STRUCTURE OF THE MODIFIED NUCLEOSIDE 2',3'-DIDEOXY-3'-FLUOROCYTIDINE

1-(2,3-Dideoxy-3-fluoro-beta-D-erythro-pento-furanosyl)cytosine, C9H12FN3O3, M(r) = 229.21, triclinic, P1, a = 6.997 (4), b = 7.396 (4), c = 10.639 (5) angstrom, alpha = 94.48 (4), beta = 107.74 (4), gamma = 104.40 (4)-degrees, V = 500.8 (5) angstrom3, Z = 2, D(m) = 1.52, D(x) = 1.520 Mg m-3, lambda-(Mo K-alpha) = 0.71069 angstrom, mu = 0.1198 mm-1, F(000) = 240, T = 293 K, final R = 0.033 for 2321 unique observed [F greater-than-or-equal-to 4-sigma(F)] reflections. The asymmetric unit contains two molecules A and B. For molecule A, the N-glycosidic torsion angle chi has a value of - 143.5 (3)-degrees, the sugar pucker is mixed 2T1/2 E with P = 154 (1) (C2' endo) and psi-m = 40 (1)-degrees, and the O5' A--C5' A--C4' A--C3' A torsion angle gamma = 63.4(4)-degrees. For molecule B, chi = - 153.0 (3), gamma = -71.4 (4)-degrees and the sugar pucker is 2E with P = 164 (1) (C2' endo) and psi-m = 36 (1)-degrees. The packing of the crystal is determined by a network of hydrogen bonds. Base pairing between A and B occurs, and in this way a pseudo-inversion centre is formed between the two bases. The conformational parameters are in accordance with the IUPAC-IUB Joint Commission on Biochemical Nomenclature [Pure Appl. Chem. (1983), 55, 1273-1280] guidelines.