Heptamolybdate Ion Catalyzed Epimerization of Monosaccharides. CD Study of Binding of Monosaccharides and Polyols to Molybdate and TungstateReportar como inadecuado




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Croatica Chemica Acta, Vol.64 No.3 December 1991. -

Conformational properties and weak interactions of monosaccharides in the oxo-complexes with ammonium heptamolybdate AHM and ammonium paratungstate APT are studied by the CD spectroscopy, and their relation to the catalytic C2 epimerization step is discussed. It is found for molybdate complexes that there is unequivocal correlation between the torsional angles around the 1,2,3-triol subunit in the bound monosaccharide and the Cotton effects between 325 and 300 nm, 286 and 260 nm, and that around 235 nm.

A positive torsional angle in l,2-cis-2,3-cts triols leads to a positive first and second, and a negative third mentioned Cotton effect. Assuming a 1:2 ratio for the monosaccharide to MoVI in the complex, as strongly indicated by a recent 95Mo-NMR study, binding in the -C4 conformation can be predicted for the monosaccharides from the yxo-series, whereas for those of ribo-series binding in 4Ci conformation predominates. The first type of hexoses seems to engage the C6H20H group, if present, whereas the second one binds via 1,2,3-OH groups in cis-configuration. The absence of CD effects with the cyclic polyols 1—6, derived from various pyranoses, revealed that either three OH groups with cis-configuration or two cis-hydroxy groups and an axially arranged CIS-CH2OH group represent the minimal structural requirement for the formation of the CD active oxo-complexes. Two hexitols, D-glucitol 7, and D-mannitol 8, also form 1:2 complexes, engaging four hydroxy groups »from the end« of the chain C6-C3, as found by the X ray analysis for the crystalline complex of D-mannitol. This complexation mode leaves unbound C2, the only chiral center with the opposite configuration in these two ligands, leading to very similar CD spectra of the two complexes.



Autor: Gunther Snatzke - ; Chair of Structural Chemistry, Ruhr University, P. O. Box 102148, D W 4630 Bochum, Deutschland Jia Guo - ; Ch

Fuente: http://hrcak.srce.hr/



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