The tridentate metal-binding sites of the common glycoses

Abstract

Co^III(tacn) and Ga^III(tacn) fragments (tacn = 1,4,7-triazacyclononane) are suitable metal probes for the detection of the tridentate chelating sites of a glycose. Three moles of hydroxide per mole of cobalt or gallium support triple deprotonation of the chelating triol functions at a glycose's backbone. The individual chelating sites are detected using 1D and 2D NMR techniques. The metal-binding sites always include the hydroxy function at the anomeric carbon atom. Chelators are derived from both the pyranose and the furanose isomers while forming five- and six-membered chelate rings by the use of cis,cis-1,2,3-triol functions. The assignment of less frequently occurring ligand isomers are supported by a DFT approach. Crystal-structure analysis on Na₄[Cr(β-D-Manf1,2,3H₋₃)₂]NO₃·8.5H₂O (Man = mannose) additionally upholds the NMR assignments.