Conformational disorder in the crystal structure of methyl 2-acetamido-2-deoxy-β-D-glucopyranosyl-(1→4)–2-acetamido-2-deoxy-β-D-glucopyranoside (methyl β-chitobioside) methanol monosolvate

Methyl 2-acetamido-2-deoxy-β-D-glucopyranosyl-(1→4)-2-acetamido-2-deoxy-β-D-glucopyranoside (methyl β-chitobioside), (IV), crystallizes from aqueous methanol at room temperature to give a structure (C 17 H 30 N 2 O 22 ·CH 3 OH) containing conformational disorder in the exocyclic hydroxymethyl group of one of its βGlcNAc residues. As observed in other X-ray structures of disaccharides containing β-(1→4) O -glycosidic linkages, inter-residue hydrogen bonding between O3H of the βGlcNAc bearing the OCH 3 aglycone and O5 of the adjacent βGlcNAc is observed based on the 2.79 Å internuclear distance between the O atoms. The structure of (IV) was compared to that determined previously for 2-acetamido-2-deoxy-β-D-glucopyranosyl-(1→4)-2-acetamido-2-deoxy-β-D-glucopyranose (β-chitobiose), (III). The O -glycosidic linkage torsion angles, phi (ϕ) and psi (ψ), in (III) and (IV) differ by 6–8°. The N -acetyl side chain conformation in (III) and (IV) shows some context dependence, with the C1—C2—N—C car torsion angle 10–15° smaller for the βGlcNAc residue involved in the internal O -glycosidic linkage. In (IV), conformational disorder is observed in the exocyclic hydroxymethyl (–CH 2 OH) group in the βGlcNAc residue bearing the OCH 3 aglycone, and a fitting of the electron density indicates an approximate 50:50 distribution of the gauche – gauche ( gg ) and gauche – trans ( gt ) conformers in the lattice. Similar behavior is not observed in (III), presumably due to the different packing structure in the vicinity of the –CH 2 OH substituent that affects its ability to hydrogen bond to proximal donors/acceptors. Unlike (IV), a re-examination of the previously reported electron density of (III) revealed conformational disorder in the N -acetyl side chain attached to the reducing-end βGlcNAc residue caused by rotation about the C2—N bond.