Comparison of observed torsion angles of β-D-GlcpNAc-(1–N)-Asn (a) and α-D-GlcpNAc-(1–N)-Asn (b) linkages. The latter linkage is not known to occur in nature, so that its presence in the PDB is probably based on erroneous coordinates. This assumption is supported by the relatively large scatter of the α-linkages in comparison to the β-linkages. The occurrence of these structures in the PDB might be based on improper or lacking chirality restraints on the linking C atom. The plots were generated with glyTorsion (see the legend to Fig. 2).