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

摘要

Carbohydrates containing galactopyranosyl and mannopyranosyl units with vicinal cis-diols were treated with NaVO_3 in D_2O,and complexation was determined by ~(51)V NMR spectroscopy.Me alpha-Gap,Me beta-Galp (3,4-cis-diols),and Me alpha-Manp (2,3-cis-diol) complexed,but Me beta-Manp barely did so.This low degree of complexation also occurred with a (beta-mannan containing alternate (1->3)- and (1->4)-linkages and an alginate having beta-ManpA blocks.In contrast,branched alpha-mannans complexed readily,although the ~(51)V resonances for one with side chains terminated with alpha-Manp-(l->3)-alpha-Manp-(l-> differed from another with only alpha-Manp-(l->2)-alpha-Manp-(l-> groups.The anomeric configuration of Me alpha-Galp and Me beta-Galp,each with 3,4-cis-diols remote from C-l,gave rise to three ~(5l)V signals of complexes with similar shifts and proportions.The shifts of a galactomannan with terminal alpha-Galp-(l->2)-alpha-Manp- were the same as those with alpha-Galp-(l->6)-beta-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 (delta-515 region,consistent with the dimeric trigonal-bipyramidal structure.