Disorder and conformational analysis of methyl [beta]-D-galactopyranosyl-(1[rightwards arrow]4)-[beta]-D-xylopyran­oside

Methyl [beta]-D-galactopyranosyl-(1[rightwards arrow]4)-[beta]-D-xylopyran­oside, C₁₂H₂₂O₁₀, (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 [varphi]' torsion angle (C2'_Gal-C1'_Gal-O1'_Gal-C4_Xyl) of 156.4 (5)° and a [psi]' 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 (-CH₂OH) C atoms in the [beta]-Xyl and [beta]-Gal residues, respectively. By comparison, the inter­nal linkage conformation in the crystal structure of the structurally related disaccharide, methyl [beta]-lactoside [methyl [beta]-D-galactopyrano­syl-(1[rightwards arrow]4)-[beta]-D-glucopyran­oside], (III) [Stenutz, Shang & Serianni (1999). Acta Cryst. C55, 1719-1721], is characterized by [varphi]' = 153.8 (2)° and [psi]' = 78.4 (2)°. A comparison of [beta]-(1[rightwards arrow]4)-linked disaccharides shows considerable variability in both [varphi]' and [psi]', 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).

Methyl 4-O-β-D-mannopyranosyl β-D-xylopyran­oside

Methyl β-D-mannopyranosyl-(1→4)-β-D-xylopyran­oside, C₁₂H₂₂O₁₀, (I), crystallizes as colorless needles from water, with two crystallographically independent mol­ecules, (IA) and (IB), comprising the asymmetric unit. The inter­nal glycosidic linkage conformation in mol­ecule (IA) is characterized by a φ′ torsion angle (O5′_Man—C1′_Man—O1′_Man—C4_X­yl; Man is mannose and Xyl is xylose) of −88.38 (17)° and a ψ′ torsion angle (C1′_Man—O1′_Man—C4_Xyl—C5_Xyl) of −149.22 (15)°, whereas the corresponding torsion angles in mol­ecule (IB) are −89.82 (17) and −159.98 (14)°, respectively. Ring atom numbering conforms to the convention in which C1 denotes the anomeric C atom, and C5 and C6 denote the hy­droxy­methyl (–CH₂OH) C atom in the β-Xylp and β-Manp residues, respectively. By comparison, the inter­nal glycosidic linkage in the major disorder component of the structurally related disaccharide, methyl β-D-galactopyranosyl-(1→4)-β-D-xylo­pyran­oside), (II) [Zhang, Oliver & Serriani (2012). Acta Cryst. C68, o7–o11], is characterized by φ′ = −85.7 (6)° and ψ′ = −141.6 (8)°. Inter-residue hydrogen bonding is observed between atoms O3_Xyl and O5′_Man in both (IA) and (IB) [O3_Xyl...O5′_Man inter­nuclear distances = 2.7268 (16) and 2.6920 (17) Å, respectively], analogous to the inter-residue hydrogen bond detected between atoms O3_Xyl and O5′_Gal in (II). Exocyclic hy­droxy­methyl group conformation in the β-Manp residue of (IA) is gauche–gauche, whereas that in the β-Manp residue of (IB) is gauche–trans.