Exploiting Uniformly 13C‑Labeled Carbohydrates for Probing Carbohydrate–Protein Interactions by NMR Spectroscopy

NMR of a uniformly 13C-labeled carbohydrate was used to elucidate the atomic details of a sugar–protein complex. The structure of the 13C-labeled Manα(1–2)­Manα(1–2)­ManαOMe trisaccharide ligand, when bound to cyanovirin-N (CV-N), was characterized and revealed that in the complex the glycosidic linkage torsion angles between the two reducing-end mannoses are different from the free trisaccharide. Distances within the carbohydrate were employed for conformational analysis, and NOE-based distance mapping between sugar and protein revealed that Manα(1–2)­Manα(1–2)­ManαOMe is bound more intimately with its two reducing-end mannoses into the domain A binding site of CV–N than with the nonreducing end unit. Taking advantage of the 13C spectral dispersion of 13C-labeled carbohydrates in isotope-filtered experiments is a versatile means for a simultaneous mapping of the binding interactions on both, the carbohydrate and the protein.