Revealing Single-Bond Anomeric Selectivity in Carbohydrate-Protein Interactions

The noncovalent binding of proteins to glycans is amazingly selective to the isoforms of carbohydrates, including alpha/beta anomers that coexist in solution. We isolate in the gas phase and study at the atomic level the simplest model system: noncovalent complexes of monosaccharide alpha/beta-GalNAc and protonated aromatic molecule tyramine. IR/UV cold ion spectroscopy and quantum chemistry calculations jointly solve the structures of the two complexes. Although the onsets of the measured UV absorptions of the complexes differ significantly, the networks of H bonds in both complexes appear identical and do not include the anomeric hydroxyl. The detailed analysis reveals that, through inductive polarization, the alpha- to beta-reorientation of this group nevertheless reduces the length of one remote short intermolecular H-bond by 0.03 angstrom. Although small, this change substantially strengthens the bond, thus contributing to the anomeric selectivity of the binding.