Furanose vs. acyclic forms of carbohydrate ligands. A multinuclear NMR spectroscopy study of the molybdate and tungstate complexes of d-glycero-l-manno-heptose

The formation of dinuclear tungstate and molybdate complexes of d-glycero-l-manno-heptose was studied in aqueous solution by 13C and 183W NMR spectroscopy. The chelating aldose is always tetradentate and occurs exclusively in furanose or acyclic hydrated forms, the proportions of which depend on the pH and nature of the metal ion. In the tungstate species, two types of major complexes, noted M (O-2,3,5,6) and L (O-1,2,3,5) according to the site of chelation, involve the ligand in furanose form. In one of the minor tungstate complexes, chelation occurs at the galacto (O-3,4,5,6) site of the acyclic heptose. In the molybdate species, the complex of type M does not exist, and besides the complex of type L, the major species involve the acyclic ligand with either the galacto (O-3,4,5,6) or arabino (O-1,2,3,4) sites of chelation. Multinuclear NMR data are provided for the identification of the various types of complexes. Marked differences were noticed with respect to the complexes of d-mannose, in which species of type L prevailed with both molybdate and tungstate.