Crystal structure of methyl α-l-rhamno-pyranosyl-(1→2)-α-l-rhamno-pyran-oside monohydrate.

The title compound, CHO·HO, a structural model for part of bacterial O-anti-gen polysaccharides from and , crystallizes with four independent disaccharide mol-ecules and four water mol-ecules in the asymmetric unit. The conformation at the glycosidic linkage joining the two rhamnosyl residues is described by the torsion angles φ of 39, 30, 37 and 37°, and ψ of -32, -35, -31 and -32°, which are the major conformation region known to be populated in an aqueous solution. The hexo-pyran-ose rings have the chair conformation. In the crystal, the disaccharide and water mol-ecules are associated through O-H⋯O hydrogen bonds, forming a layer parallel to the plane. The layers stack along the axis hydro-phobic inter-actions between the methyl groups.