Structure Cristalline et Moléculaire d'une Forme N(7)-Protonée de la Guanosine: le Bromhydrate de Guanosine Hémihydraté

A three-dimensional X-ray analysis of guanosine hydrobromide hemihydrate was undertaken in order to study the effect of N(7) protonation on the molecular conformation of guanosine. The crystals are monoclinic, space group P21, with cell dimensions: a = 6.29, b = 15.42, c = 14.96 A; β = 102°; Z = 4. 2500 observed reflexions were collected by the multiple-film equi-inclination Weissenberg technique, and measured manually with a microdensitometer. The structure was solved by the heavy-atom method. The atomic parameters of the guanosine hydrobromide hemihydrate structure were refined by least-squares calculations to an R index of 0.058 for 2500 observed reflexions. There are two bromine atoms, two guanosine molecules and one water molecule in the asymmetric unit. The two guanosine molecules are in anti conformations with χcn equal to − 1.8 and 31.2°. The sugar conformations are C(3′)endo-C(4′)exo, and C(3′)endo-C(2′)exo respectively. The conformations of C(5′)-O(5′) bonds are gauche-gauche in the two cases. Associations are formed between the bromine ions and the pyrimidine part of the purines, while protonation occurs on the N(7) of the imidazole rings.