Calcium binding to D-glucuronate residues: crystal structure of a hydrated calcium bromide salt of D-glucuronic acid.

Three-dimensional X-ray diffraction data were used to determine the crystal structure of alpha-D-glucuronate CaBr times 3H20, a model system for investigating the factors involved in the binding of calcium ions to D-glucuronate residues of oligo-and poly-saccharides. Crystals of the salt are monoclinic, space group P21, having a = 6.410 (1), b = 10.784 (2), c = 8.879 (1) A, betta = 92.07 (1)degrees, and Z = 2. Instensity data for 1082 reflections were measured with an automated diffractometer. A trial structure, obtained by the heavy-atom method, was refined by least squares to R = 0.025. The absolute configuration was confirmed by anomalous-dispersion effects. An outstanding feature of the crystal packing is the interaction of D-glucuronate anions with calcium ions. The calcium ion is coordinated to three symmetry-related D-glucuronate anions and to two water molecules. The D-glucuronate anion binds calcium cations through three chelation sites: one that involves a carboxyl-oxygen atom combined with O-5; one that includes the second carboxyl-oxygen atom acting in concert with O-4, and one composed of the O-1-O-2 pair of hydroxyl groups.