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-glyc ero-L-manno-heptose was studied in aqueous solution by C-13 and W-183 NMR spectroscopy. The chelating aldose is always tetradentate and occu rs exclusively in furanose or acyclic hydrated forms, the proportions of which depend on the pH and nature of the metal ion. In the tungstat e species, two types of major complexes, noted M (0-2,3,5,6) and L (0- 1,2,3,5) according to the site of chelation, involve the ligand in fur anose form. In one of the minor tungstate complexes, chelation occurs at the galacto (0-3,4,5,6) site of the acyclic heptose. In the molybda te species, the complex of type M does not exist, and besides the comp lex of type L, the major species involve the acyclic ligand with eithe r the galacto (0-3,4,5,6) or arabino (0-1,2,3,4) sites of chelation. M ultinuclear NMR data are provided for the identification of the variou s 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. (C) 1996 Elsevier Science Ltd.