Complexation of vanadium(V) oxyanions with hexopyranose- and mannopyranoseuronic acid-containing polysaccharides: stereochemical considerations Original Research Article

Carbohydrates containing galactopyranosyl and mannopyranosyl units with vicinal cis-diols were treated with NaVO3 in D2O, and complexation was determined by 51V NMR spectroscopy. Me α-Galp, Me β-Galp (3,4-cis-diols), and Me α-Manp (2,3-cis-diol) complexed, but Me β-Manp barely did so. This low degree of complexation also occurred with a β-mannan containing alternate (1 → 3)- and (1 → 4)-linkages and an alginate having β-ManpA blocks. In contrast, branched α-mannans complexed readily, although the 51V resonances for one with side chains terminated with α-Manp-(1 → 3)-α-Manp-(1 → differed from another with only α-Manp-(1 → 2)-α-Manp-(1 → groups. The anomeric configuration of Me α-Galp and Me β-Galp, each with 3,4-cis-diols remote from C-1, gave rise to three 51V signals of complexes with similar shifts and proportions. The shifts of a galactomannan with terminal α-Galp-(1 → 2)-α-Manp- were the same as those with α-Galp-(1 → 6)-β-Manp- groups, but fewer complexes were formed with the former structure, probably due to greater steric crowding of the vanadate esters. Most of the complexes gave rise to a signal in the δ −515 region, consistent with the dimeric trigonal–bipyramidal structure.