A novel 13C isotopic labelling strategy for probing the structure and dynamics of glycan chains in situ on glycoproteins.

A protocol is described for uniform 13C labelling of terminal galactose residues of the glycan chains of glycoproteins, using an enzymatic method which does not perturb the protein. The technique is illustrated by application to the biantennary N-linked glycan chains attached at Asn 297 of immunoglobulin G (IgG). Isotope-edited NMR experiments on this glycoprotein yield data which suggest that the galactose residues on the glycan exist in two discrete environments, with the galactose in one environment having greater mobility than that in the other. These data are qualitatively consistent with crystallographic data on an Fc fragment, which suggest that one arm of the glycan is in contact with the protein, while the other projects into the space between the C gamma 2 domains. Quantitatively, however, these data cannot be rationalized with the crystallographic data, which implies subtle differences in oligosaccharide structure and dynamics between the solution and crystal states of Fc.