A simplified NMR-based method to assign the absolute configuration of aldose monosaccharides

A straightforward, aqueous-based procedure is described to determine the absolute configuration of aldose monosaccharides, a critical task in carbohydrate analysis. This approach leverages the reaction of aldose monosaccharide enantiomers with L-cysteine, a readily-available chiral amino acid, to form cyclic thiazolidine diastereomers. These derivatives can be analyzed using Heteronuclear Single Quantum Coherence (HSQC) NMR spectroscopy, specifically looking at the H-1 - C-1 proton-carbon single bond correlations. This NMR technique provides the necessary data to determine the absolute configuration of the parent monosaccharide. Several monosaccharide D-/L-enantiomeric pairs are analyzed as examples, and the technique is utilized to determine the compositional stereochemistry of complex polysaccharides (gellan and xanthan gums). We have also established the previously unknown stereochemistry of the arabinosyl residues found in frost grape polysaccharide (FGP) isolated from vines of the grape species, Vitis riparia Michx. The generalized method described is anticipated to be valuable to analytical carbohydrate chemists for the facile assignment of carbohydrate stereochemistry.