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.

Abstract

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 (C₁₇H₃₀N₂O₂₂·CH₃OH) 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 OCH₃ 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 (-CH₂OH) group in the βGlcNAc residue bearing the OCH₃ 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 -CH₂OH 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.