Structure and conformation of 2′,5′-anhydroarabinosylcytosine: X-ray, ¹H nmr and ¹³C nmr analyses

Abstract

Crystals of 2',5'-anhydroarabinosylcytosine hemihydrate belong to the monoclinic space group P21. The cell dimensions are a = 9.643(2), b = 10.328(1), c = 10.544(2) A, P = 94.55!1)". X-ray intensity data were measured on a diffractometer and the structure was determined by direct methods. Least-squares refinement, which included all nucleoside hydrogen atoms, converged at R = 0.041 for 2298 observed reflections. The asymmetric unit contains two molecules of the nucleoside and one molecule of water. In both nucleoside molecules, the conformation about the glycosyl bond is anti, with XCN values of 15.5(3) and 26.3(3)", respectively. In the bicyclic sugar moiety, the arabinofuranose rings adopt a C(3')exo/C(2')endo conformation and are highly puckered (7, = 57"). The solution conformation was studied by 'H and I3C nmr spectroscopy. A difference nOe proton nmr spectrum and 3~(~,~) coupling constants reveal an anti conformation in solution, with torsion angles very similar to those obtained from X-ray analysis. A comparison of observed 3J(H,H) coupling constants with those calculated on the basis of a modified Kitrplus equation shows significant differences, probably due to the presence of the bicyclic system. Une comparaison des constantes de couplage 3~( H ,~) observkes avec celles calculCes en se basant sur une Cquation de Karplus modifiee prksente des differences notables qui sont probablement dues i la presence du systkme bicyclique.[Traduit par la revue]