Methyl 3-O-α-d-mannopyranosyl β-d-glucopyranoside tetrahydrate

The title compound, C13H24O11·4H2O, forms extended hydrogen-bonded networks. These are present between disaccharides, but not as inter-residue hydrogen bonds, as well as to water molecules that in addition form an intermolecular chain of hydrogen bonds. The conformation of the disaccharide is described by the glycosidic torsion angles ϕH = −34° and ψH = −5°. Macroscopically, the disaccharide was observed to be hygroscopic.


Experimental
Crystal data H atoms treated by a mixture of independent and constrained refinement Á max = 0.13 e Å À3 Á min = À0.14 e Å À3 Table 1 Selected torsion angles ( ).
are formed by oligosaccharides present as glycoconjugates. The information contents in carbohydrate structures are indeed very large as a consequence of the immense numbers of permutations possible by combining different linkages and anomeric configurations of the sugar residues. It is of particular importance that the often weak carbohydrate interactions function by resorting to multivalent interactions upon cell-cell recognition (Huskens, 2006).
The major degrees of freedom in an oligosaccharide are described by the torsion angles φ H , ψ H , and ω. For the title compound the two former are present at the glycosidic α-(1 → 3)-linkage with φ H being defined by H1m-C1m-O3g-C3g and ψ H by C1m-O3g-C3g-H3g. The ω torsion angle, defined by O5-C5-C6-O6, refers to the conformation of the hydroxymethyl group of each sugar residue. The structure is described as the exo-anomeric conformation with φ H = -34°, which, as a result of stereoelectronic effects, is characteristic of sugars in a cyclic form (Fig. 1). For the title compound the presence of the endo-anomeric effect (Juaristi & Cuevas, 1992) is evident from the difference in C-O bond lengths at the anomeric positions of the α-D-Manp residue having the axial bond C1m-O3g = 1.409 (2) Å and the β-D-Glcp residue having the equatorial bond C1g-O1g = 1.402 (2) Å, i.e., the bond with the axial electronegative atom is longer than the corresponding equatorial one, in complete agreement with ab initio data of model compounds (Odelius et al., 1995). At the glycosidic linkage ψ H = -5°, leading to an almost eclipsed conformation and as a result the inter-residue distance across the glycosidic linkage for the proton pair H1m-H3g becomes short, only 2.12 Å.
The conformations of the hydroxymethyl groups are described by one of the three rotamers, gauche-trans, gauchegauche, or trans-gauche with respect to the orientation of C6-O6 to C5-O5 and to C5-C4, respectively. In the present case both the mannopyranosyl and the glucopyranosyl residues show the gg conformation for their hydroxymethyl groups with ω = -64.9 (2)° and ω = -69.7 (2)°, respectively. This conformation is one of the two anticipated rotamers for the monosaccharides in the title compound, since both have an equatorial hydroxyl group at C4, which precludes the tg rotamer as a result of a non-favorable 1,3-diaxial interaction known as the Hassel-Ottar effect (Hassel & Ottar, 1947). The title compound was quite hygroscopic. This fact is consistent with the relatively high water content in the crystal of the title disaccharide. In our previous structural studies on disaccharide crystals the number of water molecules ranged from zero to three per disaccharide (Eriksson et al. 1997(Eriksson et al. , 2000(Eriksson et al. , 2002(Eriksson et al. , 2005Färnbäck et al. 2003Färnbäck et al. , 2008. All hydroxyl groups and all H atoms of the four water molecules are hydrogen bond donors and the structure is stabilized by an elaborate hydrogen bond network. The four water molecules can be considered as lying in channels along the b-direction between the sugar supplementary materials sup-2 residues as shown in Fig. 2. Previous conformational studies on the title compound that focused on solution patterns in binary aqueous solvent mixtures indicated that an inter-residue hydrogen bond was present between O6m as the donor atom and O2g as the acceptor atom (Vishnyakov et al. 2000). This was possible when the ω torsion angle of the mannosyl residue had the gt conformation. However, in the present crystal structure the exo-cyclic hydroxymethyl groups of the glucosyl residue as well as that in the mannosyl residue have the gg conformation, the latter of which precludes the intra-molecular hydrogen bond. Further analysis of the hydrogen bonding patterns showed that O6m acts as a donor to OW3. The O6g atom, on the other hand, acts as a donor to OW1, which acts as a donor to OW2, continued in a donor-acceptor relationship to OW3, and in an analogous way to OW4. Finally, the latter water molecule acts as a donor to the acceptor O6g in another molecule. Thus, the water-mediated chain starts from one glucosyl residue and ends at a symmetry related glucosyl residue.
Along the 'chain of water molecules' various atoms of the sugar residues act as hydrogen bond donors and acceptors. The close proximity of O2g in one molecule and O3m and O4m in a symmetry related molecule at distances of 3.140 (2) Å and 2.848 (2) Å, respectively, indicate that a bifurcated hydrogen bond is present with O2g as the donor atom. The triangle formed by the three oxygen atoms is almost isosceles with an O3m v -O2g-O4m v [symmetry code (v): -x + 1/2,y + 1/2,-z] angle of 56.09 (4)°.

Experimental
The synthesis of the title compound was described by Jansson et al. (1990). The disaccharide was crystallized by slow evaporation from a mixture of water, ethanol and acetonitrile (1:1:1) at ambient temperature. The absolute configuration of each sugar residue is known from the starting compounds used in the synthesis.

Refinement
The hydrogen atoms were geometrically placed and constrained to ride on the parent atom. The C-H bond distances are 0.96 Å for CH 3 , 0.97 Å for CH 2 , 0.98 Å for CH. The U iso (H) = 1.5 U eq (C) for the CH 3 and 1.2 U eq (C) for all other H atoms. Due to the abscence of significant anomalous scatterers, the value of the Flack parameter (Flack, 1983) was not meaningful, thus the 1707 Friedel equivalents were included in the merging process (MERG 3 in SHELXL97). The H atoms of the water molecule were located from difference density map and the d(O-H) were restrained to retain the previously known geometry of the water molecule. The hydrogen atoms of the water molecule were given U iso (H) = 1.5U eq (O).   supplementary materials sup-9