGREES) to 5(DEGREES). The difference between the two C-S bond distances in 4\u27-thionucleosides is very small. A most interesting feature in these two structures is a vertical interaction between the thiolane sulfur and an adjacent base, reminiscent of the O(4\u27)(.)(.)(.)base stacking interaction which is observed frequently in nucleoside and nucleotide crystal structures. The study of the crystal structure of 2,2\u27-anhydro-3\u27-O-acetyl-2\u27-thio-1-(beta)-D-arabinofuranosylcytosine hydrochloride showed that the alterations in the geometry of the 2,2\u27-anhydro-1-(beta)-D-arabinofuranosylcytosine cation when the oxygen bridge is replaced by a sulfur bridge are localized in the region of the fused ring. The angle at the bridge atom decreases to nearly 90(DEGREES), with concomitant enlargements of about 6(DEGREES)-8(DEGREES) in the angles opposite to the bridge atom. Angle C(1\u27)-N(1)-C(6) decreases by about 4(DEGREES). Also, the amount of double bond character in the bond formed by C(2) and the bridge atom decreases. A most interesting feature in this crystal structure is a sandwich formed between the positive base and two electronegative atoms: atom O(5\u27) of the same molecule on one side of it ((psi) = g(\u27+)) and the carbonyl oxygen of a symmetrically related molecule on the other side. The conformational features of S,2\u27-cyclonucleosides are summarized and compared with those of O,2\u27-cyclonucleosides. The preferred conformations of the two classes of compounds are the same. However, S,2\u27-cyclonucleosides exhibit a somewhat wider range of P and X values than O,2\u27-cyclonucleosides, perhaps suggesting a greater ease for the sulfur-containing five-membered ring to pucker as compared to the oxygen-cyclo ring