The crystal structure and absolute configuration of 2'-deoxycytidine hydrochloride

2'-Deoxycytidine hydrochloride (C_9H_(14)N_3Q_4Cl) was crystallized from water as monoclinic needles, space group P2_1, with ɑ = 6-561, b= 17·659, c=5·125 A, β = 108·08° and two molecules per cell. The crystal structure has been determined by a three-dimensional X-ray diffraction analysis. A complete hemisphere of data (positive and negative values of both hand k) was collected with a General Electric XRD-5 automatic diffractometer and nickel-filtered Cu Kɑ radiation. The structure was solved by the heavy-atom method. Full-matrix least-squares refinement of both the D and L enantiomorphs has led to respective R values of 0·035 and 0·061; the configuration that gave the lower R value confirms previous conclusions regarding the absolute configuration of the D sugar. The atoms in the pyrimidine ring exhibit small but significant deviations from planarity. The torsion angle describing the conformation about the glycosidic bond is 0°. The least-squares planes through any four ring atoms of the sugar residue, normally used to describe its conformation, are unsatisfactory in that the deviations of the atoms from the planes are rather large. The conformation can, perhaps, be best described relative to the plane through C(l'), C(4'), and 0(1'), with respect to which C(3') is displaced endo by 0·361 A and C(2') is 0·245 A exo.