Interactions of cations with sugar anions. Part I. Crystal structures of calcium sodium galacturonate hexahydrate and strontium sodium galacturonate hexahydrate

Abstract

The crystal structures have been determined for the isomorphous pair, calcium sodium galacturonate (I) and strontium sodium galacturonate (II), as models for the biologically important interactions between Ca2+ and the pyranose forms of sugar carboxylic acids. The space group is P63 and the cell dimensions are: a= 13·56(1), c= 9·70(1)Å(I): and a= 13·50(1), c= 9·63(1)Å(II). There are six galacturonate anoions, twelve water molecules, two sodium ions, and two alkaline-earth cations in the hexagonal unit cell. The structures were solved by Fourier methods from photographic data, and refined by least-squares techniques to R 12·2 [(I), 837 observed reflections] and 8·4%[(II), 946 observed reflections]. Each divalent cation is co-ordinated by three sugar anions and three water molecules in a nine-co-ordinate trigonal prism arrangement. The sugar oxygen atoms which co-ordinate to the cation are those of the carboxylate and the sugar ring. We suggest that sugar anions will in general co-ordinate to Ca2+ through the carboxylate oxygen and the oxygen function on the α-carbon atom. With these two oxygens in an eclipsed or near-eclipsed conformation. In solution, however, the system may deviate from the eclipsed conformation to facilitate a terdentate complex with its entropic advantage.