Selective C-13-Labels on Repeating Glycan Oligomers to Reveal Protein Binding Epitopes through NMR: Polylactosamine Binding to Galectins

A combined chemo-enzymatic synthesis/NMR-based methodology is presented to identify, in unambiguous manner, the distinctive binding epitope within repeating sugar oligomers when binding to protein receptors. The concept is based on the incorporation of C-13-labels at specific monosaccharide units, selected within a repeating glycan oligomeric structure. No new chemical tags are added, and thus the chemical entity remains the same, while the presence of the C-13-labeled monosaccharide breaks the NMR chemical shift degeneracy that occurs in the non-labeled compound and allows the unique identification of the different components of the oligomer. The approach is demonstrated by a proof-of-concept study dealing with the interaction of a polylactosamine hexasaccharide with five different galectins that display distinct preferences for these entities.