Disorder and conformational analysis of methyl β-D-galactopyranosyl-(1→4)-β-D-xylopyranoside

Abstract

Methyl β-D-galactopyranosyl-(1→4)-β-D-xylopyranoside, C(12)H(22)O(10), (II), crystallizes as colorless needles from water with positional disorder in the xylopyranosyl (Xyl) ring and no water molecules in the unit cell. The internal glycosidic linkage conformation in (II) is characterized by a φ' torsion angle (C2'(Gal)-C1'(Gal)-O1'(Gal)-C4(Xyl)) of 156.4 (5)° and a ψ' torsion angle (C1'(Gal)-O1'(Gal)-C4(Xyl)-C3(Xyl)) of 94.0 (11)°, where the ring atom numbering conforms to the convention in which C1 denotes the anomeric C atom, and C5 and C6 denote the hydroxymethyl (-CH(2)OH) C atoms in the β-Xyl and β-Gal residues, respectively. By comparison, the internal linkage conformation in the crystal structure of the structurally related disaccharide, methyl β-lactoside [methyl β-D-galactopyranosyl-(1→4)-β-D-glucopyranoside], (III) [Stenutz, Shang & Serianni (1999). Acta Cryst. C55, 1719-1721], is characterized by φ' = 153.8 (2)° and ψ' = 78.4 (2)°. A comparison of β-(1→4)-linked disaccharides shows considerable variability in both φ' and ψ', with the range in the latter (~38°) greater than that in the former (~28°). Inter-residue hydrogen bonding is observed between atoms O3(Xyl) and O5'(Gal) in the crystal structure of (II), analogous to the inter-residue hydrogen bond detected between atoms O3(Glc) and O5'(Gal) in (III). The exocyclic hydroxymethyl conformations in the Gal residues of (II) and (III) are identical (gauche-trans conformer).