Interactions of cations with sugar anions. Part II. Crystal structure of strontium 4-O-(4-deoxy-β-L-threo-hex-4-enosyl)-α-D-galacturonate–4.5 water

Abstract

The crystal structure has been determined for the title compound (1) from 1 928 visually estimated reflections. The structure was solved by the heavy-atom method and refined by block-diagonal least-squares to R 0.11. Two molecules of the disaccharide exist in each asymmetric unit and, since the space group is P2₁, each unit cell [a= 12.81(1), b= 11.29(1), c= 14.81(2)Å; β= 119.3(4)°] contains 4 disaccharide residues, 4 Sr ions, and 18 H₂O molecules. One of the unsaturated residues in the asymmetric unit adopts the ²H₁, half-chair conformation, whereas the other adopts the ¹H₂, so that the glycosidic linkages in the two independent disaccharides are axial-axial and equatorial-axial respectively. The two independent strontium ions are co-ordinated similarly by three water molecules and the same five oxygen atoms of three saturated residues, in a distorted square antiprism. Unsaturated residues do not co-ordinate to Sr²⁺, even though they carry a formal negative charge at the carboxylate.

The glycosidic bond angles are rather smaller than expected : 112 in the axial-axial and 115° in the equatorialaxial conformer. Glycosidic torsion angles in the axial-axial form adopt values close to those in our model for sodium pectate. Carboxylate and ring oxygens form bidentate arrangements which co-ordinate to Sr²⁺.