Methyl 3-O-α-l-fucopyranosyl α-d-galactopyranoside: a synchrotron study

The title compound, C13H24O10 is the methyl glycoside of a structural element α-l-Fucp-(1→ 3)-α-d-Galp making up two thirds of the repeating unit in the capsular polysaccharide of Klebsiella K63. The conformation of the title compound is described by the glycosidic torsion angles ϕH = 55 (1)° and ψH = −24 (1)°. The hydroxymethyl group in the galactose residue is present in the gauche–trans conformation. In the crystal, O—H⋯O hydrogen bonds connect the disaccharide units into chains along the a-axis direction and further hydrogen bonds cross-link the chains.

The title compound, C 13 H 24 O 10 is the methyl glycoside of a structural element -l-Fucp-(1! 3)--d-Galp making up two thirds of the repeating unit in the capsular polysaccharide of Klebsiella K63. The conformation of the title compound is described by the glycosidic torsion angles ' H = 55 (1) and H = À24 (1) . The hydroxymethyl group in the galactose residue is present in the gauche-trans conformation. In the crystal, O-HÁ Á ÁO hydrogen bonds connect the disaccharide units into chains along the a-axis direction and further hydrogen bonds cross-link the chains.
Polysaccharides are often built of repeating units of oligosaccharides having two to seven sugar residues in their repeats.
To understand the physicochemical properties and immunological specificity of the polymers it is essential to obtain information on their structures, both the primary and the three-dimensional structures. The torsion angles φ H , ψ H , and ω describe the major degrees of freedom in an oligosaccharide and for the title compound (I) the two former are present at the glycosidic α-(1 → 3)-linkage. In addition, for the galactose residue the φ H torsion angle is also of interest. The ω torsion angle refers to the conformation of the hydroxymethyl group in the galactose residue. Both of the φ H torsion angles in the structure are described by the exo-anomeric conformation with φ H = 55 (1)° for the fucose residue and φ H = -53 (1)° for the galactose residue (Fig. 1). The ψ H torsion angle may in solution populate more that one conformational state (see below); for title compound (I) ψ H = -24 (1)°. The conformation of the hydroxymethyl group is described by one of the three rotamers, gauche-trans, gauche-gauche, or trans-gauche with respect to the conformation of C6-O6 to C5-O5 and to C5-C4, respectively. In the present case the galactose residue has the gt conformation with ω = 70 (1)°, shifted away slightly from an ideal gauche conformation.
supplementary materials sup-2 In the study of Fucogel the conformational space of the constituent disaccharides were investigated by molecular mechanics and Ramachandran maps. Two low energy regions were identified from the adiabatic map of α-L-Fucp-(1→ 3)-α-D-Galp with essentially equal potential energy at their minima being (i) φO5 = 279.6° and ψC4 = 140.4° and (ii) φO5 = 260.2° and ψC4 = 70.2°, in which the former torsion angle is defined by O5f-C1f-O3g-C3g and the latter by C1g-O3g-C3g-C4g. Interresidue hydrogen bonding was not present for these two conformations although it was identified for a significantly higher-energy conformation.
The conformation of the title compound I and the corresponding glycosidic torsion angles in the polysaccharide are indeed quite similar. The resemblance of the crystal structure and the two low-energy minima of the adiabatic map suggests that torsion angle information from XRD data may be suitable as starting points for molecular modeling of oligo-and polysaccharides.
Interestingly, a fiber X-ray diffraction study of the Klebsiella K63 CPS shows that it forms an extended 2-fold helix (Elloway et al., 1980).

Experimental
The synthesis of (I) was described by Baumann et al. (1988) in which the fucose and galactose residues have the L and D absolute configurations, respectively. The compound was crystallized by slow evaporation of a mixture of water and ethanol (1:1) at ambient temperature.

Refinement
The hydrogen atoms were refined in riding mode with U iso (H) = 1.5U eq (X), where X = C or O. The coverage at 0.8 Å resolution = 0.738 but already at 0.9 Å resolution the coverage has increased to 0.922 and at 1.0 Å resolution the coveragẽ 0.995. The refinement with reflection data up to 1.0 Å resolution converged at R1 = 0.0466. It should be noted that the reflection data diminishes at high resolution as shown in Fig 2; thus the low coverage to 0.8 or 0.9 Å is of minor importance.