Calcium-carbohydrate bridges composed of uncharged sugars. Structure of a hydrated calcium bromide complex of alpha-fucose.

X-ray diffraction data were used to determine the crystal structure of a hydrated CaBr2 complex of alpha-fucose, a common terminal sugar of oligosaccharide chains on glycoproteins. Crystals of C6H12O5-CaBr2-3H2O are orthorhombic, space group P212121, with A equals 14.360(2), B equals 12.896(3), and C equals 8.043(1) A. Intensity data for 1442 independent reflections were measured with an automated diffractometer. A trial structure, obtained by the heavy-atom method, was refined by least-squares to R equals 0.052. Ca-2+ is chelated by a pair of hydroxyl groups from each of tow symmetry-related fucose molecules and is coordinated to three water molecules. Thus the structure consists of hydrated fucose-calcium-fucose bridges. The bridge geometry, which is dictated by the coordination requirements of Ca-2+, is like that of other calcium-carbohydrate complexes. Our results indicate that calcium-fucose interactions can provide an effective, sterospecific mechanism for cross-linking carbo hydrate chains. Similar calcium-carbohydrate bridges may be involved in a variety of Ca-2+-dependent agglutination and adhesion processes.