Complexes of carbohydrates with tungstate and other transition metal ions

Abstract

Electrophoresis of polyhydroxy-compounds in a sodium tungstate buffer (pH 5) has shown that cyclic polyols with six ring atoms will complex with tungstate when they possess three adjacent hydroxyl groups in a cis.cis,1,2,3-triol configuration. Acyclic polyols will complex with tungstate when they possess four adjacent hydroxyl groups But the configuration of these is not critical. 4-Substituted sorbitols are shown to Be exceptional in their behaviour; this is believed to Be due to a weaker type of oomplexing involving only the hydroxyl groups on carbon atoms 1, 2 and 3 of sorbitol. A study of the oxidation of hexitols in the presence of tungstate using a deficiency of periodate has confirmed that four hydroxyl groups are involved in the complex formation and has provided the basis for a synthesis of D-arabinose from mannitol in high yield. The change of rotation with pH of polyhydroxy-compounds in the presence of tungstate has Been examined. The maximum changes occur in the pH range 5.5 to 6.0. Polarimetric measurements indicate that the complexes contain two tungsten atoms per molecule of polyol. The complexes of 4 substituted sorbitols may contain only one tungsten atom per molecule of polyol. The tungstate is shown to he present in the complexesas the ion and two fundamental structures for theseare postulated in which each tungsten atom, tetrahedral in WO42-, expands to octahedral covalency by co-ordination with the oxygen atoms of adjacent glycol groups. In the complexes of cyclic polyols, where only three hydroxyl groups are involved, it is suggested that the fourth oxygen atom is provided by a molecule of water. The existence of an unstable intermediate type of complex is discussed. Some applications of the complexes are described. The complexes of polyhydroxy-compounds with molybdate are discussed in the light of the conclusions drawn from the analogous tungstate complexes.