alpha-Nucleosides in biological systems. Crystal structure and conformation of alpha-cytidine.

The structure of alpha-cytidine, C9H13N3O5, monoclinic with space group C2 and cell parameters a = 20.064 (3) A, b = 7.100 (1) A, c = 7.860 (2) A, beta = 104.60 (2) degrees, Z = 4, was determined by X-ray diffraction using a combination of direct methods, Patterson and difference Fourier techniques and refined by block-diagonal least-squares to a final R of 0.033 for 1002 reflections measured on a diffractometer. The glycosidic torsional angle, chiCN = -28.4 degrees, is in the anti region; the sugar pucker is C(2')exo-C(3')endo in a nearly pure 32H twist; and the conformation of C(4')-C(5') is gauche-gauche. The molecules are bound by hydrogen bonds in the lattice with little likelihood of base-stacking interactions. The molecular features of the compound are compared and contrasted with those of its naturally occurring beta-anomer, and some biological implications of this structure, and alpha-nucleosides in general, are discussed.